- Email: [email protected]
Competitiveness of negative tone resists for nanoimprint lithography
Microelectronic Engineering 123 (2014) 43–47
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Competitiveness of negative tone resists for nanoimprint lithography Khalid Dhima ⇑, Christian Steinberg, Andre Mayer, Si Wang, Marc Papenheim, Hella-Christin Scheer University of Wuppertal, Wuppertal D-42119, Germany
a r t i c l e
i n f o
Article history: Received 23 October 2013 Received in revised form 3 April 2014 Accepted 9 May 2014 Available online 17 May 2014 Keywords: Negative tone resist Swing curves Thermal nanoimprint Residual layer removal
a b s t r a c t This paper presents a semi-analytical approach for the calculation of the mean relative intensity within photoresist layers of a given thickness on silicon – the mean intensity is the one obtained after a post exposure bake to remove standing wave effects. The approach is based on the handling of analytically determined intensity values in a matrix form. Transmission, reflection and absorption in an air/resist/ substrate configuration are considered to calculate the intensity for a single wavelength or a multiple wavelength exposure. Swing curves of the mean relative intensity as a function of the total resist thickness indicate a novel application in the context of nanoimprint: a residual layer-free imprint can be obtained with residual layers of up to about 30 nm, as such thin layers always remain underexposed. Thus, when a negative tone photoresist is imprinted and is flood exposed after the imprint, any thin residual layer will be removable in a simple development step, thus avoiding any breakthrough etch. This is of particular interest for a further use of the imprinted structures with lift-off. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Residual layer-free nanoimprint is of general interest, as a break-through etch is avoided, which might affect the pattern dimensions and increase the lithography cost. One possibility is to apply UV-nanoimprint with a stamp that is non-transparent in the elevated part of the patterns [1]; this technique requires non-conventional and costly imprint masters. Another possibility makes use of thermal nanoimprint with a bi-layer system, where the bottom layer is a lift-off resist and the top layer consists of a polymer like PMMA [2]; this technique is hard to control as the imprint will not proceed reliably down to the bottom layer. A further possibility is to imprint into layers that provide the exact amount of polymer to fill the cavities (volume conservation) and to rely on the de-wetting of ultra-thin residues [3,4]; this method suffers from potential non-uniformities of the residual layer with mixed pattern sizes and the potential formation of physical selfassembly defects with mixed cavity sizes. Therefore its applicability is limited to special cases. This study exploits the potential of photoresists as imprint materials. They are thermoplastic, so that they can be imprinted at a relatively low temperature. In addition, they are photosensitive, so that an additional exposure can be used for changing the properties of the material in a developer environment. Here, negative tone resists are addressed, where the solubility is decreased by
exposure. With an exposure of a pre-imprinted pattern the intensity may differ locally, resulting in a locally differing stabilization. Based on a theoretical investigation of the intensity within the resist layer in a typical imprint stack (air/resist/Si-substrate) and assuming homogeneous layers of infinite lateral geometry, the swing curves for the mean intensity in the resist are calculated, taking into account either a mono-energetic or a broadband illumination. In addition, the absorption within the resist is accounted for. In a semi-analytic approach the average intensity is determined with the aid of an intensity matrix. This method offers the benefit of being scalable for any layer thickness range of interest, for high layer thicknesses as well as for small layer thicknesses. In particular the latter is important to realistically assess the exposure situation with thin layers: In particular with layers below 50 nm the dose differs substantially from the often used approach of simply considering the variations due to constructive and destructive interference. The theoretical result obtained opens-up a unique, new possibility for residual layer-free nanoimprint with negative tone photoresists. It shows that a residual layer below a thickness of 30 nm is by far under-exposed in comparison to any thicker resist layer and will thus not be stabilized during exposure. Therefore, a residual layer in this thickness range is easily removed by development without any dry etch step required. 2. Theoretical background
⇑ Corresponding author. Tel.: +49 202 439 1802; fax: +49 202 439 1804. E-mail address: [email protected] (K. Dhima). http://dx.doi.org/10.1016/j.mee.2014.05.017 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
