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A study of stitch line formation during high speed laser patterning of thin film indium tin oxide transparent electrodes
Applied Surface Science 256 (2010) 7276–7284
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A study of stitch line formation during high speed laser patterning of thin film indium tin oxide transparent electrodes Paul M. Harrison ∗ , Nick Hay, Duncan P. Hand School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK
a r t i c l e
i n f o
Article history: Received 13 February 2010 Received in revised form 16 May 2010 Accepted 17 May 2010 Available online 24 May 2010 Keywords: ITO thin film PDP transparent electrode Laser patterning Stitch line Redeposition Transparent conductive oxide
a b s t r a c t High speed laser patterning of indium tin oxide thin films on glass is part of the production method used to produce transparent conductive electrodes for plasma display panels. Such a design consists of rows of repeating electrode structures which cover the active area of the display. Whilst the patterning process for such electrode structures exceeds the industrial acceptance criteria there are certain features that are yet to be fully understood. The visible line that occurs in-between two adjacent laser processed areas, commonly known as a stitch line, is one such feature. Previously published research claimed that the stitch line was caused by incomplete removal of the thin film however experimental results presented within this paper demonstrate that this cannot be the case and show that the stitch line is formed by redeposition of the plume of ablated material within the area of overlap with the previous pulse, and that heating of the sample by the second pulse plays a key role in stitch line formation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Indium tin oxide (ITO) thin films have many industrial uses, including numerous applications in the flat panel displays (FPDs) industry. These films are used within plasma display panels (PDPs) to form the transparent electrode which is required in order to allow the light generated by each pixel in the display to reach the viewer [1]. Most candidate thin films for this application are found within the group of transparent conductive oxides (TCOs) and for FPD applications ITO is one of the most commonly used [2]. In order to manufacture such transparent electrode structures it is typical to coat the whole of the glass sheet with an appropriate thin film and then selectively etch to form the electrode design. In the past, wet etch lithography has been the method of choice for patterning large areas such as displays but this technology has several financial and environmental drawbacks including the use of toxic chemicals which generate contaminated effluent, the need for many process stations and the use of expensive, consumable, large area masks [3]. Recently, laser based systems have been developed for processing ITO thin films on glass substrates for integration into PDP production lines [4,5]. In order to fully process a TCO coated glass panel for use in a PDP, two separate patterns must be etched:
∗ Corresponding author. Tel.: +44 1342 322 398. E-mail addresses: [email protected], [email protected] (P.M. Harrison). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.05.064
electrode structures and fan-out designs [6]. The requirements for these two patterning operations are quite different and although the basic operation of selective removal of a TCO thin film using a pulsed laser is common to both, there are many differences, particularly the spatial intensity profile of the incident laser beam and the pulse to pulse overlap. Electrode designs require the same basic electrode cell design to be repeated many times over a large area array, generally once for every pixel in the display. Single pulse processing is typically used in order to obtain high area coverage rates in order to process a PDP panel fast enough for a production line. Since the pixel size is quite large (for a 42 display size the pixel size is in the region of 850 m × 850 m), a high pulse energy (>30 mJ) is required to be able to pattern a complete pixel with a single laser pulse. In order to introduce the electrode shape into the beam delivery system a mask projection system is used which requires the mask illumination beam to have square spatial profile of uniform intensity, which is normally achieved either by the use of a beam homogeniser or by delivery through a multi-mode square optical fibre. On the other hand, fan-out designs, which are located within the border area of the panel, consist of long relatively thin lines of ITO which are non-symmetrical and non-repeating and are used to provide an electrical connection to each electrode row. They are produced by removing the unwanted ITO thin film in-between such tracks, and it is typical to use a finely focussed Gaussian beam to produce these designs, which requires a much lower pulse energy and high beam quality. ITO films have also been used for photo-voltaic [PV] applications, particularly for thin film designs on glass where this layer is
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used for the “front side” (sun facing) transparent conductive contact surface, and in most cases it is the layer which is closest to the glass substrate [6,7]. In order to increase the output voltage of the PV panel the active area is divided into many cells which are electrically connected in series. This involves the creation of a set of electrically isolating scribe lines at selected positions within each layer of the design. The ITO layer must be scribed with many long, thin parallel lines typically 30–50 m wide which are known as P1 scribe lines. It is very common for this operation to be carried out using high beam quality, ns pulse duration laser systems where the output is finely focussed onto the work-piece. Since this scribing operation occurs when only the ITO layer has been deposited onto the glass (i.e. before any other layer has been applied), it is very similar to the patterning of PDP panels, in particular the generation of fan-out designs. For both P1 scribing and the creation of fan-out designs the main performance criteria is electrical isolation rather than optical transparency, and in both cases stitch lines are tolerated provided that they do not cause electrical issues. The earliest research into laser patterning of ITO thin films used KrF excimer lasers operating at 248 nm with ns pulse duration which showed that the ITO layer could be removed by laser irradiation [8,9]. The removal mechanism was found to be thermal vaporisation which was considered to be dependant on both optical absorption and heat diffusion at this wavelength. However there were two drawbacks of using an ultra-violet wavelength – firstly it was found that multiple pulses were required to fully remove the coating, and secondly the glass substrate was strongly absorbing at this wavelength so that once the ITO layer was fully removed the glass would begin to etch if the process was allowed to continue, leading to significant levels of glass damage. This meant that careful process control was required to avoid glass damage, a potential cause of premature PDP failure. Yavas and co-workers examined laser processing of ITO thin films using pulsed DPSS lasers through a series of papers published between 1994 and 1999 [10–13]. The authors used a diode-pumped Q-switched Nd:YLF laser having a FWHM pulse duration of 15 ns and operating at a choice of either the fundamental wavelength of 1047 nm or second, third and fourth harmonics of 523 nm, 349 nm and 262 nm, respectively. The research concentrated on the production of fan-out designs, using Gaussian beam profiles with high pulse to pulse overlap for all experimental trials, and showed that for all wavelengths under test the ITO thin film can be removed and electrical isolation can be achieved. The work identified the material removal mechanism as thermal vaporisation for all wavelengths under test and shoulders were observed around the perimeter of the etched areas that were approximately 100 nm higher than the original surface of the ITO layer. The formation of these shoulders was attributed to surface tension gradients that caused liquid motion in an area near the edge of the incident Gaussian beam shape where it was claimed that melting was established for an extended time due to the poor thermal conductivity of the glass substrate. The appearance of the laser processed region was observed to vary according to the wavelength used which was attributed to differences in absorption in both the ITO coating and the substrate. Tests at the fundamental wavelength showed a series of “ripplelike lines” (which are also called “stitch lines” elsewhere) which were seen to occur at regular intervals and whose separation was linked to the pulse to pulse spacing. The appearance of these lines decreased as the wavelength decreased to the extent that at = 262 nm they were no longer visible. The formation of these “ripplelike” lines was attributed to incomplete removal of the ITO coating, and substrate absorption was demonstrated to aid in their removal. Since the experimental work used a multiple-pulse processing method with a Gaussian beam profile, the pulse intensity continuously varied over the processed area which had the drawback that it was difficult to distinguish between features caused by
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a single pulse and those features caused by two or more laser pulses processing the same area at different times. In addition it was not possible to make an assessment of how the beam intensity profile caused the various observed features due to the lack of an accurate spatial pulse fluence description. Furthermore, the claim that the “ripplelike” lines are caused by partial film removal has been found to be incorrect. Experimental results presented within this report using a flat-top beam intensity profile demonstrate that this is not the case, and show that initially the ITO coating is fully removed by the first incident pulse and that the stitch line is formed by a method of redeposition by partially overlapping pulses. Furthermore, the solution that was presented for removal of the “ripplelike” lines, i.e. the use of UV wavelengths, would be hard to incorporate into a high speed production environment for the manufacture of transparent electrodes for several reasons: firstly a multiple-pulse processing method would be too slow, secondly there would be positional registration problems due to the need to align several laser pulses with high accuracy at high speed and thirdly there would be a risk of significant localised glass substrate heating which is likely to cause micro-cracking, a major failure mechanism in any product using sheet-glass, particularly a device such as a PDP that undergoes thermal cycling during normal everyday use. Ashkensai et al. performed trials with a range of pulse durations from 150 fs to 5 ps at a wavelength of 800 nm using glass substrate samples coated with 150 nm thick ITO layer [14]. The results showed that the ITO coating could be cleanly removed using a multi-pulse process strategy, and confirmed that the ablation process at short pulse durations remained a thermal mechanism as opposed to a coulomb explosion due to the conductive nature of the film. Perimeter ridges were still evident but were significantly reduced and were, at the worst case, typically 20 nm high, which is much smaller than the ridges obtained using ns pulse processing. No mention was made of the existence of a stitch line. Park et al. investigated the use of femtosecond laser etching for ITO patterning applications for organic light emitting diode (OLED) devices where experimental trials used laser pulses at a wavelength of 810 nm and pulse duration 150 fs to process a 200-nm thick ITO coating on a glass substrate [15]. The results showed that the ITO etching process could be effectively self-limiting due to the large difference in threshold removal fluence between the thin film and the substrate, however multiple pulses were needed to fully remove the ITO coating in order to create isolated patterns. No mention was made of whether a stitch line existed. Crucially, the ITO coating could be removed with very little formation of the perimeter ridge, one of the characteristics of ns pulse duration laser processing and a feature that is detrimental to OLED performance. The absence of perimeter shoulders was attributed to the low removal threshold in the case of ultrafast ablation leading to the lack of large areas of melt surrounding the ablated region. Raˇciukaitus et al. published the results of ITO patterning trials using picosecond lasers at visible and ultra-violet wavelengths of 532 nm, 355 nm and 266 nm [16]. Clean removal of the ITO coating was reported at all tested wavelengths using multi-pulse processing and best results were obtained using the shortest wavelength. However, even at this wavelength perimeter ridges with a height of 30–50 nm were observed, and glass damage was noted to sometimes occur when using ultra-violet wavelengths if the pulse fluence was too high. The authors found that the perimeter ridge and surface contamination could be minimised using a pulse fluence which is close to the ablation threshold. No mention of a stitch line was made but the pulse to pulse overlap was noted to be very high during the experimental work. Research using pulsed laser sources for ITO etch applications is generally aimed at the production of fan-out designs where a multiple-pulse process method can be employed using a Gaussian beam profile from a high beam quality laser source. Stitch line
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Fig. 1. A sample electrode structure made using the workstation described below. The ITO thin film can be seen as a tan or brown area whilst the laser processed area is more white/grey. A stitch line, seen in the figure as a vertical line, is visible in-between each laser pulse. Fig. 2. Details of beam delivery using during experimental trials.
