- Email: [email protected]
Laser machining of GaN-on-diamond wafers
Diamond & Related Materials 20 (2011) 675–681
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Laser machining of GaN-on-diamond wafers Dubravko I. Babić ⁎, Quentin Diduck, Firooz Faili, John Wasserbauer, Frank Lowe, Daniel Francis, Felix Ejeckam Group 4 Labs, Inc. 39500 Stevenson Place, Suite 207, Fremont, CA 94539, United States
a r t i c l e
i n f o
Article history: Received 15 July 2010 Received in revised form 25 February 2011 Accepted 13 March 2011 Available online 23 March 2011
a b s t r a c t The commercialization of gallium-nitride microwave circuits on diamond substrates requires chip-dicing technology and via formation process compatible with standard semiconductor processes. This paper discusses issues related to dicing and drilling of GaN-on-diamond wafers for RF power transistor applications (die size b 1 mm2) using laser micromachining. © 2011 Elsevier B.V. All rights reserved.
Keywords: Gallium nitride on diamond Laser machining AlGaN/GaN high-electron mobility transistors Via processing in diamond Diamond wafer dicing
1. Introduction The integration of gallium nitride epilayers with chemical-vapor deposited (CVD) diamond substrates presents an ideal combination of materials for manufacturing thermally efficient high-power electronics and optoelectronics. Microwave and millimeter power amplifier modules based on AlGaN/GaN high-electron mobility transistors (HEMTs) on diamond substrates are expected to have double the thermal conductance over those built on silicon carbide [1]. Consequently, circuits employing this technology can expect a substantial increase in power handling capability for the same junction and ambient temperatures or significantly reduced cooling requirements (i.e., higher ambient temperature for the same junction temperature and power). This fact has spurred numerous investigations of GaN integration with diamond substrates [2–8]. Presently the closest to commercial introduction is the “GaN-on-diamond” technology in which GaN epilayers are atomically attached to chemical-vapor deposited (CVD) diamond substrates [9,10]. Using this approach we have demonstrated 100-mm GaN-on-diamond wafers [9], record operation of GaN-on-diamond HEMTs [11], and amplifier modules [12]. The commercialization of electronic and optoelectronic components based on GaN-on-diamond engineered wafers requires the development of a wafer-dicing process and, in the case of microwave and millimeterwave circuits, a process for the formation of vias for connecting the front of a chip to its metalized back. Diamond is the hardest material known to man and its micromachining presents a unique challenge (also present in the development of diamond-based electronics [13,14]). Wafer mechan⁎ Corresponding author. Tel.: +1 408 398 2484; fax: +1 408 689 9674. E-mail address: [email protected] (D.I. Babić). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.03.017
ical dexterity is critical for compliance with standard semiconductor device processing techniques. Unlike silicon and typical III–V semiconductors that are typically processed at 300–500 μm wafer thickness and subsequently lapped down for back metallization and dicing, GaNon-diamond wafers are generally not thinned and have thicknesses between 50 and 100 μm throughout the process. There are two approaches to diamond cutting and drilling: plasma etching and laser drilling/cutting. Reactive-ion etching of diamond has been investigated in recent years [15–18]. However, no etching of 100μm deep structures with high-aspect ratios (critical for vias and dicing) has been reported. Additionally, issues such as side-wall slope control, mask material, and overall practicality of etching such deep structures has not been properly studied or evaluated. With the increasing interest in diamond wafer use and applications, reactive ion etching may hold the future for high volume micromachining once the quality of the etched structure, the total wafer processing times and equipment overhead become more economical than laser machining. At this time, laser micromachining is the economical as well as the technological choice to meet die processing requirements. In this paper, we discuss several issues and findings related to dicing and drilling of GaN-on-diamond wafers. The ultimate result of this work is the successful fabrication of hybrid GaN-on-diamond RF amplifier modules described elsewhere [12]. 2. Wafer structure GaN-on-diamond engineered wafers are presently available in 25, 50, and 100 mm diameters. The manufacturing process, epilayer structure, and substrate parameters have been previously described in reference [9], while here we only provide the parameters relevant to laser
676
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 1. Cross-sectional schematic of GaN-on-diamond wafer. The thickness of the graded-heat-conductivity (GHC) region varied depending on the process conditions, but of the order of 10–15 μm.