When a uniform and infinitely-extended photoresist layer over silicon is exposed to UV light, the propagating light interferes with
44
K. Dhima et al. / Microelectronic Engineering 123 (2014) 43–47
the reflected one, resulting in distinct standing waves within the resist layer. The ratio between the maxima and minima of the relative intensity within the resist layer may reach values up to 17. The situation is well described by the mathematical approach of C. Mack [5], where the relative local intensity in the photoresist with standing waves is analytically described by: Irel ðzÞ ¼
d=0
(a)
n2 n1
js12 j2 eaz þ jq23 j2 eað2dzÞ þ 2jq23 jead cos 4pkn2 ðd zÞ þ u23 : 1 þ jq12 j2 jq23 j2 e2ad þ 2jq12 jjq23 jead cos 4pkn2 d þ u12 þ u23
u12 u23
2ðn2 j1 n1 j2 Þ ¼ arctan ðn21 þ j21 Þ ðn22 þ j22 Þ
;
2ðn3 j2 n2 j3 Þ þ p; ¼ arctan ðn22 þ j22 Þ ðn23 þ j23 Þ
ð2Þ
ð3Þ
at the air-resist and resist-Si-interface. j is the extinction coefficient (imaginary part of the complex index of refraction), with a = 4pjk. The shift p accounts for the correct definition of the arctangent function (principal value). Eq. (1) gives the relative intensity as a function of the depth into the resist, namely as a function of the position z for one single resist thickness d. Integration of Eq. (1) and averaging over d provides the mean intensity within the resist for a given thickness d. The mean intensity is a measure of the exposure state of the resist layer after a post exposure bake that is typically applied to minimize solubility differences due to standing wave effects [6]. The mean value of the relative intensity within the resist varies due to constructive and destructive interference within the layer [7]. In other words, the mean intensity depends on the layer thickness d exposed; with destructive interference (d = 2kk/4nR) the mean intensity is minimal and with constructive interference (d = (2k 1)k/4nR) the mean intensity becomes maximum (with k = 1, 2, 3, . . .). This intensity ‘swing’ affects many experimental parameters, e.g. the dose to clear with a positive tone resist or the dose to stabilize/crosslink with a negative tone photoresist and also the development rates. Furthermore, it has consequences for the line width obtained. Actually, the relative intensity as it is given by Eq. (1) depends on two parameters, namely on the thickness d and on the position z. Our approach is to handle these two dependencies in a matrix form as it is illustrated in Fig. 1. A handling in such a matrix allows to easily consider different exposure modes (i-line and/or broadband exposure) with and without absorption; the matrix is upscalable to any resist thickness of interest. Each column of the matrix represents the relative intensity as a function of the position z; the maximal z value within one column is the resist thickness d. Therefore, the intensity-matrix is a triangular matrix. Each line of the matrix represents the relative intensity as a function of resist thicknesses d in one position. For example, the first line for z = 0 presents the relative intensity at the resist surface (air-resist-interface) for a differing thickness d. The diagonal of the matrix reflects the relative intensity at the Si substrate (resist-Si-interface); though being quite low, the intensity still
d=2
...
d=k
z=0
I00
I01
I02
...
I0k
z=1
0
I11
I12
...
I1k
z=2 .. .
0
0
I22
...
I2k
z=k
0
(nm)
(b)
0
.
d=0
(nm) relative intensity as function of resist thickness at the resist surface
.
. ...
0
relative intensity as function of resist thickness d at the Si substrate
ð1Þ
Here, n is the real part of the refractive index, a is the absorption coefficient, s is the transmission coefficient, q is the reflection coefficient and / is the phase shift at the respective interface (air-resist and resist-silicon), s and q being complex. The indices 1/2/3 refer to the air/resist/Si-stack. The geometry z indicates the depth into the resist, with z = 0 being the air-resist-interface. Eq. (1) refers to one single exposure wavelength k; all optical parameters depend on the wavelength. The normalization is done with respect to the intensity incident to the resist. The phase shifts are defined by:
d=1
Ikk
relative intensity as function of depth into resist z for one thickness
d=1
d=2
...
d=k
z=0
I00
I01
I02
...
I0k
z=1
0
I11
I12
...
I1k
z=2 .. .
0
0
I22
...
I2k
z=k
0
0
I00
I01+I11
.
0
(nm)
.
. ...
Ikk k
Imean (d) =
2
I02+I12+I22 3
...
i=0
Iik
k+1
Fig. 1. Semi-analytical approach to determine the relative intensity in an exposed resist layer (handling in matrix form). The relative intensity depends on two parameters, the position z (the depth into the resist, the maximum z-value is the resist thickness, zmax = d) and the resist thickness d. Each column represents the relative intensity as a function of the position z, each line represents the relative intensity in one position z as a function of the resist thickness (a). The average value of all intensities within one column gives the mean relative intensity for this layer thickness (b).