issues for fan-out designs are not so important for display manufacturers since fan-outs are not located within the active areas of the display however for electrode structures stitch lines are much more of an issue and require an explanation and a solution. Furthermore, an industrially viable process demands single pulse processing using shaped beam profiles, and as such the proposed solution from the fan-out patterning research to remove the stitch line is not appropriate. As an illustration of this point Fig. 1 shows a sample transparent electrode structure that has been etched into ITO on glass, and the stitch lines are clearly visible. Therefore the first step in finding possible solutions for this problem is to understand how the stitch line develops. In this work, a series of trials using a transparent electrode patterning workstation have been made in order to understand what constitutes a stitch line, where it is located and how it is formed. 2. Experimental details Several sets of tests were made using commercially available ITO coated glass. All samples were 2.8-mm thick PDP grade glass coated with 130-nm thick ITO manufactured by Corning. The experimental work was carried out using a workstation that featured a Powerlase Starlase AO4 q-switched DPSS laser with a nominal power output of 400 W and an output wavelength of 1064 nm. At a 6 kHz pulse repetition rate the laser generated a pulse energy of 53 mJ and duration of 35 ns (full width half maximum). The laser beam was attenuated externally and the beam spatially homogenised to form a uniform square flat-top shape using a microlens beam homogeniser manufactured by LIMO GmbH. This was used to illuminate an adjustable rectangular mask which is imaged onto the ITO coated glass sample. Fig. 2 shows a layout of this beam delivery system. The distances X and Y shown in the figure are calculated from Eq. (1) which is the Gaussian lens formula, where f is the focal length of the lens [17]. The demagnification factor of the mask image onto the object plane is determined by the ratio (X/Y). 1 1 1 + = X Y f
(1)
A galvanometric scanner (model HurryScan 25, manufactured by Scanlab GmbH) is used to move the beam in relation to the sample, and the scanner objective lens is used as the projection lens of the demagnification system. Throughout all of the experi-
mental work the size of the beam at the work-piece was fixed at 295 m × 414 m, and the laser repetition rate was set to 6 kHz. The pulse energy at the work-piece is critical to the analysis of the experimental work and measurements were made by removing the work-piece and positioning a power meter (model PM5200, manufactured by Molectron Detector Inc.) under the scanner so that measurements taken represented the actual power at the workpiece. The spot size at the work-piece was measured using the microscope described below and the pulse fluence (unit J/cm2 ) was determined by dividing the pulse energy by the spot size area. After processing, the samples were assessed with a Nikon Eclipse optical microscope using reflected illumination which allowed measurements to be made against a Nikon calibration standard. Further assessment of the ITO ablation was made using a Veeco Explorer Atomic Force Microscope (AFM) and transmission measurements were made using an Ocean Optics USB4000 fibre-coupled spectrometer. 2.1. Experimental test details In order to investigate the stitch line in more detail, four sets of tests were made. Each set consisted of a series of lines of laser pulses where, line by line, the pulse fluence was stepped through the range 1.0–5.0 J/cm2 in 0.2 J/cm2 intervals. Single pulse processing was used throughout the trials and the principal scan direction was along the horizontal axis, parallel to the short edge of the rectangular pulse profile. The test sets are illustrated in Fig. 3 and described below. (a) “Individual pulses” – horizontal lines of pulses with adjacent pulses spaced more than 500 m apart. (b) “Stitched pulses” – horizontal lines of pulses with adjacent pulses having 15 m overlap. (c) “Stitched, staggered pulses” – inclined lines where adjacent pulses have 80 m (25%) horizontal overlap and also a vertical step of 50 m (12%) per pulse. (d) “Tests using selectively coated glass” – horizontal lines of pulses with no overlap where half of the area of ITO coating was previously removed using a wet etch method. Each line in the set started in an uncoated region and ended in a coated region.
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Fig. 3. Illustration of the pulse layout for each experimental test set.
3. Results and discussion The results of the trials are summarised below. All microscope images show results with the earliest laser pulse on the left, i.e. the line of pulses always runs from left to right. Microscope images and equivalent AFM images and profiles from the same set series at the same fluence were compared in order to understand how height related details from the AFM profile corresponded to the various features and colours of the microscope image. One such comparison is shown in Fig. 4 for the case of the 1.4 J/cm2 test from the “stitched pulses” test series which examines the area of overlap of 2 pulses in order to see the maximum amount of height profile detail. The height of the ITO layer at particular positions in the profile is compared to the colours observed in the microscope picture, allowing other microscope images to be interpreted in a similar manner. In the figure, the area labelled “bare glass” (or at least having minimal residue) is seen in the corresponding microscope image as a grey colour and the area labelled “thin residue”, which is seen to be up to 30 nm higher than the “bare glass” area, is seen as a whitish area. As the height profile rises towards 50 ± 20 nm above the level
Fig. 4. Equivalent AFM and microscope images of the overlap of 2 pulses at 1.4 J/cm2 from the “stitched pulses” test set.
of the “bare glass” area a brown colour is observed, and cyan and purple colours are observed with a further height profile increase to within the range 85 ± 15 nm. 3.1. Individual pulses The aim of this test was to observe the effect of a single laser pulse without having any interaction from adjacent pulses. The
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Fig. 7. Transmission spectra for a laser etched ITO thin film sample. Note that the data is normalised compared to the transmission measured through the bare glass substrate. Fig. 5. ITO removal using a single laser pulse at fluence 3.0 J/cm2 .
result obtained with a pulse fluence of 3.0 J/cm2 , a typical fluence setting for maximum thin film removal, is shown in Fig. 5. It can be seen that the whitish colour of the laser processed area is fairly uniform with a slight darkening at the edges and the corners. Fig. 6 shows an AFM profile around the edge of this test and the characteristic perimeter ridge can be clearly seen as a 1-m wide ridge typically 70–100 nm higher than the unprocessed film which has been described in previous work [5]. The difference in height between the unprocessed ITO film and the laser irradiated area is typically measured to be 125 nm, indicating that a thin residue may cover the laser processed area since the ITO coating is known to have a thickness of 130 nm. In order to confirm the presence of a residue, the optical transmission through the sample was measured using a fibre-coupled spectrometer with a 200 m circular spot positioned at the centre of the irradiated area, and the result is shown in Fig. 7. The result is normalised against a glass substrate without ITO coating which represents 100% transmission at all wavelengths which accounts for losses in the system and intensity variations of the illumination source at different wavelengths. The figure shows that compared to bare glass there is a transmission loss which must be caused by some kind of residue on the glass – however, note that the decreased transmission at lower wavelengths may be caused by an increase in scattering rather than a reduction in optical transmission.
Fig. 6. AFM profile of the edge of an individual pulse.