micromachining. The cross-sectional schematic of wafers used in this work is shown in Fig. 1. The wafers are intended for building RF power transistors and consist of 1.9-μm thick GaN/AlGaN high-electronmobility-transistor (HEMT) epilayers atomically attached on a 100-μm thick CVD diamond wafer. The HEMT epilayers include a buffer and GaN nucleation (strain relief) layers. The first-grown region of the CVD diamond wafer is adjacent to the GaN/AlGaN epilayers, while the rough, last-grown regions of the diamond wafer become the back of the chip. Diamond substrates were grown using microwave plasma and exhibit columnar-grain growth with grain size on the last-grown surface ≈20 μm. The bulk thermal conductivity (average value for the entire wafer) has been measured to be κD ≥ 1,500 W/cmK. The heat conductivity of CVD diamond wafers is not uniform: the first-grown diamond layers have heat conductivity that is lower than the ultimate “bulk” value due to smaller grain size (diamond micro-crystals) and possible presence of amorphous carbon [19,20] at the beginning of the diamond growth. The region within which the heat conductivity is lower and varies with depth will be referred to as the graded-heat-conductivity (GHC) region. The wafers used in this work are optically transparent. Heat conductivity correlates with the optical absorption [21] and hence the GHC region is expected to have light absorption higher than bulk CVD diamond. Both lower heat conductivity and higher optical absorption close to the surface of the diamond wafer play a role in successful laser drilling and cutting of this type of wafer. Some of the experiments described in this work were performed on bare GaN-ondiamond wafers (with structure described above), while others were performed on wafers with completed high-electron mobility transistors (as indicated with the device features in Fig. 1). 3. Wafer micromachining 3.1. Laser cutting/drilling of diamond Diamond has fundamental absorption gap around 230 nm, and therefore is transparent for wavelengths used in most commercial lasers. Laser machining using visible and infrared lasers relies on thermal ablation and/or laser-assisted chemical etching, rather than photochemical ablation [22]. In a simplified view, laser drilling/cutting of diamond occurs as a two step process: First, the high-energy laser pulse thermally converts the top surface of diamond into a thin layer of graphite which in turn absorbs the subsequent laser pulses and, before it is thermally ablated, converts the diamond below it to graphite [23–26]. This process sequentially drills the diamond while an intermediate graphite layer persists on top of the diamond being drilled; the thickness of this graphite layer varies with the pulse duration and heat diffusivity of the material around it. To start this process efficiently it is advantageous to coat the diamond with an absorbing film: a layer of metal or graphite. In this work, the energy required for initial graphitization is
provided by the light absorbed in the GHC region and by free-carrier absorption in the GaN/AlGaN epilayers. Both of these exhibit light absorption greater than bulk diamond. Chemical etching of diamond in oxygen while producing CO and CO2 (i.e., diamond burning), dominates below the graphite ablation threshold temperature (≈4000 °C), provided there is sufficient oxygen for this reaction. Under conditions of insufficient oxygen, drilling of diamond is accompanied with spallation of graphitized carbon due to thermal shock. For cutting in air, the residue on the surface of diamond around the cut/drill has been identified as graphite and glassy carbon (nanocrystalline graphite) [27]. When chips are large (several tens of mm2) and sufficiently thick, they can be handled individually and cleaned in bulk (for example, commercially-available diamond heatsinks and heat-spreaders). However, working with small semiconductor dies (several mm2) that have fragile top surface (eg. devices with airbridges and metal contacts) requires cleaning at the wafer level. The two processes we are interested in are via drilling and laser dicing. These processes generally occur at different stages in a microwave chip fabrication (described below), but both require precision cutting/drilling location, minimal heat damage around the cut, and clean surfaces after laser machining. 3.2. Heat damage In order to maintain the damage around the cut/drilled region to a minimum, the penetration of heat from the drill location into the surrounding volume should be kept to a minimum or at least within the region allowed for kerf. Higher light absorption and lower heat conductivity in the top layers of the GaN-on-diamond structure (GHC and GaN/AlGaN layers) facilitate lower heat diffusion and more efficient drilling. Using typical values for thermal diffusivity of CVD diamond grown by microwave deposition technique as α ≈ 4 cm2/s (using κ≈7 W/Kcm) and laser pulse duration τL = 90 ns used in most of this work, we estimate the thermal diffusion length L2D = ατL for CVD diamond relevant to our work to be of the order of tens of micrometers. For picosecond lasers, the heat penetration can be well below a micrometer. Fig. 2 shows an example of a via drilled with 90 ns (this work) and 10 ps [28] Nd:YAG laser. Clearly, for via processing, processing with shorter pulses is preferred because it allows placing the vias in close proximity of the ohmic contacts. The thermal time constant of the drilled volume can be estimated as the ratio of the energy E dissipated within a heated region to the heat flux P flowing out of the heated region into the substrate: τTH ≡E/P (assumes Newtonian cooling). We estimate the thickness of GHC region to be at least 10 to 15 μm. Since the beam diameters used in this experiment are typically double that depth (20–30 μm), it is reasonable to represent the heated volume as a half-sphere with r~ 15 μm submerged in the diamond surface. In the simplest model, the half-sphere is where the power is dissipated and the temperature is high, while the rest of the half-space (into the substrate) is the volume into which the heat is being dissipated. The thermal time constant τTH is then given as τTH =VθTHκ/α, where, V volume of the half-sphere, θTH thermal resistance seen by the half-sphere when cooling into the half-space below it, and κ the thermal conductivity of the surrounding space. The thermal resistance of an iso-thermal halfsphere with radius r immersed in a semi-infinite space is approximately given with θTH = 1/4rκ [29] giving an approximate expression for the thermal time constant: 3ατTH =r2. (Note that this is close to the approximate expressions used in laser interaction with tissue [22]: 4ατTH = d2, where d= depth of absorption.) For CVD diamond and r ≈15 μm we get τTH ≈0.2 μs. Once the drilling progresses into the body of the substrate, the thermal time constant reduces further because the heat conductivity increases (increasing the heat diffusivity), while the heated volume reduces: the laser beam only gets absorbed in the graphitized layer whose thickness depends on the pulse duration and is typically significantly less than 10 μm [23]. Typical thermal diffusivity values for bulk diamond are α≈7 −8 cm2/s [30]. Finally, it is essential
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
677
Fig. 2. Vias in GaN-on-diamond using 1064 nm Nd:YAG Q-switched laser with pulse duration (a) 35 ns and (b) 10 ps.