varies there. With thin layers in the range of 300 nm the average intensity at the resist-Si-interface varies between 0.1 and 0.21, whereas it varies between 0.1 and 3.6 at the resist-air-interface. The average value of each column of the matrix gives the mean relative intensity for the respective resist thickness (Fig. 1b). With a step increase of 1 nm of the resist thickness (from column to column) and of the position z as well (from line to line) an excellent accuracy of the integration and thus of the mean relative intensity is obtained. Fig. 2a displays the mean relative intensity as a function of the resist thickness for mono-energetic exposure (i-line) and for broadband UV exposure (g + h + i-line) as well. For the latter, the intensities of the three lines were weighted with the factors g:h:i = 0.32:0.28:0.4. The maxima of the swing curves are obtained with layer thicknesses meeting constructive interference, the 0minima with layer thicknesses meeting destructive interference conditions. Under broadband exposure these minima and maxima correlate with different layer thicknesses for the three wavelengths, resulting in a decreasing modulation around 300 nm (beat). Independent of the exposure situation (i-line or broadband), the intensity swing is always the highest with thin layers, decreasing with increasing thickness. This is typical, as with thin layers only parts of a full standing wave period (about 100 nm with i-line) can prevail in the resist. A stationary mean value varying between (almost) constant minimum and maximum values can only be obtained with thick layers (more than 1 micron), but then absorption within the resist may dominate. In any case, the mean relative intensity within thin layers, 30 nm and below, is always very low. This opens a new and unique possibility of application with T-NIL (thermal nanoimprint), namely residual layer-free nanoimprint lithography. Though promising because of its simplicity, nanoimprint still suffers from
45
mean relative intensity
K. Dhima et al. / Microelectronic Engineering 123 (2014) 43–47
resist
(a)
2
i-line
Si stamp
1 g+h+i-line
Si
mean relative intensity
0
(b)
2
g+h+i-line
1
Si 35 nm 0 0
50
100
150
200
250
300
resist thickness / nm Fig. 2. Mean relative intensity as a function of the resist layer thickness. (a) Curves calculated for i-line exposure only and for broad-band exposure with a weighting factor of about 40% for i-line, 28% for h-line and 32% for g-line. (b) Curve for broadband exposure; the mean intensity within a residual layer of up to 35 nm is lower than the mean intensity for any layer in the thickness range of 100–300 nm (dashdotted line). With a choice of the exposure dose adequate to stabilise a thick layer (here: 280 nm), a thin residual layer (here: 30 nm) always remains underexposed and thus still removable by development.
the need of an etching step to remove the residual layer, which is affected by pattern density-induced non-uniformity. Residual layer-free processing makes nanoimprint more competitive for production and improves time- and cost-effectiveness. It further avoids defects and pattern loss due to etching. As a residual layer below 30 nm is aimed at the imprint provides fully filled stamp cavities without any risk for defects resulting from physical selfassembly. Fig. 3 displays the process proposed to benefit from the considerations discussed before. The imprinted resist is flood exposed to UV light and, after a post exposure bake step to level-out the standing waves, the under-exposed residual layer can simply be removed in a developer solution. Fig. 2b illustrates that with layers up to 35 nm the dose in a thin layer is lower than in any thicker layer. In particular, with an upper polymer level of 280 nm and a lower polymer level of 30 nm the exposure dose within the thin layer is only about 60% of the dose in the thick layer. This is the situation that is characteristic for the experiments performed. The thin layer is the residual layer after imprint. 3. Experimental The resist ma-N 405 (micro resist technology), a commercial negative tone photoresist, was investigated for thermal nanoimprint lithography and subsequent exposure. ma-N 405 is a novolac-based photoresist; its solubility is decreased by exposure to UV-light; the spectral response of the resist lies in the range of 300–380 nm (data sheet). As its solubility change does not result from crosslinking, the resist can be removed after a full exposure in an appropriate remover (ma-R 404S). The thermal stability limit is at about 110 °C (material data sheet) and should be respected in any process step. Negative tone resists that are still removable
Si Fig. 3. Process sequence for a residual layer-free nanoimprint with a negative tone photoresist. After pre-treatment of the substrate with HMDS the resist is spincoated and imprinted with a remaining residual layer of about 10–30 nm. UV floodexposure is applied, the exposure dose should be adequate to stabilize the filled cavities but leaving the residual layer under-exposed. Thereafter a post exposure bake at 100 °C is applied to reduce standing waves effects within the resist layer. In the final development step the residual layer is removed, without any need for dry etching.
after exposure are of general interest as materials for further processing by metal deposition and lift-off. The structures were lifted after sputtering with Cr and Au in the remover in an ultra sonic bath at 50 °C within 5 min. The Si-stamp used for the imprint was 1.5 1.5 cm2 in size and was treated with a tri-chloro-silane-based anti-sticking layer in a temperature driven gas phase process [8]. The stamp features line fields of different geometries (line width s, cavity width w), the size of each field amounts to about 5 mm in length and 220 lm in width; the elevated stamp pattern is about 240 nm high. An initial layer thickness h0 of 170 nm was chosen for the experiments. With 260 nm wide lines and 430 nm wide cavities the imprint results in fully filled cavities. Only pattern sizes enabling thin residual layers and fully filled cavities were evaluated (stamp line width s = 260 nm), as with negative tone resists only thin residual layers can be removed by development. Exposure was performed in a commercial contact printer (EV 620, EV group) and thermal imprint was performed in a laboratory system (Weber). In this study, the imprint parameters were fixed to the values T = 90 °C, t = 5 min and p = 100 bar (corresponding to 2.25 kN for a stamp area of 1.5 1.5 cm2). The temperature chosen is lower than during earlier experiments with this material [9]; it was found that with the imprint system used, the actual processing temperature is about 10% higher than the control temperature of the hotplates; thus, the real imprint temperature amounts to about 100 °C with a set temperature of 90 °C; this reduced temperature set-point safely avoids to touch the stability limit of the resist, so that solubility is not affected. The thickness of the initial layer after spin-coating was measured via profilometry (Dektak II, Veeco) and was subsequently validated by spectral ellipsometry (ES4-G, Sopra); the height of residual layer after imprint was determined from SEM (scanning electron microscopy) cross-sections.