3.2. Stitched pulses The aim of this test series was to observe the shape of the stitch line. Within this test set, for tests with pulse fluence in the range 2.4–4.2 J/cm2 the results appear to be quite similar, and Fig. 8 shows a typical result from this test series at the pulse fluence 3.0 J/cm2 . This figure clearly shows a regularly spaced white stitch line which is related to the pulse to pulse spacing. It has been examined in more detail in Fig. 9 which shows that the white line is a raised feature that is typically 25 nm higher that the nominal area of the first pulse and there is a darker section located within the area of pulse 2 which is lower than the nominal surface level. 3.3. Stitched, staggered pulses This test series is used to investigate the alignment of the stitch line in relation to individual pulses. Fig. 10 shows a result from this set at 3.0 J/cm2 which shows that the white section of the stitch line occurs close to the perimeter of the first pulse but only where it is overlapped by the second pulse. Fig. 11 shows a magnified view of the test result at 3.0 J/cm2 with an indication of the boundary location of the central pulse. The position of both regions of the stitch line can be seen in relation to the indicated pulse perimeter and the highlighted area A shows that these are on either side of the boundary of the pulse. Within this test set, the stitch line has an internal corner and in addition to the white lines following the perimeter of the first pulse, another thin white line can be seen which bisects the angle of this corner, as highlighted in area B of the figure. This feature is seen to occur on all tests within
Fig. 8. The result of the 3.0 J/cm2 test from the “stitched pulses” test series. Note that the pulse sequence moves from left to right.
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Fig. 9. A microscope image of the stitch line with corresponding AFM height profile of the 3.0 J/cm2 pulse fluence test of the “stitched pulses” test series.
the fluence range 2.2–4.2 J/cm2 . We postulate that the stitch line is formed from material ablated by the second pulse where it is incident upon unprocessed ITO. The pressure gradient formed by the plume of ablated material will expand upwards and outwards from this area and near the edge the horizontal pressure gradient will be perpendicular to the perimeter as a result of the uniform intensity profile. In the case of the two straight sides forming the internal corner pressure gradients from each side, as indicated in the enlarged view of area B of Fig. 11, will form the appropriate white sections of the “L-shaped” stitch line. Since these gradients
Fig. 10. Result from the “stitched, staggered pulses” test set at pulse fluence 3.0 J/cm2 .
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Fig. 11. Alignment of stitch line features relative to the edge of each pulse at fluence 3.0 J/cm2 .
are perpendicular to each other in the horizontal plane and will be equal in magnitude, the third white line will form as a result of the intersection of these pressure gradients. 3.4. Trials using selectively coated glass This test series was used to examine more closely whether redeposition can occur during laser patterning of ITO thin films. The position of the rectangular laser spot was chosen such that it impinged partly on coated and partly on uncoated glass. Fig. 12 shows a result at 5.0 J/cm2 from this test set where the region of bare glass is shown on the left hand side and the ITO coated glass on the right (the edge of the coated area is seen as a vertical line positioned towards the centre of the image). Each test line started within the bare glass section and ended in the coated region and is positioned so that a laser pulse straddles the edge of the coated area. The results show expected typical laser patterning within the ITO coated region. In the uncoated region, whilst at lower fluence levels there is limited redeposition, at a fluence in excess of 4.4 J/cm2 there is visible redeposition from the coated area onto the uncoated area where it appears as a white deposit. Fig. 13 shows a magnified view of the 5.0 J/cm2 test of this set. The coated region of the sample is shown on the right hand side of the picture and the uncoated area on the left. The main picture in the figure shows the lower section of a laser pulse that straddles the intersection between coated and uncoated regions. On the left hand side of the coated/uncoated intersection is an area of redeposited ITO material which is located on the uncoated region of the sample and appears as a white deposit and takes the shape of the incident
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P.M. Harrison et al. / Applied Surface Science 256 (2010) 7276–7284 Table 1 Optical and material properties of ITO used for the numerical model [9,13]. Property
Unit
ITO
−1
Cp
J kg
−1
Solid: (340.5 + 0.0904 × T) Melt: 521.3 7120 5.65 2 × 106 2300 3600 301.4 1281
K
−3
K ˛ (1064 nm) T (melting) T (evaporation) L (melting) L (evaporation)
kg m W m−1 K−1 m−1 K K kJ kg−1 kJ kg−1
4. Mathematical model
Fig. 12. Result from the “tests using selectively coated glass” test set at fluence 5.0 J/cm2 .
laser beam. The clearly defined shape of the redeposition could be caused either by the interaction of the laser beam with the ITO plume or with the substrate. In the lower left hand corner of the figure it is possible to see an additional area of redeposited material at the location of the previous pulse, even though this pulse was confined to the uncoated glass. We postulate that surface heating and/or charging of the substrate by this first pulse results in a region to which the plume generated by the subsequent pulse will stick.
A numerical model of the laser ablation process has been developed in order to examine the removal of the absorbing ITO coating from a glass substrate that is considered to be optically transparent. In the case of stitch line formation the aim of this thermal analysis is to investigate the heating effect of a typical laser pulse (i.e. fluence 3.0 J/cm2 , 40 ns duration) with an ITO layer which is thinner than the unprocessed coating thickness of 130 nm. Of particular interest is the region of overlap with the previous pulse where experimental results have shown that the thickness of the ITO layer is in the region of 5–30 nm. Experimental results have also shown that material can be preferentially deposited onto regions adjacent to an irradiated area that has an increased surface temperature, so our model was also used to investigate whether this is the case within the region of overlap of two adjacent pulses. The approach will be to determine for a given thickness of ITO, say in the region 20–30 nm, the peak temperature that is reached and whether vaporisation would be expected in that region. The laser beam has uniform intensity over the incident area and the ITO coating has a uniform thickness so a 1D thermal diffusion equation was used for this analysis as shown in Eq. (2) [18]. FlexPDE software was used to implement the thermal model which enables temperature related terms within the thermal differential equation, allowing temperature dependant material properties to be used, increasing the accuracy of the model. ∂T Q ∂ = + Cp ∂t ∂z
D∂T ∂z
(2)
where T is the temperature, t is the time, is the density, Cp is the specific heat, D is the thermal diffusivity and z is the depth into target material. Q(z,t) is the power density absorbed by the sample as a function of depth and is described by Eq. (3). Q (z, t) = P(t)(1 − R)˛e−˛z
(3)
where R is the material reflectivity, ˛ is the linear absorption coefficient of the material and P(t) is the temporal description of the incident laser beam power density as described by Eq. (4).
P(t) = PMAX exp
Fig. 13. Magnified view of the 5.0 J/cm2 test from the “tests using selectively coated glass” test set showing redeposited material on previously uncoated areas of the glass substrate.
−(t − to )2 4 ln 2 (t)
2
(4)
where to is the start time, t is the full width half maximum (FWHM) pulse duration of the laser pulse and PMAX is the peak power density. The optical and material properties of ITO and glass were taken from the literature [9,13] and are shown in Tables 1 and 2. Note that the thermal diffusivity, D, is obtained from D = (K/Cp ), where K is the thermal conductivity. Fig. 14 shows the calculated peak temperature for ITO layer thicknesses up to 130 nm using a laser pulse of 40 ns duration, fluence 3.0 J/cm2 . The figure shows two regions, the first indicated by a solid line showing peak temperatures below the vaporisation temperature (3600 K) and the second region indicated by a dotted
P.M. Harrison et al. / Applied Surface Science 256 (2010) 7276–7284 Table 2 Optical and material properties for glass used for the numerical model [9,13]. Property Cp K ˛ (1064 nm)
Unit −1
Glass substrate −1
J kg K kg m−3 W m−1 K−1 m−1
((0.275 × T) + 587.5) 2760 ((1.71 × 10−3 ) × T) + 0.6 79.7
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pulse is also shown for clarity. Each profile corresponds to a different ITO thickness, from 10 nm to 50 nm in steps of 5 nm. The graph shows that as the ITO layer thickness increases the peak temperature increases, and if the ITO layer is thicker than 25 nm the melting temperature is exceeded and the effect of the latent heat of melting can be seen as “shoulders” on the temperature profile. However for this range of ITO thickness the vaporisation temperature is not reached so it is expected that there would be no material removal since thermal vaporisation is the main removal mechanism when laser processing ITO thin films with the pulse characteristics used in this study. This means that within the area of overlap of two adjacent laser pulses, the first pulse will remove the majority of the ITO layer leaving a residual layer which will be heated by the second pulse but without material removal, making this region an ideal location for redeposition.