that the pulse duration should be kept below the thermal time constant of the drilled volume [26,31,32]. 3.3. Cutting from the front versus the back of the wafer The front of processed GaN-on-diamond wafers is epi-smooth, while the back is rough. Laser drilling/scribing may be performed from either side, but the quality of the cut will be different as we describe in this section. Drilling from the back is potentially of interest because the graphite debris formed during the drilling would not coat the completed devices. However, the debris would coat the backside and would have to be cleaned for proper die attachment. When drilling from the uncoateddiamond back, we consistently find that the beam-outgoing (front) surface gets damaged: The beam entering from the back of the wafer refracts on the transparent diamond micro-crystals and is then redirected randomly toward the front of the wafer where it exits in multiple places around the central hole. Upon exiting, the laser beam burns holes in the GHC region due to higher absorption, as shown in Fig. 3(b). We believe laser beam scattering is quite pronounced in these experiments because the size of diamond micro-crystals on the back of the diamond wafer is comparable to the size of the laser beam spot. Fig. 4 illustrates this explanation. One could potentially prevent light beam refraction and penetration into the crystal by coating the back of the wafer with an absorber. We have experimented with TiAu coating as a part of the process (not used specifically to absorb the laser beam), but discovered that it melts and balls up, while neither preventing graphite debris deposition nor light penetration into the wafer. When the beam is incident from the front of the sample, the flat surface, higher absorption, and finer diamond grains easily facilitate graphitization and diamond ablation — results shown in Fig. 3(a). For this reason, the drilling proceeds straight into the sample exiting clearly at one location on the back making it the preferred approach for both via drilling and wafer scribing/dicing. 3.4. Drill/scribe profile All of the drilling and dicing experiments performed in this work exhibit similar drill profiles as shown in Figs. 5 and 9: The width w of the trench as a function of the depth z exhibits convex curvature (d2w/ dz2 N 0). This shape is a result of the Gaussian beam-shape: the middle of
Fig. 3. Photographs of (a) the back side of the wafer when laser-drilled from the front, and (b) the front side when laser-drilled from the back.
678
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 4. Sketch explaining the difference between drilling from the back and the front.
the beam drills faster than the edges [26]. Convex drill profile is advantageous for efficient metallization coverage by evaporating or sputtering metal from the front. We have successfully coated vias using TiW sputtering. However, an angled scribe trench results in die with sloped edges. This presents a challenge in die picking; using vacuum to pick die will be preferred. 3.5. Graphite debris formation and removal An ideal diamond laser-machining recipe is the one that leaves no graphite debris. To accomplish this, drilling conditions have to allow sufficient oxygen presence to convert all (or nearly all) of the carbon into volatile products. The rate of oxygen supply at the drilling location is determined by oxygen partial pressure, diffusion of volatile reactants, and the size of the drilling location (determining the efficiency of oxygen diffusion to the burning location). Based on this simple hypothesis, we expect that graphite debris formation will be reduced if drilling is performed under oxygen atmosphere and using average laser powers low enough to allow all of the carbon to convert to volatile products. These conditions would ensure that chemical etching of diamond is reaction limited rather than diffusion limited. In addition, a number of previous reports of laser-assisted etching in oxygen suggest increased etch rates [25,27,33], although they do not address the issue of graphite debris. To illustrate that laser machining under oxygen atmosphere improves the cleanliness, we performed several trench drilling experiments using Nd:YAG 1064 nm laser with 5 kHz pulse repetition frequency (PRF), pulse duration of 35 ns, and average power of 7 W [34]. We laser-scribed GaN-on-diamond wafers (structure shown in Fig. 1) with rectangular
Fig. 5. Typical (a) dicing profile with a through cut (the wafer is 50 μm thick), and (b) thruvias on a 100-μm wafer (the wafer has been cleaved through the vias).