46
K. Dhima et al. / Microelectronic Engineering 123 (2014) 43–47
4. Results Fig. 4 displays the imprint result obtained with the negative tone photoresist ma-N 405; the imprint temperature at a set point of 90 °C is well above the glass transition temperature of about 50 °C and still well below the thermal stability limit of 110 °C. The height of the residual layer after the imprint amounts to about 30 nm. According to the swing curve (Fig. 2b) this is an acceptable value to safely provide under-exposure. The dose chosen for flood exposure was 300 mJ/cm2, adequate to fully expose the elevated imprinted patterns of 280 nm in height. Development for 3 min completely removed the 30 nm thick residual layer, thus proving the concept. The imprinted pattern was transferred into a metal pattern of reverse tone by lift-off. Fig. 5 shows the lift-off result, here 30 nm Cr and 30 nm Au were sputtered and lifted in the remover within a time of 5 min. For complete residual layer removal the exposure dose must not be too high. Fig. 6a shows that the residual layer of 30 nm has completely been removed after 3 min development time, when the resist was exposed at 300 mJ/cm2 and post baked at 100 °C for 1 min. As indicated by the results in Fig. 6b, an over-exposure should be avoided; otherwise the thin residual layer may also become stabilized and remains resistant against the developer; even with prolonged development time (20 min) a complete removal of the residual layer was not possible after an exposure at 750 mJ/cm2; the residual layer was slightly attacked from the top but was not completely removed. Our experiments show that residual-free processing works well with residual layers of up to 30 nm over large areas. Though the experiments performed were done with imprinted patterns of 260 nm there should not be a lower limit. Even when the imprinted pattern is not ‘imaged’ (as it is the case for subhalf-wavelength patterns) a low exposure near the substrate will
5 m Fig. 5. Lift-off result obtained after sputtering. Process steps: flood exposure with a dose of 300 mJ/cm2, PEB at 100 °C for 1 min, development for 3 min; sputtering with 30 nm Cr and 30 nm Au, lift-off in remover, supported by an ultrasonic bath at 50 °C.
(a)
200 nm
(b)
200 nm
1 m
Fig. 6. Impact of the correct choice of the exposure dose for residual layer-free imprint with negative tone resists. (a) Desired result obtained with an adequate exposure dose of 300 mJ/cm2 and a development time of 3 min. (b) Defective result obtained at an over-exposure (750 mJ/cm2) and a prolonged development time of 20 min; with over-exposure the residual layer cannot be completely removed.
always prevail, so that any thin layer is removable as long as a local developer supply is provided for dissolving it.
30 nm
200 nm Fig. 4. Nanoimprint result obtained with the negative tone resist ma-N 405; the imprint was performed into an initial layer of about 170 nm thickness, resulting in a remaining residual layer of about 30 nm (T = 90 °C, t = 5 min, p = 100 bar). (a) Survey, (b) detail.
5. Summary Thermal nanoimprint lithography with negative tone photoresists opens a new and effective possibility, a residual layer-free processing. The idea relies on an imprint with a residual layer height below 30 nm. After a flood exposure adequate to stabilise the elevated pattern, a thin residual layer always remains underexposed and can be removed by development. To determine the correct exposure dose and to evaluate the requirements and
K. Dhima et al. / Microelectronic Engineering 123 (2014) 43–47
boundary conditions, a novel approach to determine the mean relative intensity in the resist for exposed layers of different thickness (swing curves) based on a matrix type of handling was proposed, which can be applied in any exposure situation; it is scalable for any desired thickness of the resist and allows to consider different exposure situations (single wavelength, multiple wavelength) with and without absorption in the resist in an easy way. Acknowledgements Funding by the Deutsche Forschungsgemeinschaft (DFG) is highly acknowledged. The authors are indebted to Y. Hirai, Osaka Prefecture University, for making available the stamp used in this investigation and to A. Voigt micro resist technology for discussion of material properties.