5. Discussion
Fig. 14. Thermal modelling results showing peak surface temperature in response to a 3.0 J/cm2 laser pulse for varying thickness of ITO on glass. Note that in the dotted region vaporisation occurs.
line which shows peak temperatures above the vaporisation temperature. However, it is hard to accurately model the vaporisation process of nanosecond duration laser pulses using a numerical simulation since there are many interdependent processes occurring simultaneously including evaporation, melt ejection and plasma formation, etc., so there is less confidence in the results in this region. The figure shows that the vaporisation temperature will be reached with a minimum ITO layer thickness of between 57 nm and 58 nm. At the wavelength of the incident light the optical absorption depth of ITO is 500 nm which is significantly larger than the thickness of any ITO layer under consideration, and with a 130-nm thick ITO layer at room temperature approximately 23% of the laser pulse is absorbed. Within the range of ITO thickness under consideration in this paper, i.e. up to 130 nm, absorption of the laser pulse will increase with thickness according to the Beer–Lambert law and the time to reach the vaporisation temperature will decrease. For the case of stitch line analysis the model is used to examine situations where vaporisation does not occur, i.e. with an ITO layer thickness of less than 57 nm, and a set of results is shown in Fig. 15. The figure shows a set of surface temperature profiles that simulate the response to a 40 ns duration, 3.0 J/cm2 fluence laser pulse acting on the sample. The temporal evolution of the laser heating
Fig. 15. Thermal modelling results showing surface temperature profile in response to a 3.0 J/cm2 laser pulse for varying thickness of ITO on glass.
For transparent electrode laser patterning applications, the stitch line formation has been analysed. The “individual pulses” test series showed that removal of the ITO layer is almost complete over the whole irradiated area and only a thin residue remains. The “stitched pulses” test series showed that the white stitch line is a raised line of material which is 20–30 nm higher than the typical area level. The results from the “stitched, staggered pulses” test set showed that the white stitch line only occurs within the area of overlap of two adjacent laser pulses and indicated that the pressure gradient of the ablated plume plays a part in forming the stitch line. The effect of redeposition of the ITO thin film has been observed using partly coated samples and it has been seen that material ablated from the coated region can be deposited onto the uncoated area. The area of redeposition is strongly controlled by the spatial profile of the current and previous laser pulses incident on the uncoated area indicating that laser-induced heating and/or charging of the surface aids the process. The redeposition process is also dependent on the plume that forms above the work-piece. The evaporation recoil pressure will push the plume upwards and outwards in all directions, but material is only visibly deposited at locations where the previous pulse was incident meaning that there must be some kind of interaction between the ablated plume and the irradiated area of overlap with the previous pulse. These trials showed that significant redeposition only occurred during higher fluence tests whereas for fully coated samples the stitch line is observed at much lower fluence levels. In the case of the trials on partially coated glass the bare glass has poor absorption at this wavelength resulting in lower surface temperatures meaning that higher intensities were needed to produce a sufficient surface temperature to enable the redeposition process. However, when processing the fully coated samples, in the area of overlap between two laser pulses there will be a layer of residue remaining after the first pulse. This means that during the second pulse there will be increased absorption compared to that of the bare substrate and as a result the surface temperature within this overlap area will be much higher, as shown by the thermal modelling results. Consequently, it is likely that lower pulse fluences would be capable of generating sufficiently high temperatures within the overlap region to enable the redeposition process, provided that the pulse has an intensity above the threshold to generate a plume of evaporated material in the fully coated region. The temperature rise in the overlap region caused by the second pulse has been analysed using the thermal model which indicates that at a modest fluence level of 3.0 J/cm2 , a fluence level where trials have shown that a single laser pulse is capable of removing the majority of the ITO coating, there would be a considerable temperature rise but would not be sufficient to reach the vaporisation temperature for the ITO
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thin film. This means that within this overlap region the second pulse will cause a temperature increase that would aid material redeposition but is unlikely to be sufficient to cause any material removal by vaporisation. Previous research has claimed that the stitch line for fan-out designs is caused by incomplete removal of the thin film [12,13], but it is difficult to see how this could occur. It is possible to achieve partial removal of a thin coating, for instance a metal layer, when the optical absorption is thinner than the film thickness so that a top layer of the film can be heated to the vaporisation temperature whilst the remaining depth of the film is not heated so much and remains at a low temperature. However, in this case the optical absorption depth for ITO at this wavelength is much larger than the film thickness, meaning that the full depth of the film will be heated simultaneously. At this wavelength it is not possible to heat only the surface of the ITO film in order to promote a regime of partial film removal. Similarly, if the incident fluence is reduced, as could be the case within the outer regions of a Gaussian beam profile, partial film removal by limited vaporisation would not be expected to occur since no part of the ITO film would be heated to the vaporisation temperature. Partial ITO film removal is therefore unlikely, and the most probable method of forming the stitch line during ITO electrode patterning is redeposition. 6. Conclusion The stitch line that occurs between two adjacent laser pulses used for patterning electrode structures on ITO thin films has been examined in detail and the location and formation of the line has been analysed. It has been found to consist of two regions: a thin white section located within the area of overlap of the adjacent pulses and a wider darker region that forms in the area processed by the second pulse only. AFM analysis has shown that in general the laser processed area is covered with a thin layer of residue and the white section of the stitch line is typically 10–30 nm higher that the nominal surface of the residue whilst the darker region is slightly lower. Experimental trials have shown that initially the ITO coating is fully removed by the first incident pulse and the edges of the irradiated area are slightly darker than the centre. Trials have also shown that the white section of the stitch line is formed by redeposition of material via the plume generated by the second pulse and is deposited within the region of overlap of the first and second pulse, an area which has an increased surface temperature at the time that the redeposition occurs due to interaction of the second pulse with the ITO residue remaining after the first pulse. The
whitish appearance of the stitch line may result from the imperfect crystal structure likely to arise from this type of deposition, and hence increased optical scattering. The results of numerical simulation of the surface temperature within the area of overlap due to the action of the second laser pulse have shown that within a given range of residual coating thicknesses there will be a significant temperature rise that is not sufficient to cause material removal by thermal vaporisation, and indeed it can have the opposite effect by enabling redeposition. Previous research has attributed the formation of the stitch line in fan-out designs to partial removal of the ITO thin film, but in this case it is unlikely since at this wavelength the optical absorption depth is significantly larger than the thin film thickness. Experimental trials have also demonstrated that single isolated laser pulses are capable of removing the ITO layer to an extent that the stitch line is not observed, and it is the occurrence of the second, slightly overlapped pulse that causes the stitch line to form. In this way partial removal of the ITO layer cannot be responsible for the stitch line formation and redeposition of material ablated by the second pulse is the most likely cause. References [1] J.P. Beouf, J. Phys. D: Appl. Phys. 36 (2003) R53–R79. [2] T. Minami, Semicond. Sci. Technol. 20 (2005) S35–S44. [3] M. Henry, J. Wendland, P.M. Harrison, D. Hand, Proceedings of the 26th International Congress on Applications of Lasers and Electro-Optics, 29th October–1st November, 2007, pp. 119–128. [4] S. Venkat, C. Dunsky, Proceedings of Photon Processing in Microelectronics and Photonics V, Photonics West Conference, SPIE, vol. 6106, San Jose, 2006, p. 610602. [5] M. Henry, P.M. Harrison, J. Wendland, J. Laser Micro/Nanoeng. 2 (2007) 49–56. [6] H.J. Booth, Thin Solid Films 453–454 (2004) 450–457. [7] J. Bovatsek, A. Tamhankar, R.S. Patel, N.M. Bulgavoka, J. Bonse, Thin Solid Films 518 (2010) 2897–2904. [8] J.G. Lunney, R.R. O’Neill, K. Schulmeister, Appl. Phys. Lett. 59 (6) (1991) 647–649. [9] T. Szörényi, L.D. Laude, I. Bertóti, Z. Kántor, Zs. Geretovsky, J. Appl. Phys. 78 (1995) 6211–6219. [10] M. Takai, D. Bollman, K. Haberger, Appl. Phys. Lett. 64 (1994) 2560–2562. [11] O. Yavas, M. Takai, Appl. Phys. Lett. 73 (1998) 2558–2560. [12] O. Yavas, C. Ochiai, M. Takai, Appl. Phys. A: Mater. Sci. Process 69 (1999) S875–S878. [13] O. Yavas, M. Takai, J. Appl. Phys. 85 (1999) 4207–4212. [14] D. Ashkensai, G. Müller, A. Rosenfeld, R. Stoian, I.V. Hertel, N.M. Bulgakova, E.E.B. Campbell, Appl. Phys. A 77 (2003) 223–228. [15] M. Park, B.H. Chon, H.S. Kim, S.C. Jeoung, D. Kim, J.I. Lee, H.Y. Chu, H.R. Kim, Opt. Lasers Eng. 44 (2006) 138–146. [16] G. Raˇciukaitus, M. Brikas, M. Gedvilas, T. Rakickas, Appl. Surf. Sci. 253 (2007) 6570–6574. [17] E. Hecht, Optics, 4th edition, Addison Wesley, 2002 (Chapter 5). [18] I.W. Boyd, Laser Processing of Thin Films and Microstructures, 1st edition, Springer Verlag, 1987 (Section 2.5).