Fig. 6. GaN-on-diamond wafer surface after laser scribing and cleaning.
grid pattern with 400-μm pitch in each direction with 10 scans each line. The laser spot size had a 50-μm diameter. The scribing was performed under the presence of forced nitrogen (N2), forced oxygen (O2), and stationary air (78% N2/21% O2). A ¼″ PVC tubing nozzle was placed about 7 mm from drilling location and the gas flow was adjusted to be about 20,000 sccm from a tank; there was no enclosure. Three samples were scribed. The micrographs of the GaN surface for the three conditions are shown in the far left column of Fig. 6. It is clear that the presence of oxygen has a very beneficial effect on reducing debris. We subsequently investigated ways to clean the debris: using isopropanol or water while agitated in an ultrasonic bath or manual scrubbing did not produce good results. Better results were obtained using 300 MIF® AZ Developer (containing tetramethylammonium hydroxide) and ultrasonic agitation, shown in Fig. 6 under “solvent clean”. We also used a downstream ash tool (Gasonics Aura 1000) for 200 s at 1 kW RF power, and found that it produced the best results as shown in Fig. 6 and labeled “oxygen clean”. Evidently, the combination of oxygen during drilling and atomic oxygen during cleaning fully cleans the samples with no visible residue. 3.6. Debris-free dicing The average power of 7 W, used in the above experiment, clearly results in a diffusion limited laser-assisted chemical etching of diamond. To explore low power laser cutting we used a Q-switched Nd:YAG 1064 nm laser [35][36] with PRF 1.5–5 kHz, pulse duration from 65 ns to 90 ns and average beam powers ranging from 100 mW to 2 W. The laser spot diameter was estimated to be 18 μm. Inasmuch as the laser operates in a pulsed mode, in order to obtain smooth scribes, the
Fig. 7. Etch rate (drill rate) as a function of fluence as determined in this work (in the presence of oxygen overpressure).
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
679
Fig. 8. The edge of a through-cut of 100-μm thick GaN-on-diamond wafer (a) in air and (b) under oxygen overpressure. The conditions were PAVE = 150 mW, f = 1.5 kHz.
scribing velocity v (beam spot relative to the wafer motion) has to be selected so that two subsequent pulses overlap. The pulses overlap if v b df/2. Selecting v = 6.25 mm/s (constant in all experiments) meets this condition by a factor of two. The average beam power is given by PAVE = PRF⋅ F ⋅ A, where A [cm2] is the area of the beam spot and F the fluence. Fluence is defined as radiative flux integrated over time (i.e., electromagnetic energy) per unit area delivered by a laser beam (typical units are J/cm2). We use beam diameter as the width of the hole at halfway through a 100 μm substrate since the beam diameter is not clearly defined due to the non-uniformity of the intensity over the beam profile. Above a certain threshold, diamond removal rate monotonically increases with the fluence [23,25,26]. For any fixed beam profile, the fluence is proportional to the average power and the proportionality depends on the beam profile, and pulse duration. The measured etch rate versus the fluence is shown in Fig. 7. The etching threshold was found to be around F ≈ 30 J/cm2, which is in agreement with previously published values for diamond laser drilling [23]. The two main constraints for realizing debris-free laser drilling of diamond are (a) maintaining consistent drilling by keeping the fluence above the threshold (typically 50% above the threshold) and (b) maintaining the average beam power sufficiently low to allow carbon burning rather than ablating. We were successful in scribing a large number of samples under the above constraints, but found that it was not possible to eliminate all of the debris from the neighborhood of the scribe. We performed a number of experiments over a
Fig. 9. Side-view of a scribed and cleaved GaN-on-diamond wafer. One un-cleaved scribe trench with depth of 50 μm is shown. The marks from the multiple passes of the laser beam are visible in the top part of cross section, while the bottom part is the jagged part that was cleaved.
range of average laser powers and concluded that the best results (subjectively established acceptable amount of graphite debris) were obtained when the average laser power was below 1/3 W. Fig. 8 shows an optical micrograph of the effect of oxygen presence in a straight cut through a wafer. We consider debris shown in Fig. 8(a) unacceptable, while the debris shown in Fig. 8(b) barely acceptable. The optimal conditions for scribing were established by reducing the average power to a minimum value for which etching occurs consistently: average power equal to 200 mW, PRF=1.5 kHz, τL =35ns. Under these conditions, the processing time averaged about 2 s/die. Incidentally, the mentioned chemical-etching component makes diamond laser machining significantly more efficient than machining of silicon or silicon carbide, the other two common choices for substrates for GaN growth. If the wafer surface could be protected before scribing, the diamond wafer would have to be scribed in such a way that cleaning could be performed in wafer form rather than chip form. This means that the scribed wafer would have to remain sufficiently rigid to be handled prior to its attachment to dicing tape and cleaving. 4. Cleaving Laser-scribing involves cutting partially through a wafer to facilitate controlled cleaving. CVD diamond used for making the GaN-ondiamond wafers is polycrystalline and it does not have clearly defined cleaving planes as do single crystal silicon or III–V semiconductors: Cleaving CVD diamond wafers does not result in atomically flat surfaces, but in jagged edges and, most importantly, diamond dust. Die separation is typically done by attaching a wafer to a dicing tape, sawing or scribing, and cleaving by applying pressure from below. For this to work well, the adhesion of the wafer (and the chip) to the dicing tape has to be greater than the force necessary to break the wafer. This means that the diamond growth surface (bottom of structure shown in Fig. 1) must adhere well to the dicing tape. We find that, due to diamond back-surface roughness, this is not the case: The adhesion to standard medium-tack dicing tape (“blue tape”) [37] is insufficient and the wafer detaches from the dicing tape during cleaving even when the trench depth is ~90%. Ultra-violet dicing tape has better adhesion, but is still insufficient to hold the wafers in place. To cleave wafers reliably, we resorted to a two-dicing-tape approach: a first blue tape layer is stretched on a dicing hoop with the adhesive up (normal orientation) and the GaN-on-diamond wafers is attached to it with the device side up (back side to adhesive). A second dicing tape piece is stretched and
680
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 10. View of a 380 μm × 400 μm die (a) before scribing, and (b) after scribing, cleaving, and die separation.