47
References [1] X. Cheng, L.J. Guo, Microelectron. Eng. 71 (2004) 277–282. [2] Y. Chen, D.S. Macintyre, E. Boyd, D. Moran, I. Thayne, S. Thoms, Microelectron. Eng. 67–68 (2003) 245–251. [3] C. Auner, U. Palfinger, H. Gold, J. Kraxner, A. Haase, T. Haber, M. Sezen, W. Grogger, G. Jakopic, J.R. Krenn, G. Leising, B. Stadlober, Org. Electron. 10 (2009) 1466–1472. [4] N. Bogdanski, M. Wissen, S. Möllenbeck, H.-C. Scheer, J. Vac. Sci. Technol. B 24 (2006) 2998–3001. [5] C. Mack, Fundamental principles of optical lithography, Wiley, 2010. [6] D. Widmann, H. Mader, A. Friedrich, Technologie hochintegrierter Schaltungen, Springer, 1996. [7] D.W. Widmann, H. Binder, IEEE Trans. Electron Devices 22 (1975) 467–471. [8] H.-C. Scheer, W. Häfner, A. Fidler, S. Möllenbeck, N. Bogdanski, J. Vac. Sci. Technol. B 26 (2008) 2380. [9] K. Dhima, C. Steinberg, S. Möllenbeck, A. Mayer, S. Wang, H.-C. Scheer, J. Vac. Sci. Technol. B 30 (2012). 06FB08-1-6.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Competitiveness of negative tone resists for nanoimprint lithography Khalid Dhima ⇑, Christian Steinberg, Andre Mayer, Si Wang, Marc Papenheim, Hella-Christin Scheer University of Wuppertal, Wuppertal D-42119, Germany
a r t i c l e
i n f o
Article history: Received 23 October 2013 Received in revised form 3 April 2014 Accepted 9 May 2014 Available online 17 May 2014 Keywords: Negative tone resist Swing curves Thermal nanoimprint Residual layer removal
a b s t r a c t This paper presents a semi-analytical approach for the calculation of the mean relative intensity within photoresist layers of a given thickness on silicon – the mean intensity is the one obtained after a post exposure bake to remove standing wave effects. The approach is based on the handling of analytically determined intensity values in a matrix form. Transmission, reflection and absorption in an air/resist/ substrate configuration are considered to calculate the intensity for a single wavelength or a multiple wavelength exposure. Swing curves of the mean relative intensity as a function of the total resist thickness indicate a novel application in the context of nanoimprint: a residual layer-free imprint can be obtained with residual layers of up to about 30 nm, as such thin layers always remain underexposed. Thus, when a negative tone photoresist is imprinted and is flood exposed after the imprint, any thin residual layer will be removable in a simple development step, thus avoiding any breakthrough etch. This is of particular interest for a further use of the imprinted structures with lift-off. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Residual layer-free nanoimprint is of general interest, as a break-through etch is avoided, which might affect the pattern dimensions and increase the lithography cost. One possibility is to apply UV-nanoimprint with a stamp that is non-transparent in the elevated part of the patterns [1]; this technique requires non-conventional and costly imprint masters. Another possibility makes use of thermal nanoimprint with a bi-layer system, where the bottom layer is a lift-off resist and the top layer consists of a polymer like PMMA [2]; this technique is hard to control as the imprint will not proceed reliably down to the bottom layer. A further possibility is to imprint into layers that provide the exact amount of polymer to fill the cavities (volume conservation) and to rely on the de-wetting of ultra-thin residues [3,4]; this method suffers from potential non-uniformities of the residual layer with mixed pattern sizes and the potential formation of physical selfassembly defects with mixed cavity sizes. Therefore its applicability is limited to special cases. This study exploits the potential of photoresists as imprint materials. They are thermoplastic, so that they can be imprinted at a relatively low temperature. In addition, they are photosensitive, so that an additional exposure can be used for changing the properties of the material in a developer environment. Here, negative tone resists are addressed, where the solubility is decreased by
exposure. With an exposure of a pre-imprinted pattern the intensity may differ locally, resulting in a locally differing stabilization. Based on a theoretical investigation of the intensity within the resist layer in a typical imprint stack (air/resist/Si-substrate) and assuming homogeneous layers of infinite lateral geometry, the swing curves for the mean intensity in the resist are calculated, taking into account either a mono-energetic or a broadband illumination. In addition, the absorption within the resist is accounted for. In a semi-analytic approach the average intensity is determined with the aid of an intensity matrix. This method offers the benefit of being scalable for any layer thickness range of interest, for high layer thicknesses as well as for small layer thicknesses. In particular the latter is important to realistically assess the exposure situation with thin layers: In particular with layers below 50 nm the dose differs substantially from the often used approach of simply considering the variations due to constructive and destructive interference. The theoretical result obtained opens-up a unique, new possibility for residual layer-free nanoimprint with negative tone photoresists. It shows that a residual layer below a thickness of 30 nm is by far under-exposed in comparison to any thicker resist layer and will thus not be stabilized during exposure. Therefore, a residual layer in this thickness range is easily removed by development without any dry etch step required. 2. Theoretical background