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A study of stitch line formation during high speed laser patterning of thin film indium tin oxide transparent electrodes Paul M. Harrison ∗ , Nick Hay, Duncan P. Hand School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK
a r t i c l e
i n f o
Article history: Received 13 February 2010 Received in revised form 16 May 2010 Accepted 17 May 2010 Available online 24 May 2010 Keywords: ITO thin film PDP transparent electrode Laser patterning Stitch line Redeposition Transparent conductive oxide
a b s t r a c t High speed laser patterning of indium tin oxide thin films on glass is part of the production method used to produce transparent conductive electrodes for plasma display panels. Such a design consists of rows of repeating electrode structures which cover the active area of the display. Whilst the patterning process for such electrode structures exceeds the industrial acceptance criteria there are certain features that are yet to be fully understood. The visible line that occurs in-between two adjacent laser processed areas, commonly known as a stitch line, is one such feature. Previously published research claimed that the stitch line was caused by incomplete removal of the thin film however experimental results presented within this paper demonstrate that this cannot be the case and show that the stitch line is formed by redeposition of the plume of ablated material within the area of overlap with the previous pulse, and that heating of the sample by the second pulse plays a key role in stitch line formation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Indium tin oxide (ITO) thin films have many industrial uses, including numerous applications in the flat panel displays (FPDs) industry. These films are used within plasma display panels (PDPs) to form the transparent electrode which is required in order to allow the light generated by each pixel in the display to reach the viewer [1]. Most candidate thin films for this application are found within the group of transparent conductive oxides (TCOs) and for FPD applications ITO is one of the most commonly used [2]. In order to manufacture such transparent electrode structures it is typical to coat the whole of the glass sheet with an appropriate thin film and then selectively etch to form the electrode design. In the past, wet etch lithography has been the method of choice for patterning large areas such as displays but this technology has several financial and environmental drawbacks including the use of toxic chemicals which generate contaminated effluent, the need for many process stations and the use of expensive, consumable, large area masks [3]. Recently, laser based systems have been developed for processing ITO thin films on glass substrates for integration into PDP production lines [4,5]. In order to fully process a TCO coated glass panel for use in a PDP, two separate patterns must be etched:
∗ Corresponding author. Tel.: +44 1342 322 398. E-mail addresses: [email protected], [email protected] (P.M. Harrison). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.05.064
electrode structures and fan-out designs [6]. The requirements for these two patterning operations are quite different and although the basic operation of selective removal of a TCO thin film using a pulsed laser is common to both, there are many differences, particularly the spatial intensity profile of the incident laser beam and the pulse to pulse overlap. Electrode designs require the same basic electrode cell design to be repeated many times over a large area array, generally once for every pixel in the display. Single pulse processing is typically used in order to obtain high area coverage rates in order to process a PDP panel fast enough for a production line. Since the pixel size is quite large (for a 42 display size the pixel size is in the region of 850 m × 850 m), a high pulse energy (>30 mJ) is required to be able to pattern a complete pixel with a single laser pulse. In order to introduce the electrode shape into the beam delivery system a mask projection system is used which requires the mask illumination beam to have square spatial profile of uniform intensity, which is normally achieved either by the use of a beam homogeniser or by delivery through a multi-mode square optical fibre. On the other hand, fan-out designs, which are located within the border area of the panel, consist of long relatively thin lines of ITO which are non-symmetrical and non-repeating and are used to provide an electrical connection to each electrode row. They are produced by removing the unwanted ITO thin film in-between such tracks, and it is typical to use a finely focussed Gaussian beam to produce these designs, which requires a much lower pulse energy and high beam quality. ITO films have also been used for photo-voltaic [PV] applications, particularly for thin film designs on glass where this layer is
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used for the “front side” (sun facing) transparent conductive contact surface, and in most cases it is the layer which is closest to the glass substrate [6,7]. In order to increase the output voltage of the PV panel the active area is divided into many cells which are electrically connected in series. This involves the creation of a set of electrically isolating scribe lines at selected positions within each layer of the design. The ITO layer must be scribed with many long, thin parallel lines typically 30–50 m wide which are known as P1 scribe lines. It is very common for this operation to be carried out using high beam quality, ns pulse duration laser systems where the output is finely focussed onto the work-piece. Since this scribing operation occurs when only the ITO layer has been deposited onto the glass (i.e. before any other layer has been applied), it is very similar to the patterning of PDP panels, in particular the generation of fan-out designs. For both P1 scribing and the creation of fan-out designs the main performance criteria is electrical isolation rather than optical transparency, and in both cases stitch lines are tolerated provided that they do not cause electrical issues. The earliest research into laser patterning of ITO thin films used KrF excimer lasers operating at 248 nm with ns pulse duration which showed that the ITO layer could be removed by laser irradiation [8,9]. The removal mechanism was found to be thermal vaporisation which was considered to be dependant on both optical absorption and heat diffusion at this wavelength. However there were two drawbacks of using an ultra-violet wavelength – firstly it was found that multiple pulses were required to fully remove the coating, and secondly the glass substrate was strongly absorbing at this wavelength so that once the ITO layer was fully removed the glass would begin to etch if the process was allowed to continue, leading to significant levels of glass damage. This meant that careful process control was required to avoid glass damage, a potential cause of premature PDP failure. Yavas and co-workers examined laser processing of ITO thin films using pulsed DPSS lasers through a series of papers published between 1994 and 1999 [10–13]. The authors used a diode-pumped Q-switched Nd:YLF laser having a FWHM pulse duration of 15 ns and operating at a choice of either the fundamental wavelength of 1047 nm or second, third and fourth harmonics of 523 nm, 349 nm and 262 nm, respectively. The research concentrated on the production of fan-out designs, using Gaussian beam profiles with high pulse to pulse overlap for all experimental trials, and showed that for all wavelengths under test the ITO thin film can be removed and electrical isolation can be achieved. The work identified the material removal mechanism as thermal vaporisation for all wavelengths under test and shoulders were observed around the perimeter of the etched areas that were approximately 100 nm higher than the original surface of the ITO layer. The formation of these shoulders was attributed to surface tension gradients that caused liquid motion in an area near the edge of the incident Gaussian beam shape where it was claimed that melting was established for an extended time due to the poor thermal conductivity of the glass substrate. The appearance of the laser processed region was observed to vary according to the wavelength used which was attributed to differences in absorption in both the ITO coating and the substrate. Tests at the fundamental wavelength showed a series of “ripplelike lines” (which are also called “stitch lines” elsewhere) which were seen to occur at regular intervals and whose separation was linked to the pulse to pulse spacing. The appearance of these lines decreased as the wavelength decreased to the extent that at = 262 nm they were no longer visible. The formation of these “ripplelike” lines was attributed to incomplete removal of the ITO coating, and substrate absorption was demonstrated to aid in their removal. Since the experimental work used a multiple-pulse processing method with a Gaussian beam profile, the pulse intensity continuously varied over the processed area which had the drawback that it was difficult to distinguish between features caused by
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a single pulse and those features caused by two or more laser pulses processing the same area at different times. In addition it was not possible to make an assessment of how the beam intensity profile caused the various observed features due to the lack of an accurate spatial pulse fluence description. Furthermore, the claim that the “ripplelike” lines are caused by partial film removal has been found to be incorrect. Experimental results presented within this report using a flat-top beam intensity profile demonstrate that this is not the case, and show that initially the ITO coating is fully removed by the first incident pulse and that the stitch line is formed by a method of redeposition by partially overlapping pulses. Furthermore, the solution that was presented for removal of the “ripplelike” lines, i.e. the use of UV wavelengths, would be hard to incorporate into a high speed production environment for the manufacture of transparent electrodes for several reasons: firstly a multiple-pulse processing method would be too slow, secondly there would be positional registration problems due to the need to align several laser pulses with high accuracy at high speed and thirdly there would be a risk of significant localised glass substrate heating which is likely to cause micro-cracking, a major failure mechanism in any product using sheet-glass, particularly a device such as a PDP that undergoes thermal cycling during normal everyday use. Ashkensai et al. performed trials with a range of pulse durations from 150 fs to 5 ps at a wavelength of 800 nm using glass substrate samples coated with 150 nm thick ITO layer [14]. The results showed that the ITO coating could be cleanly removed using a multi-pulse process strategy, and confirmed that the ablation process at short pulse durations remained a thermal mechanism as opposed to a coulomb explosion due to the conductive nature of the film. Perimeter ridges were still evident but were significantly reduced and were, at the worst case, typically 20 nm high, which is much smaller than the ridges obtained using ns pulse processing. No mention was made of the existence of a stitch line. Park et al. investigated the use of femtosecond laser etching for ITO patterning applications for organic light emitting diode (OLED) devices where experimental trials used laser pulses at a wavelength of 810 nm and pulse duration 150 fs to process a 200-nm thick ITO coating on a glass substrate [15]. The results showed that the ITO etching process could be effectively self-limiting due to the large difference in threshold removal fluence between the thin film and the substrate, however multiple pulses were needed to fully remove the ITO coating in order to create isolated patterns. No mention was made of whether a stitch line existed. Crucially, the ITO coating could be removed with very little formation of the perimeter ridge, one of the characteristics of ns pulse duration laser processing and a feature that is detrimental to OLED performance. The absence of perimeter shoulders was attributed to the low removal threshold in the case of ultrafast ablation leading to the lack of large areas of melt surrounding the ablated region. Raˇciukaitus et al. published the results of ITO patterning trials using picosecond lasers at visible and ultra-violet wavelengths of 532 nm, 355 nm and 266 nm [16]. Clean removal of the ITO coating was reported at all tested wavelengths using multi-pulse processing and best results were obtained using the shortest wavelength. However, even at this wavelength perimeter ridges with a height of 30–50 nm were observed, and glass damage was noted to sometimes occur when using ultra-violet wavelengths if the pulse fluence was too high. The authors found that the perimeter ridge and surface contamination could be minimised using a pulse fluence which is close to the ablation threshold. No mention of a stitch line was made but the pulse to pulse overlap was noted to be very high during the experimental work. Research using pulsed laser sources for ITO etch applications is generally aimed at the production of fan-out designs where a multiple-pulse process method can be employed using a Gaussian beam profile from a high beam quality laser source. Stitch line
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Fig. 1. A sample electrode structure made using the workstation described below. The ITO thin film can be seen as a tan or brown area whilst the laser processed area is more white/grey. A stitch line, seen in the figure as a vertical line, is visible in-between each laser pulse. Fig. 2. Details of beam delivery using during experimental trials.