attached from the top to cover the device wafer. The side of the second dicing tape that had no adhesive is adjacent to the device wafer. Pressure onto the die to be separated is applied using a cleaving wedge from below, while additional pressure is applied on adjacent dies from the top. The top tape generally touches only edge of the chip and does not damage the circuitry on the chip. Using this method, we were able to reliably cleave die as small as 400 × 400 μm2 that has been scribed at least 40% (die width to remaining thickness ratio ~7×). The ability to cleave a wafer using the cleaving wedge depends on the difference in torque that can be exerted on two adjacent dies on the wafer. Our experience is that the ratio between the die size and the remaining uncut thickness of the diamond should be at least 15:1 for an efficient cleave. Higher ratios are preferred, but the bottom of the cut should still not approach the rough back side of the diamond. If the cut enters the rough side of the substrate, the wafer may break apart (cleave) while it is being laser-scribed and the alignment may be lost. Cleaving scribed diamond wafers produces dust. Unless precautions are taken to minimize it or protect the chip surface, the diamond micro-crystals, which form an excellent abrasive, will scratch the chip surface during cleaving and handling. One way to avoid this is to cut through the entire wafer and avoid cleaving altogether. Although this is possible by dicing the wafer while it is mounted on a dicing tape, it requires precise cutting depth to avoid damaging the dicing tape. We prefer to maintain the ratio between the die size and the remaining uncut thickness of the diamond at least 15:1, which minimizes the dust generation and makes the cleaving easy. Fig. 10 shows optical micrographs of a completed high-electron mobility transistor (a) prior to scribing and (b) after cleaving and die separation. The scribing was performed at average laser power equal to 200 mW, f = 1.5 kHz, τL = 65 ns, and the cleaving using two-dicing tape approach described above. The scribing trench depth was ~40% of the wafer thickness (die size to remaining thickness ratio ~7). The first noteworthy feature of this example is that the short pulse duration and low power resulted in minimal heat damage. The transistors were distanced by 50 μm and 150 μm in the two directions, and the dicing trench was almost 50-μm wide, while there is no evidence of heat damage (gold metallization remained unchanged) and there was no graphite debris correlated to the die edge. The second noteworthy feature is that diamond dust is present at random places (the dust does not track the edge of the die, but is distributed unevenly) and that it has produced some surface damage during cleaving.
5. Discussion Via drilling and dicing steps generally do not occur at the same stage in the device manufacturing: via drilling is followed by via coating (and associated lithography) and backside processing, while dicing occurs at the end of the wafer processing after which the die is separated. As we showed in Section 2, cleaning of the graphite debris is possible and straightforward on the wafer level. Dicing, on the other hand, requires a high degree of cleanliness to facilitate efficient die attach and visually clean shippable devices. The approaches for accomplishing this are either (a) cleaning individual die or (b) scribing/dicing under conditions that minimize debris. We have successfully employed (b) in this work and the previous work [12]. The difficulty, however, with this latter approach is the long processing time resulting from using low average laser power. In laser machining, cost reduction is accomplished primarily by increasing the throughput. Operating at reduced laser powers makes the scribing take longer time and increases the die cost. One alternative to meeting both requirements (faster processing and clean final die) is to protect the devices on the wafer, drill the vias and scribe at the same stage in the process, and clean the debris by dissolving the protect layer while the wafers are not yet cleaved. For this to be successful, the scribe depth has to be carefully managed to maintain the wafers sufficiently rigid to allow cleaning and further limited processing (e.g., lithography), while still allowing it to be efficiently cleaved. For example, a 100-um-thick diamond wafer could be scribed to around 50% still enabling an efficient final cleave step. We find that patterning of the protective coating (on the devices) and leaving scribe alleys and via locations unprotected, is necessary to prevent laser-charred protect-layer residue which can be very difficult to remove (typically, we would use thick photoresist). Using laser-assisted etching in oxygen as described in Section 2 and subsequent cleaning in atomic oxygen produced best results. Finally, we explored laser dicing using 355 nm tripled Nd:YAG laser [38]. We found that the etch rate is not very wavelength dependent, and confirmed that shorter pulses and reduced average laser powers lead to cleaner scribing. This can be seen in the example shown in Fig. 11 where a 600 μm × 600 μm die was laser-scribed at PAVE = 330 mW, τL = 35 ns, and PRF = 5 kHz. 6. Conclusion Dicing and via drilling of GaN-on-diamond wafers to obtain clean shippable die present a number of opposing constraints owing to
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 11. Optical micrograph of an array of 600 × 600 μm2 dies after laser scribing using 355 nm Q-switched Nd:YAG laser with pulse duration 35 ns, f = 5 kHz, and average power equal to 330 mW [38].