⇑ Corresponding author. Tel.: +49 202 439 1802; fax: +49 202 439 1804. E-mail address: [email protected] (K. Dhima). http://dx.doi.org/10.1016/j.mee.2014.05.017 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
When a uniform and infinitely-extended photoresist layer over silicon is exposed to UV light, the propagating light interferes with
44
K. Dhima et al. / Microelectronic Engineering 123 (2014) 43–47
the reflected one, resulting in distinct standing waves within the resist layer. The ratio between the maxima and minima of the relative intensity within the resist layer may reach values up to 17. The situation is well described by the mathematical approach of C. Mack [5], where the relative local intensity in the photoresist with standing waves is analytically described by: Irel ðzÞ ¼
d=0
(a)
n2 n1
js12 j2 eaz þ jq23 j2 eað2dzÞ þ 2jq23 jead cos 4pkn2 ðd zÞ þ u23 : 1 þ jq12 j2 jq23 j2 e2ad þ 2jq12 jjq23 jead cos 4pkn2 d þ u12 þ u23
u12 u23
2ðn2 j1 n1 j2 Þ ¼ arctan ðn21 þ j21 Þ ðn22 þ j22 Þ
;
2ðn3 j2 n2 j3 Þ þ p; ¼ arctan ðn22 þ j22 Þ ðn23 þ j23 Þ
ð2Þ
ð3Þ
at the air-resist and resist-Si-interface. j is the extinction coefficient (imaginary part of the complex index of refraction), with a = 4pjk. The shift p accounts for the correct definition of the arctangent function (principal value). Eq. (1) gives the relative intensity as a function of the depth into the resist, namely as a function of the position z for one single resist thickness d. Integration of Eq. (1) and averaging over d provides the mean intensity within the resist for a given thickness d. The mean intensity is a measure of the exposure state of the resist layer after a post exposure bake that is typically applied to minimize solubility differences due to standing wave effects [6]. The mean value of the relative intensity within the resist varies due to constructive and destructive interference within the layer [7]. In other words, the mean intensity depends on the layer thickness d exposed; with destructive interference (d = 2kk/4nR) the mean intensity is minimal and with constructive interference (d = (2k 1)k/4nR) the mean intensity becomes maximum (with k = 1, 2, 3, . . .). This intensity ‘swing’ affects many experimental parameters, e.g. the dose to clear with a positive tone resist or the dose to stabilize/crosslink with a negative tone photoresist and also the development rates. Furthermore, it has consequences for the line width obtained. Actually, the relative intensity as it is given by Eq. (1) depends on two parameters, namely on the thickness d and on the position z. Our approach is to handle these two dependencies in a matrix form as it is illustrated in Fig. 1. A handling in such a matrix allows to easily consider different exposure modes (i-line and/or broadband exposure) with and without absorption; the matrix is upscalable to any resist thickness of interest. Each column of the matrix represents the relative intensity as a function of the position z; the maximal z value within one column is the resist thickness d. Therefore, the intensity-matrix is a triangular matrix. Each line of the matrix represents the relative intensity as a function of resist thicknesses d in one position. For example, the first line for z = 0 presents the relative intensity at the resist surface (air-resist-interface) for a differing thickness d. The diagonal of the matrix reflects the relative intensity at the Si substrate (resist-Si-interface); though being quite low, the intensity still
d=2
...
d=k
z=0
I00
I01
I02
...
I0k
z=1
0
I11
I12
...
I1k
z=2 .. .
0
0
I22
...
I2k
z=k
0
(nm)
(b)
0
.
d=0
(nm) relative intensity as function of resist thickness at the resist surface
.
. ...
0
relative intensity as function of resist thickness d at the Si substrate
ð1Þ
Here, n is the real part of the refractive index, a is the absorption coefficient, s is the transmission coefficient, q is the reflection coefficient and / is the phase shift at the respective interface (air-resist and resist-silicon), s and q being complex. The indices 1/2/3 refer to the air/resist/Si-stack. The geometry z indicates the depth into the resist, with z = 0 being the air-resist-interface. Eq. (1) refers to one single exposure wavelength k; all optical parameters depend on the wavelength. The normalization is done with respect to the intensity incident to the resist. The phase shifts are defined by:
d=1
Ikk
relative intensity as function of depth into resist z for one thickness
d=1
d=2
...
d=k
z=0
I00
I01
I02
...
I0k
z=1
0
I11
I12
...
I1k
z=2 .. .
0
0
I22
...
I2k
z=k
0
0
I00
I01+I11
.
0
(nm)
.
. ...