issues for fan-out designs are not so important for display manufacturers since fan-outs are not located within the active areas of the display however for electrode structures stitch lines are much more of an issue and require an explanation and a solution. Furthermore, an industrially viable process demands single pulse processing using shaped beam profiles, and as such the proposed solution from the fan-out patterning research to remove the stitch line is not appropriate. As an illustration of this point Fig. 1 shows a sample transparent electrode structure that has been etched into ITO on glass, and the stitch lines are clearly visible. Therefore the first step in finding possible solutions for this problem is to understand how the stitch line develops. In this work, a series of trials using a transparent electrode patterning workstation have been made in order to understand what constitutes a stitch line, where it is located and how it is formed. 2. Experimental details Several sets of tests were made using commercially available ITO coated glass. All samples were 2.8-mm thick PDP grade glass coated with 130-nm thick ITO manufactured by Corning. The experimental work was carried out using a workstation that featured a Powerlase Starlase AO4 q-switched DPSS laser with a nominal power output of 400 W and an output wavelength of 1064 nm. At a 6 kHz pulse repetition rate the laser generated a pulse energy of 53 mJ and duration of 35 ns (full width half maximum). The laser beam was attenuated externally and the beam spatially homogenised to form a uniform square flat-top shape using a microlens beam homogeniser manufactured by LIMO GmbH. This was used to illuminate an adjustable rectangular mask which is imaged onto the ITO coated glass sample. Fig. 2 shows a layout of this beam delivery system. The distances X and Y shown in the figure are calculated from Eq. (1) which is the Gaussian lens formula, where f is the focal length of the lens [17]. The demagnification factor of the mask image onto the object plane is determined by the ratio (X/Y). 1 1 1 + = X Y f
(1)
A galvanometric scanner (model HurryScan 25, manufactured by Scanlab GmbH) is used to move the beam in relation to the sample, and the scanner objective lens is used as the projection lens of the demagnification system. Throughout all of the experi-
mental work the size of the beam at the work-piece was fixed at 295 m × 414 m, and the laser repetition rate was set to 6 kHz. The pulse energy at the work-piece is critical to the analysis of the experimental work and measurements were made by removing the work-piece and positioning a power meter (model PM5200, manufactured by Molectron Detector Inc.) under the scanner so that measurements taken represented the actual power at the workpiece. The spot size at the work-piece was measured using the microscope described below and the pulse fluence (unit J/cm2 ) was determined by dividing the pulse energy by the spot size area. After processing, the samples were assessed with a Nikon Eclipse optical microscope using reflected illumination which allowed measurements to be made against a Nikon calibration standard. Further assessment of the ITO ablation was made using a Veeco Explorer Atomic Force Microscope (AFM) and transmission measurements were made using an Ocean Optics USB4000 fibre-coupled spectrometer. 2.1. Experimental test details In order to investigate the stitch line in more detail, four sets of tests were made. Each set consisted of a series of lines of laser pulses where, line by line, the pulse fluence was stepped through the range 1.0–5.0 J/cm2 in 0.2 J/cm2 intervals. Single pulse processing was used throughout the trials and the principal scan direction was along the horizontal axis, parallel to the short edge of the rectangular pulse profile. The test sets are illustrated in Fig. 3 and described below. (a) “Individual pulses” – horizontal lines of pulses with adjacent pulses spaced more than 500 m apart. (b) “Stitched pulses” – horizontal lines of pulses with adjacent pulses having 15 m overlap. (c) “Stitched, staggered pulses” – inclined lines where adjacent pulses have 80 m (25%) horizontal overlap and also a vertical step of 50 m (12%) per pulse. (d) “Tests using selectively coated glass” – horizontal lines of pulses with no overlap where half of the area of ITO coating was previously removed using a wet etch method. Each line in the set started in an uncoated region and ended in a coated region.
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Fig. 3. Illustration of the pulse layout for each experimental test set.
3. Results and discussion The results of the trials are summarised below. All microscope images show results with the earliest laser pulse on the left, i.e. the line of pulses always runs from left to right. Microscope images and equivalent AFM images and profiles from the same set series at the same fluence were compared in order to understand how height related details from the AFM profile corresponded to the various features and colours of the microscope image. One such comparison is shown in Fig. 4 for the case of the 1.4 J/cm2 test from the “stitched pulses” test series which examines the area of overlap of 2 pulses in order to see the maximum amount of height profile detail. The height of the ITO layer at particular positions in the profile is compared to the colours observed in the microscope picture, allowing other microscope images to be interpreted in a similar manner. In the figure, the area labelled “bare glass” (or at least having minimal residue) is seen in the corresponding microscope image as a grey colour and the area labelled “thin residue”, which is seen to be up to 30 nm higher than the “bare glass” area, is seen as a whitish area. As the height profile rises towards 50 ± 20 nm above the level
Fig. 4. Equivalent AFM and microscope images of the overlap of 2 pulses at 1.4 J/cm2 from the “stitched pulses” test set.
of the “bare glass” area a brown colour is observed, and cyan and purple colours are observed with a further height profile increase to within the range 85 ± 15 nm. 3.1. Individual pulses The aim of this test was to observe the effect of a single laser pulse without having any interaction from adjacent pulses. The
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Fig. 7. Transmission spectra for a laser etched ITO thin film sample. Note that the data is normalised compared to the transmission measured through the bare glass substrate. Fig. 5. ITO removal using a single laser pulse at fluence 3.0 J/cm2 .
result obtained with a pulse fluence of 3.0 J/cm2 , a typical fluence setting for maximum thin film removal, is shown in Fig. 5. It can be seen that the whitish colour of the laser processed area is fairly uniform with a slight darkening at the edges and the corners. Fig. 6 shows an AFM profile around the edge of this test and the characteristic perimeter ridge can be clearly seen as a 1-m wide ridge typically 70–100 nm higher than the unprocessed film which has been described in previous work [5]. The difference in height between the unprocessed ITO film and the laser irradiated area is typically measured to be 125 nm, indicating that a thin residue may cover the laser processed area since the ITO coating is known to have a thickness of 130 nm. In order to confirm the presence of a residue, the optical transmission through the sample was measured using a fibre-coupled spectrometer with a 200 m circular spot positioned at the centre of the irradiated area, and the result is shown in Fig. 7. The result is normalised against a glass substrate without ITO coating which represents 100% transmission at all wavelengths which accounts for losses in the system and intensity variations of the illumination source at different wavelengths. The figure shows that compared to bare glass there is a transmission loss which must be caused by some kind of residue on the glass – however, note that the decreased transmission at lower wavelengths may be caused by an increase in scattering rather than a reduction in optical transmission.
Fig. 6. AFM profile of the edge of an individual pulse.