phenomena specific to diamond processing: graphite debris deposition around the cut, cleaning of the debris, appearance of diamond dust during cleaving, and finally the speed of wafer processing. The conditions and approaches for optimal laser micromachining will eventually be defined when these processes are integrated into a standard GaN HEMT fabrication sequence. The fact that the amount of graphite debris can be controlled with the average laser power and oxygen concentration, and that it can be efficiently cleaned, clearly favors performing wafer scribing (and subsequent processing) on the wafer level. Protecting the devices during via and scribing steps is essential. Acknowledgment This work was supported in part by generous SBIR grants from MDA and AFRL (John Blevins). Other parts were also funded by SBIR grants from NASA (Lihua Li and Arnold Silva). The authors gratefully acknowledge James Gillespie for providing some of the processed wafers that were diced during this work (Fig. 10), Brian Price, Terry Pothoven, Grey Brooks for assistance with laser micromachining, Lumera Laser for performing test via-drilling, and Alex Schreiber for the design of the mechanical tools used in this work. References [1] J.G. Felbinger, et al., IEEE Electron Device Lett. 28 (2007) 948. [2] P.R. Hageman, J.J. Schermer, P.K. Larsen, Thin Solid Films 443 (2003) 9–13. [3] P.W. Maya, H.Y. Tsai, W.N. Wang, J.A. Smith, Diam. Rel. Mat. 15 (2006) 526–530.
681
[4] M. Oba, T. Sugino, Diam. Rel. Mat. 10 (2001) 1343. [5] G.W.G. van Dreumel, J.G. Buijnsters, T. Bohnen, J.J. ter Meulen, P.R. Hageman, W.J.P. van Enckevort, E. Vlieg, Diam. Rel. Mat. 18 (2009) 1043. [6] J.W. Zimmer, Mater. Res. Soc. Symp. Proc. 956 (2007). [7] E. Piner, J.W. Zimmer, J.C. Roberts, G. Chandler, R.A. Sadler, Epi-inverted N-face GaN/Diamond for AlGaN/GaN/AlGaN FETs, 33rd WOCSDICE, Málaga, Spain (May 17–20, 2009), 2009. [8] A. Dussaigne, M. Maliverni, D. Martin, A. Castiglia, N. Grandjean, (0001) GaN grown on (111) single crystal diamond substrate for high power electronics applications, Proc. 18th European Workshop on Heterostructure Technology (HETECH 2009), 2–4 November 2009, Ulm, Germany, 2009, p. 33. [9] D. Francis, F. Faili, D. Babić, F. Ejeckam, A. Nurmikko, H. Maris, Diam. Rel. Mat. 19 (2010) 229–233. [10] D. Francis, J. Wasserbauer, F. Faili, D. Babic, F. Ejeckam, W. Hong, P. Specht, E.R. Weber, GaN HEMT epilayers on diamond substrates: recent progress, CS MANTECH, Austin, TX (May 14–17, 2007), 2007. [11] Q. Diduck, J. Felbinger, L.F. Eastman, D. Francis, J. Wasserbauer, F. Faili, D.I. Babić, F. Ejeckam, Electron. Lett 45 (2009) 758–759. [12] D.I. Babić, Q. Diduck, P. Yenigalla, A. Schreiber, D. Francis, F. Faili, F. Ejeckam, J.G. Felbinger, L.F. Eastman, GaN-on-diamond field-effect transistors: from wafers to amplifier modules, Proc. of the Symposium on Microelectronics, Electronics, and Electronic Technologies (MEET), MIPRO, Opatija, Croatia, May 20–24, 2010. [13] R.J. Trew, J.-B. Yan, P.M. Mock, IEEE Proc. 79 (1991) 598. [14] A. Aleksov, M. Kubovic, M. Kasu, P. Schmid, D. Grobe, S. Erttl, M. Schreck, B. Strizker, E. Kohn, Diam. Rel. Mat. 13 (2004) 233–240. [15] O. Dorsch, M. Werner, E. Obermeier, R.E. Harper, C. Johnton, I.M. Buckle-Golder, Diam. Rel. Mat. 1 (1992) 277–280. [16] M.P. Hiscocks, C.J. Kaalund, F. Ladouceur, S.T. Huntington, B.C. Gibson, S. Trpowski, D. Simpson, E. Ampen-Lassen, S. Prawer, J.E. Butler, Diam. Rel. Mat. 17 (2008) 1831–1834. [17] R. Otterbach, U. Hilleringmann, Diam. Rel. Mat. 11 (2002) 841–844. [18] Y. Ando, Y. Nishibayashi, K. Kobashi, T. Hirao, K. Oura, Diam. Rel. Mat. 11 (2002) 824–827. [19] H. Verhoeven, A. Flőter, H. Reiβ, R. Zachai, Appl. Phys. Lett. 71 (1997) 1329–1331. [20] J.E. Graebner, S. Jin, G.W. Kammlott, J.A. Herb, C.F. Cardinier, Appl. Phys. Lett. 60 (1992) 1576–1578. [21] J.E. Graebner, Diam. Rel. Mat. 4 (1995) 1196–1199. [22] N.B. Dahotre, S.P. Harimkar, Laser Fabrication and Machining of Materials, Springer Science + Business Media, New York, USA, 2008. [23] V.V. Kononenko, T.V. Kononenko, S.M. Pimenov, M.N. Sinyavkii, V.I. Konov, F. Dausinger, Quantum Electron. 35 (2005) 252–256. [24] T.V. Kononenko, V.G. Ralchenko, I.I. Vlasov, S.V. Garnov, V.I. Konov, Diam. Rel. Mat. 7 (1998) 1623–1627. [25] V.G. Ralchenko, K.G. Korotushenko, A.A. Smolin, E.N. Loubnin, Diam. Rel. Mat. 4 (1995) 893–896. [26] T.V. Kononenko, V.G. Ralchenko, I.I. Vlasov, S.V. Garnov, V.I. Konov, Diam. Rel. Mat. 7 (1998) 1623–1627. [27] S. Gloor, S.M. Pimenov, E.D. Obratsova, W. Lűthy, H.P. Weber, Diam. Rel. Mat. 7 (1998) 607–611. [28] Lumera laser GmbHKaiserslautern, Germany, www.lumeralaser.com. [29] S.S. Kutateladze, Fundamentals of Heat Transfer, Academic Press, New York, 1963. [30] S. Albin, W.P. Winfree, B.S. Crews, J. Electrochem. Soc. 137 (1990) 1973–1976. [31] M.D. Shirk, P.A. Molian, A.P. Malshe, J. Laser Appl. 10 (1998) 64–708 still need to get it. [32] V.V. Kononenko, M.S. Komlenok, S.M. Pimenov, V.I. Konov,“Photoinduced laser etching of a diamond surface”, Quantum Electron. 37 (2007) 1043–1046. [33] V.V. Migulin, V.G. Ralchenko, Y.-J. Baik, Proc. SPIE 3483 (1998) 175. [34] Laser Marks, Inc., Santa Clara, CA; www.lasermarks.com. [35] Laserod, Inc., Torrance, CA; www.laserod.com. [36] Lee Laser, Inc., Orlando, FL; model 712THP. [37] Medium-tack dicing tape, www.semicorp.com8 p/n 18551–7.50. [38] Coherent, Inc, Santa Clara, CA; Q-switched tripled Nd:YAG model AVIA 355.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Laser machining of GaN-on-diamond wafers Dubravko I. Babić ⁎, Quentin Diduck, Firooz Faili, John Wasserbauer, Frank Lowe, Daniel Francis, Felix Ejeckam Group 4 Labs, Inc. 39500 Stevenson Place, Suite 207, Fremont, CA 94539, United States