Ikk k
Imean (d) =
2
I02+I12+I22 3
...
i=0
Iik
k+1
Fig. 1. Semi-analytical approach to determine the relative intensity in an exposed resist layer (handling in matrix form). The relative intensity depends on two parameters, the position z (the depth into the resist, the maximum z-value is the resist thickness, zmax = d) and the resist thickness d. Each column represents the relative intensity as a function of the position z, each line represents the relative intensity in one position z as a function of the resist thickness (a). The average value of all intensities within one column gives the mean relative intensity for this layer thickness (b).
varies there. With thin layers in the range of 300 nm the average intensity at the resist-Si-interface varies between 0.1 and 0.21, whereas it varies between 0.1 and 3.6 at the resist-air-interface. The average value of each column of the matrix gives the mean relative intensity for the respective resist thickness (Fig. 1b). With a step increase of 1 nm of the resist thickness (from column to column) and of the position z as well (from line to line) an excellent accuracy of the integration and thus of the mean relative intensity is obtained. Fig. 2a displays the mean relative intensity as a function of the resist thickness for mono-energetic exposure (i-line) and for broadband UV exposure (g + h + i-line) as well. For the latter, the intensities of the three lines were weighted with the factors g:h:i = 0.32:0.28:0.4. The maxima of the swing curves are obtained with layer thicknesses meeting constructive interference, the 0minima with layer thicknesses meeting destructive interference conditions. Under broadband exposure these minima and maxima correlate with different layer thicknesses for the three wavelengths, resulting in a decreasing modulation around 300 nm (beat). Independent of the exposure situation (i-line or broadband), the intensity swing is always the highest with thin layers, decreasing with increasing thickness. This is typical, as with thin layers only parts of a full standing wave period (about 100 nm with i-line) can prevail in the resist. A stationary mean value varying between (almost) constant minimum and maximum values can only be obtained with thick layers (more than 1 micron), but then absorption within the resist may dominate. In any case, the mean relative intensity within thin layers, 30 nm and below, is always very low. This opens a new and unique possibility of application with T-NIL (thermal nanoimprint), namely residual layer-free nanoimprint lithography. Though promising because of its simplicity, nanoimprint still suffers from
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resist
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mean relative intensity
0
(b)
2
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Si 35 nm 0 0
50
100
150
200
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resist thickness / nm Fig. 2. Mean relative intensity as a function of the resist layer thickness. (a) Curves calculated for i-line exposure only and for broad-band exposure with a weighting factor of about 40% for i-line, 28% for h-line and 32% for g-line. (b) Curve for broadband exposure; the mean intensity within a residual layer of up to 35 nm is lower than the mean intensity for any layer in the thickness range of 100–300 nm (dashdotted line). With a choice of the exposure dose adequate to stabilise a thick layer (here: 280 nm), a thin residual layer (here: 30 nm) always remains underexposed and thus still removable by development.
the need of an etching step to remove the residual layer, which is affected by pattern density-induced non-uniformity. Residual layer-free processing makes nanoimprint more competitive for production and improves time- and cost-effectiveness. It further avoids defects and pattern loss due to etching. As a residual layer below 30 nm is aimed at the imprint provides fully filled stamp cavities without any risk for defects resulting from physical selfassembly. Fig. 3 displays the process proposed to benefit from the considerations discussed before. The imprinted resist is flood exposed to UV light and, after a post exposure bake step to level-out the standing waves, the under-exposed residual layer can simply be removed in a developer solution. Fig. 2b illustrates that with layers up to 35 nm the dose in a thin layer is lower than in any thicker layer. In particular, with an upper polymer level of 280 nm and a lower polymer level of 30 nm the exposure dose within the thin layer is only about 60% of the dose in the thick layer. This is the situation that is characteristic for the experiments performed. The thin layer is the residual layer after imprint. 3. Experimental The resist ma-N 405 (micro resist technology), a commercial negative tone photoresist, was investigated for thermal nanoimprint lithography and subsequent exposure. ma-N 405 is a novolac-based photoresist; its solubility is decreased by exposure to UV-light; the spectral response of the resist lies in the range of 300–380 nm (data sheet). As its solubility change does not result from crosslinking, the resist can be removed after a full exposure in an appropriate remover (ma-R 404S). The thermal stability limit is at about 110 °C (material data sheet) and should be respected in any process step. Negative tone resists that are still removable
Si Fig. 3. Process sequence for a residual layer-free nanoimprint with a negative tone photoresist. After pre-treatment of the substrate with HMDS the resist is spincoated and imprinted with a remaining residual layer of about 10–30 nm. UV floodexposure is applied, the exposure dose should be adequate to stabilize the filled cavities but leaving the residual layer under-exposed. Thereafter a post exposure bake at 100 °C is applied to reduce standing waves effects within the resist layer. In the final development step the residual layer is removed, without any need for dry etching.