3.2. Stitched pulses The aim of this test series was to observe the shape of the stitch line. Within this test set, for tests with pulse fluence in the range 2.4–4.2 J/cm2 the results appear to be quite similar, and Fig. 8 shows a typical result from this test series at the pulse fluence 3.0 J/cm2 . This figure clearly shows a regularly spaced white stitch line which is related to the pulse to pulse spacing. It has been examined in more detail in Fig. 9 which shows that the white line is a raised feature that is typically 25 nm higher that the nominal area of the first pulse and there is a darker section located within the area of pulse 2 which is lower than the nominal surface level. 3.3. Stitched, staggered pulses This test series is used to investigate the alignment of the stitch line in relation to individual pulses. Fig. 10 shows a result from this set at 3.0 J/cm2 which shows that the white section of the stitch line occurs close to the perimeter of the first pulse but only where it is overlapped by the second pulse. Fig. 11 shows a magnified view of the test result at 3.0 J/cm2 with an indication of the boundary location of the central pulse. The position of both regions of the stitch line can be seen in relation to the indicated pulse perimeter and the highlighted area A shows that these are on either side of the boundary of the pulse. Within this test set, the stitch line has an internal corner and in addition to the white lines following the perimeter of the first pulse, another thin white line can be seen which bisects the angle of this corner, as highlighted in area B of the figure. This feature is seen to occur on all tests within
Fig. 8. The result of the 3.0 J/cm2 test from the “stitched pulses” test series. Note that the pulse sequence moves from left to right.
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Fig. 9. A microscope image of the stitch line with corresponding AFM height profile of the 3.0 J/cm2 pulse fluence test of the “stitched pulses” test series.
the fluence range 2.2–4.2 J/cm2 . We postulate that the stitch line is formed from material ablated by the second pulse where it is incident upon unprocessed ITO. The pressure gradient formed by the plume of ablated material will expand upwards and outwards from this area and near the edge the horizontal pressure gradient will be perpendicular to the perimeter as a result of the uniform intensity profile. In the case of the two straight sides forming the internal corner pressure gradients from each side, as indicated in the enlarged view of area B of Fig. 11, will form the appropriate white sections of the “L-shaped” stitch line. Since these gradients
Fig. 10. Result from the “stitched, staggered pulses” test set at pulse fluence 3.0 J/cm2 .
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Fig. 11. Alignment of stitch line features relative to the edge of each pulse at fluence 3.0 J/cm2 .
are perpendicular to each other in the horizontal plane and will be equal in magnitude, the third white line will form as a result of the intersection of these pressure gradients. 3.4. Trials using selectively coated glass This test series was used to examine more closely whether redeposition can occur during laser patterning of ITO thin films. The position of the rectangular laser spot was chosen such that it impinged partly on coated and partly on uncoated glass. Fig. 12 shows a result at 5.0 J/cm2 from this test set where the region of bare glass is shown on the left hand side and the ITO coated glass on the right (the edge of the coated area is seen as a vertical line positioned towards the centre of the image). Each test line started within the bare glass section and ended in the coated region and is positioned so that a laser pulse straddles the edge of the coated area. The results show expected typical laser patterning within the ITO coated region. In the uncoated region, whilst at lower fluence levels there is limited redeposition, at a fluence in excess of 4.4 J/cm2 there is visible redeposition from the coated area onto the uncoated area where it appears as a white deposit. Fig. 13 shows a magnified view of the 5.0 J/cm2 test of this set. The coated region of the sample is shown on the right hand side of the picture and the uncoated area on the left. The main picture in the figure shows the lower section of a laser pulse that straddles the intersection between coated and uncoated regions. On the left hand side of the coated/uncoated intersection is an area of redeposited ITO material which is located on the uncoated region of the sample and appears as a white deposit and takes the shape of the incident
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P.M. Harrison et al. / Applied Surface Science 256 (2010) 7276–7284 Table 1 Optical and material properties of ITO used for the numerical model [9,13]. Property
Unit
ITO
−1
Cp
J kg
−1
Solid: (340.5 + 0.0904 × T) Melt: 521.3 7120 5.65 2 × 106 2300 3600 301.4 1281
K
−3
K ˛ (1064 nm) T (melting) T (evaporation) L (melting) L (evaporation)
kg m W m−1 K−1 m−1 K K kJ kg−1 kJ kg−1
4. Mathematical model
Fig. 12. Result from the “tests using selectively coated glass” test set at fluence 5.0 J/cm2 .
laser beam. The clearly defined shape of the redeposition could be caused either by the interaction of the laser beam with the ITO plume or with the substrate. In the lower left hand corner of the figure it is possible to see an additional area of redeposited material at the location of the previous pulse, even though this pulse was confined to the uncoated glass. We postulate that surface heating and/or charging of the substrate by this first pulse results in a region to which the plume generated by the subsequent pulse will stick.
A numerical model of the laser ablation process has been developed in order to examine the removal of the absorbing ITO coating from a glass substrate that is considered to be optically transparent. In the case of stitch line formation the aim of this thermal analysis is to investigate the heating effect of a typical laser pulse (i.e. fluence 3.0 J/cm2 , 40 ns duration) with an ITO layer which is thinner than the unprocessed coating thickness of 130 nm. Of particular interest is the region of overlap with the previous pulse where experimental results have shown that the thickness of the ITO layer is in the region of 5–30 nm. Experimental results have also shown that material can be preferentially deposited onto regions adjacent to an irradiated area that has an increased surface temperature, so our model was also used to investigate whether this is the case within the region of overlap of two adjacent pulses. The approach will be to determine for a given thickness of ITO, say in the region 20–30 nm, the peak temperature that is reached and whether vaporisation would be expected in that region. The laser beam has uniform intensity over the incident area and the ITO coating has a uniform thickness so a 1D thermal diffusion equation was used for this analysis as shown in Eq. (2) [18]. FlexPDE software was used to implement the thermal model which enables temperature related terms within the thermal differential equation, allowing temperature dependant material properties to be used, increasing the accuracy of the model. ∂T Q ∂ = + Cp ∂t ∂z
D∂T ∂z
(2)
where T is the temperature, t is the time, is the density, Cp is the specific heat, D is the thermal diffusivity and z is the depth into target material. Q(z,t) is the power density absorbed by the sample as a function of depth and is described by Eq. (3). Q (z, t) = P(t)(1 − R)˛e−˛z
(3)
where R is the material reflectivity, ˛ is the linear absorption coefficient of the material and P(t) is the temporal description of the incident laser beam power density as described by Eq. (4).
P(t) = PMAX exp
Fig. 13. Magnified view of the 5.0 J/cm2 test from the “tests using selectively coated glass” test set showing redeposited material on previously uncoated areas of the glass substrate.
−(t − to )2 4 ln 2 (t)
2
(4)
where to is the start time, t is the full width half maximum (FWHM) pulse duration of the laser pulse and PMAX is the peak power density. The optical and material properties of ITO and glass were taken from the literature [9,13] and are shown in Tables 1 and 2. Note that the thermal diffusivity, D, is obtained from D = (K/Cp ), where K is the thermal conductivity. Fig. 14 shows the calculated peak temperature for ITO layer thicknesses up to 130 nm using a laser pulse of 40 ns duration, fluence 3.0 J/cm2 . The figure shows two regions, the first indicated by a solid line showing peak temperatures below the vaporisation temperature (3600 K) and the second region indicated by a dotted
P.M. Harrison et al. / Applied Surface Science 256 (2010) 7276–7284 Table 2 Optical and material properties for glass used for the numerical model [9,13]. Property Cp K ˛ (1064 nm)
Unit −1
Glass substrate −1
J kg K kg m−3 W m−1 K−1 m−1
((0.275 × T) + 587.5) 2760 ((1.71 × 10−3 ) × T) + 0.6 79.7
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pulse is also shown for clarity. Each profile corresponds to a different ITO thickness, from 10 nm to 50 nm in steps of 5 nm. The graph shows that as the ITO layer thickness increases the peak temperature increases, and if the ITO layer is thicker than 25 nm the melting temperature is exceeded and the effect of the latent heat of melting can be seen as “shoulders” on the temperature profile. However for this range of ITO thickness the vaporisation temperature is not reached so it is expected that there would be no material removal since thermal vaporisation is the main removal mechanism when laser processing ITO thin films with the pulse characteristics used in this study. This means that within the area of overlap of two adjacent laser pulses, the first pulse will remove the majority of the ITO layer leaving a residual layer which will be heated by the second pulse but without material removal, making this region an ideal location for redeposition.