a r t i c l e
i n f o
Article history: Received 15 July 2010 Received in revised form 25 February 2011 Accepted 13 March 2011 Available online 23 March 2011
a b s t r a c t The commercialization of gallium-nitride microwave circuits on diamond substrates requires chip-dicing technology and via formation process compatible with standard semiconductor processes. This paper discusses issues related to dicing and drilling of GaN-on-diamond wafers for RF power transistor applications (die size b 1 mm2) using laser micromachining. © 2011 Elsevier B.V. All rights reserved.
Keywords: Gallium nitride on diamond Laser machining AlGaN/GaN high-electron mobility transistors Via processing in diamond Diamond wafer dicing
1. Introduction The integration of gallium nitride epilayers with chemical-vapor deposited (CVD) diamond substrates presents an ideal combination of materials for manufacturing thermally efficient high-power electronics and optoelectronics. Microwave and millimeter power amplifier modules based on AlGaN/GaN high-electron mobility transistors (HEMTs) on diamond substrates are expected to have double the thermal conductance over those built on silicon carbide [1]. Consequently, circuits employing this technology can expect a substantial increase in power handling capability for the same junction and ambient temperatures or significantly reduced cooling requirements (i.e., higher ambient temperature for the same junction temperature and power). This fact has spurred numerous investigations of GaN integration with diamond substrates [2–8]. Presently the closest to commercial introduction is the “GaN-on-diamond” technology in which GaN epilayers are atomically attached to chemical-vapor deposited (CVD) diamond substrates [9,10]. Using this approach we have demonstrated 100-mm GaN-on-diamond wafers [9], record operation of GaN-on-diamond HEMTs [11], and amplifier modules [12]. The commercialization of electronic and optoelectronic components based on GaN-on-diamond engineered wafers requires the development of a wafer-dicing process and, in the case of microwave and millimeterwave circuits, a process for the formation of vias for connecting the front of a chip to its metalized back. Diamond is the hardest material known to man and its micromachining presents a unique challenge (also present in the development of diamond-based electronics [13,14]). Wafer mechan⁎ Corresponding author. Tel.: +1 408 398 2484; fax: +1 408 689 9674. E-mail address: [email protected] (D.I. Babić). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.03.017
ical dexterity is critical for compliance with standard semiconductor device processing techniques. Unlike silicon and typical III–V semiconductors that are typically processed at 300–500 μm wafer thickness and subsequently lapped down for back metallization and dicing, GaNon-diamond wafers are generally not thinned and have thicknesses between 50 and 100 μm throughout the process. There are two approaches to diamond cutting and drilling: plasma etching and laser drilling/cutting. Reactive-ion etching of diamond has been investigated in recent years [15–18]. However, no etching of 100μm deep structures with high-aspect ratios (critical for vias and dicing) has been reported. Additionally, issues such as side-wall slope control, mask material, and overall practicality of etching such deep structures has not been properly studied or evaluated. With the increasing interest in diamond wafer use and applications, reactive ion etching may hold the future for high volume micromachining once the quality of the etched structure, the total wafer processing times and equipment overhead become more economical than laser machining. At this time, laser micromachining is the economical as well as the technological choice to meet die processing requirements. In this paper, we discuss several issues and findings related to dicing and drilling of GaN-on-diamond wafers. The ultimate result of this work is the successful fabrication of hybrid GaN-on-diamond RF amplifier modules described elsewhere [12]. 2. Wafer structure GaN-on-diamond engineered wafers are presently available in 25, 50, and 100 mm diameters. The manufacturing process, epilayer structure, and substrate parameters have been previously described in reference [9], while here we only provide the parameters relevant to laser
676
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 1. Cross-sectional schematic of GaN-on-diamond wafer. The thickness of the graded-heat-conductivity (GHC) region varied depending on the process conditions, but of the order of 10–15 μm.