after exposure are of general interest as materials for further processing by metal deposition and lift-off. The structures were lifted after sputtering with Cr and Au in the remover in an ultra sonic bath at 50 °C within 5 min. The Si-stamp used for the imprint was 1.5 1.5 cm2 in size and was treated with a tri-chloro-silane-based anti-sticking layer in a temperature driven gas phase process [8]. The stamp features line fields of different geometries (line width s, cavity width w), the size of each field amounts to about 5 mm in length and 220 lm in width; the elevated stamp pattern is about 240 nm high. An initial layer thickness h0 of 170 nm was chosen for the experiments. With 260 nm wide lines and 430 nm wide cavities the imprint results in fully filled cavities. Only pattern sizes enabling thin residual layers and fully filled cavities were evaluated (stamp line width s = 260 nm), as with negative tone resists only thin residual layers can be removed by development. Exposure was performed in a commercial contact printer (EV 620, EV group) and thermal imprint was performed in a laboratory system (Weber). In this study, the imprint parameters were fixed to the values T = 90 °C, t = 5 min and p = 100 bar (corresponding to 2.25 kN for a stamp area of 1.5 1.5 cm2). The temperature chosen is lower than during earlier experiments with this material [9]; it was found that with the imprint system used, the actual processing temperature is about 10% higher than the control temperature of the hotplates; thus, the real imprint temperature amounts to about 100 °C with a set temperature of 90 °C; this reduced temperature set-point safely avoids to touch the stability limit of the resist, so that solubility is not affected. The thickness of the initial layer after spin-coating was measured via profilometry (Dektak II, Veeco) and was subsequently validated by spectral ellipsometry (ES4-G, Sopra); the height of residual layer after imprint was determined from SEM (scanning electron microscopy) cross-sections.
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4. Results Fig. 4 displays the imprint result obtained with the negative tone photoresist ma-N 405; the imprint temperature at a set point of 90 °C is well above the glass transition temperature of about 50 °C and still well below the thermal stability limit of 110 °C. The height of the residual layer after the imprint amounts to about 30 nm. According to the swing curve (Fig. 2b) this is an acceptable value to safely provide under-exposure. The dose chosen for flood exposure was 300 mJ/cm2, adequate to fully expose the elevated imprinted patterns of 280 nm in height. Development for 3 min completely removed the 30 nm thick residual layer, thus proving the concept. The imprinted pattern was transferred into a metal pattern of reverse tone by lift-off. Fig. 5 shows the lift-off result, here 30 nm Cr and 30 nm Au were sputtered and lifted in the remover within a time of 5 min. For complete residual layer removal the exposure dose must not be too high. Fig. 6a shows that the residual layer of 30 nm has completely been removed after 3 min development time, when the resist was exposed at 300 mJ/cm2 and post baked at 100 °C for 1 min. As indicated by the results in Fig. 6b, an over-exposure should be avoided; otherwise the thin residual layer may also become stabilized and remains resistant against the developer; even with prolonged development time (20 min) a complete removal of the residual layer was not possible after an exposure at 750 mJ/cm2; the residual layer was slightly attacked from the top but was not completely removed. Our experiments show that residual-free processing works well with residual layers of up to 30 nm over large areas. Though the experiments performed were done with imprinted patterns of 260 nm there should not be a lower limit. Even when the imprinted pattern is not ‘imaged’ (as it is the case for subhalf-wavelength patterns) a low exposure near the substrate will
5 m Fig. 5. Lift-off result obtained after sputtering. Process steps: flood exposure with a dose of 300 mJ/cm2, PEB at 100 °C for 1 min, development for 3 min; sputtering with 30 nm Cr and 30 nm Au, lift-off in remover, supported by an ultrasonic bath at 50 °C.
(a)
200 nm
(b)
200 nm
1 m
Fig. 6. Impact of the correct choice of the exposure dose for residual layer-free imprint with negative tone resists. (a) Desired result obtained with an adequate exposure dose of 300 mJ/cm2 and a development time of 3 min. (b) Defective result obtained at an over-exposure (750 mJ/cm2) and a prolonged development time of 20 min; with over-exposure the residual layer cannot be completely removed.
always prevail, so that any thin layer is removable as long as a local developer supply is provided for dissolving it.
30 nm
200 nm Fig. 4. Nanoimprint result obtained with the negative tone resist ma-N 405; the imprint was performed into an initial layer of about 170 nm thickness, resulting in a remaining residual layer of about 30 nm (T = 90 °C, t = 5 min, p = 100 bar). (a) Survey, (b) detail.
5. Summary Thermal nanoimprint lithography with negative tone photoresists opens a new and effective possibility, a residual layer-free processing. The idea relies on an imprint with a residual layer height below 30 nm. After a flood exposure adequate to stabilise the elevated pattern, a thin residual layer always remains underexposed and can be removed by development. To determine the correct exposure dose and to evaluate the requirements and
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boundary conditions, a novel approach to determine the mean relative intensity in the resist for exposed layers of different thickness (swing curves) based on a matrix type of handling was proposed, which can be applied in any exposure situation; it is scalable for any desired thickness of the resist and allows to consider different exposure situations (single wavelength, multiple wavelength) with and without absorption in the resist in an easy way. Acknowledgements Funding by the Deutsche Forschungsgemeinschaft (DFG) is highly acknowledged. The authors are indebted to Y. Hirai, Osaka Prefecture University, for making available the stamp used in this investigation and to A. Voigt micro resist technology for discussion of material properties.
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