5. Discussion
Fig. 14. Thermal modelling results showing peak surface temperature in response to a 3.0 J/cm2 laser pulse for varying thickness of ITO on glass. Note that in the dotted region vaporisation occurs.
line which shows peak temperatures above the vaporisation temperature. However, it is hard to accurately model the vaporisation process of nanosecond duration laser pulses using a numerical simulation since there are many interdependent processes occurring simultaneously including evaporation, melt ejection and plasma formation, etc., so there is less confidence in the results in this region. The figure shows that the vaporisation temperature will be reached with a minimum ITO layer thickness of between 57 nm and 58 nm. At the wavelength of the incident light the optical absorption depth of ITO is 500 nm which is significantly larger than the thickness of any ITO layer under consideration, and with a 130-nm thick ITO layer at room temperature approximately 23% of the laser pulse is absorbed. Within the range of ITO thickness under consideration in this paper, i.e. up to 130 nm, absorption of the laser pulse will increase with thickness according to the Beer–Lambert law and the time to reach the vaporisation temperature will decrease. For the case of stitch line analysis the model is used to examine situations where vaporisation does not occur, i.e. with an ITO layer thickness of less than 57 nm, and a set of results is shown in Fig. 15. The figure shows a set of surface temperature profiles that simulate the response to a 40 ns duration, 3.0 J/cm2 fluence laser pulse acting on the sample. The temporal evolution of the laser heating
Fig. 15. Thermal modelling results showing surface temperature profile in response to a 3.0 J/cm2 laser pulse for varying thickness of ITO on glass.
For transparent electrode laser patterning applications, the stitch line formation has been analysed. The “individual pulses” test series showed that removal of the ITO layer is almost complete over the whole irradiated area and only a thin residue remains. The “stitched pulses” test series showed that the white stitch line is a raised line of material which is 20–30 nm higher than the typical area level. The results from the “stitched, staggered pulses” test set showed that the white stitch line only occurs within the area of overlap of two adjacent laser pulses and indicated that the pressure gradient of the ablated plume plays a part in forming the stitch line. The effect of redeposition of the ITO thin film has been observed using partly coated samples and it has been seen that material ablated from the coated region can be deposited onto the uncoated area. The area of redeposition is strongly controlled by the spatial profile of the current and previous laser pulses incident on the uncoated area indicating that laser-induced heating and/or charging of the surface aids the process. The redeposition process is also dependent on the plume that forms above the work-piece. The evaporation recoil pressure will push the plume upwards and outwards in all directions, but material is only visibly deposited at locations where the previous pulse was incident meaning that there must be some kind of interaction between the ablated plume and the irradiated area of overlap with the previous pulse. These trials showed that significant redeposition only occurred during higher fluence tests whereas for fully coated samples the stitch line is observed at much lower fluence levels. In the case of the trials on partially coated glass the bare glass has poor absorption at this wavelength resulting in lower surface temperatures meaning that higher intensities were needed to produce a sufficient surface temperature to enable the redeposition process. However, when processing the fully coated samples, in the area of overlap between two laser pulses there will be a layer of residue remaining after the first pulse. This means that during the second pulse there will be increased absorption compared to that of the bare substrate and as a result the surface temperature within this overlap area will be much higher, as shown by the thermal modelling results. Consequently, it is likely that lower pulse fluences would be capable of generating sufficiently high temperatures within the overlap region to enable the redeposition process, provided that the pulse has an intensity above the threshold to generate a plume of evaporated material in the fully coated region. The temperature rise in the overlap region caused by the second pulse has been analysed using the thermal model which indicates that at a modest fluence level of 3.0 J/cm2 , a fluence level where trials have shown that a single laser pulse is capable of removing the majority of the ITO coating, there would be a considerable temperature rise but would not be sufficient to reach the vaporisation temperature for the ITO
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thin film. This means that within this overlap region the second pulse will cause a temperature increase that would aid material redeposition but is unlikely to be sufficient to cause any material removal by vaporisation. Previous research has claimed that the stitch line for fan-out designs is caused by incomplete removal of the thin film [12,13], but it is difficult to see how this could occur. It is possible to achieve partial removal of a thin coating, for instance a metal layer, when the optical absorption is thinner than the film thickness so that a top layer of the film can be heated to the vaporisation temperature whilst the remaining depth of the film is not heated so much and remains at a low temperature. However, in this case the optical absorption depth for ITO at this wavelength is much larger than the film thickness, meaning that the full depth of the film will be heated simultaneously. At this wavelength it is not possible to heat only the surface of the ITO film in order to promote a regime of partial film removal. Similarly, if the incident fluence is reduced, as could be the case within the outer regions of a Gaussian beam profile, partial film removal by limited vaporisation would not be expected to occur since no part of the ITO film would be heated to the vaporisation temperature. Partial ITO film removal is therefore unlikely, and the most probable method of forming the stitch line during ITO electrode patterning is redeposition. 6. Conclusion The stitch line that occurs between two adjacent laser pulses used for patterning electrode structures on ITO thin films has been examined in detail and the location and formation of the line has been analysed. It has been found to consist of two regions: a thin white section located within the area of overlap of the adjacent pulses and a wider darker region that forms in the area processed by the second pulse only. AFM analysis has shown that in general the laser processed area is covered with a thin layer of residue and the white section of the stitch line is typically 10–30 nm higher that the nominal surface of the residue whilst the darker region is slightly lower. Experimental trials have shown that initially the ITO coating is fully removed by the first incident pulse and the edges of the irradiated area are slightly darker than the centre. Trials have also shown that the white section of the stitch line is formed by redeposition of material via the plume generated by the second pulse and is deposited within the region of overlap of the first and second pulse, an area which has an increased surface temperature at the time that the redeposition occurs due to interaction of the second pulse with the ITO residue remaining after the first pulse. The
whitish appearance of the stitch line may result from the imperfect crystal structure likely to arise from this type of deposition, and hence increased optical scattering. The results of numerical simulation of the surface temperature within the area of overlap due to the action of the second laser pulse have shown that within a given range of residual coating thicknesses there will be a significant temperature rise that is not sufficient to cause material removal by thermal vaporisation, and indeed it can have the opposite effect by enabling redeposition. Previous research has attributed the formation of the stitch line in fan-out designs to partial removal of the ITO thin film, but in this case it is unlikely since at this wavelength the optical absorption depth is significantly larger than the thin film thickness. Experimental trials have also demonstrated that single isolated laser pulses are capable of removing the ITO layer to an extent that the stitch line is not observed, and it is the occurrence of the second, slightly overlapped pulse that causes the stitch line to form. In this way partial removal of the ITO layer cannot be responsible for the stitch line formation and redeposition of material ablated by the second pulse is the most likely cause. References [1] J.P. Beouf, J. Phys. D: Appl. Phys. 36 (2003) R53–R79. [2] T. Minami, Semicond. Sci. Technol. 20 (2005) S35–S44. [3] M. Henry, J. Wendland, P.M. Harrison, D. Hand, Proceedings of the 26th International Congress on Applications of Lasers and Electro-Optics, 29th October–1st November, 2007, pp. 119–128. [4] S. Venkat, C. Dunsky, Proceedings of Photon Processing in Microelectronics and Photonics V, Photonics West Conference, SPIE, vol. 6106, San Jose, 2006, p. 610602. [5] M. Henry, P.M. Harrison, J. Wendland, J. Laser Micro/Nanoeng. 2 (2007) 49–56. [6] H.J. Booth, Thin Solid Films 453–454 (2004) 450–457. [7] J. Bovatsek, A. Tamhankar, R.S. Patel, N.M. Bulgavoka, J. Bonse, Thin Solid Films 518 (2010) 2897–2904. [8] J.G. Lunney, R.R. O’Neill, K. Schulmeister, Appl. Phys. Lett. 59 (6) (1991) 647–649. [9] T. Szörényi, L.D. Laude, I. Bertóti, Z. Kántor, Zs. Geretovsky, J. Appl. Phys. 78 (1995) 6211–6219. [10] M. Takai, D. Bollman, K. Haberger, Appl. Phys. Lett. 64 (1994) 2560–2562. [11] O. Yavas, M. Takai, Appl. Phys. Lett. 73 (1998) 2558–2560. [12] O. Yavas, C. Ochiai, M. Takai, Appl. Phys. A: Mater. Sci. Process 69 (1999) S875–S878. [13] O. Yavas, M. Takai, J. Appl. Phys. 85 (1999) 4207–4212. [14] D. Ashkensai, G. Müller, A. Rosenfeld, R. Stoian, I.V. Hertel, N.M. Bulgakova, E.E.B. Campbell, Appl. Phys. A 77 (2003) 223–228. [15] M. Park, B.H. Chon, H.S. Kim, S.C. Jeoung, D. Kim, J.I. Lee, H.Y. Chu, H.R. Kim, Opt. Lasers Eng. 44 (2006) 138–146. [16] G. Raˇciukaitus, M. Brikas, M. Gedvilas, T. Rakickas, Appl. Surf. Sci. 253 (2007) 6570–6574. [17] E. Hecht, Optics, 4th edition, Addison Wesley, 2002 (Chapter 5). [18] I.W. Boyd, Laser Processing of Thin Films and Microstructures, 1st edition, Springer Verlag, 1987 (Section 2.5).