micromachining. The cross-sectional schematic of wafers used in this work is shown in Fig. 1. The wafers are intended for building RF power transistors and consist of 1.9-μm thick GaN/AlGaN high-electronmobility-transistor (HEMT) epilayers atomically attached on a 100-μm thick CVD diamond wafer. The HEMT epilayers include a buffer and GaN nucleation (strain relief) layers. The first-grown region of the CVD diamond wafer is adjacent to the GaN/AlGaN epilayers, while the rough, last-grown regions of the diamond wafer become the back of the chip. Diamond substrates were grown using microwave plasma and exhibit columnar-grain growth with grain size on the last-grown surface ≈20 μm. The bulk thermal conductivity (average value for the entire wafer) has been measured to be κD ≥ 1,500 W/cmK. The heat conductivity of CVD diamond wafers is not uniform: the first-grown diamond layers have heat conductivity that is lower than the ultimate “bulk” value due to smaller grain size (diamond micro-crystals) and possible presence of amorphous carbon [19,20] at the beginning of the diamond growth. The region within which the heat conductivity is lower and varies with depth will be referred to as the graded-heat-conductivity (GHC) region. The wafers used in this work are optically transparent. Heat conductivity correlates with the optical absorption [21] and hence the GHC region is expected to have light absorption higher than bulk CVD diamond. Both lower heat conductivity and higher optical absorption close to the surface of the diamond wafer play a role in successful laser drilling and cutting of this type of wafer. Some of the experiments described in this work were performed on bare GaN-ondiamond wafers (with structure described above), while others were performed on wafers with completed high-electron mobility transistors (as indicated with the device features in Fig. 1). 3. Wafer micromachining 3.1. Laser cutting/drilling of diamond Diamond has fundamental absorption gap around 230 nm, and therefore is transparent for wavelengths used in most commercial lasers. Laser machining using visible and infrared lasers relies on thermal ablation and/or laser-assisted chemical etching, rather than photochemical ablation [22]. In a simplified view, laser drilling/cutting of diamond occurs as a two step process: First, the high-energy laser pulse thermally converts the top surface of diamond into a thin layer of graphite which in turn absorbs the subsequent laser pulses and, before it is thermally ablated, converts the diamond below it to graphite [23–26]. This process sequentially drills the diamond while an intermediate graphite layer persists on top of the diamond being drilled; the thickness of this graphite layer varies with the pulse duration and heat diffusivity of the material around it. To start this process efficiently it is advantageous to coat the diamond with an absorbing film: a layer of metal or graphite. In this work, the energy required for initial graphitization is
provided by the light absorbed in the GHC region and by free-carrier absorption in the GaN/AlGaN epilayers. Both of these exhibit light absorption greater than bulk diamond. Chemical etching of diamond in oxygen while producing CO and CO2 (i.e., diamond burning), dominates below the graphite ablation threshold temperature (≈4000 °C), provided there is sufficient oxygen for this reaction. Under conditions of insufficient oxygen, drilling of diamond is accompanied with spallation of graphitized carbon due to thermal shock. For cutting in air, the residue on the surface of diamond around the cut/drill has been identified as graphite and glassy carbon (nanocrystalline graphite) [27]. When chips are large (several tens of mm2) and sufficiently thick, they can be handled individually and cleaned in bulk (for example, commercially-available diamond heatsinks and heat-spreaders). However, working with small semiconductor dies (several mm2) that have fragile top surface (eg. devices with airbridges and metal contacts) requires cleaning at the wafer level. The two processes we are interested in are via drilling and laser dicing. These processes generally occur at different stages in a microwave chip fabrication (described below), but both require precision cutting/drilling location, minimal heat damage around the cut, and clean surfaces after laser machining. 3.2. Heat damage In order to maintain the damage around the cut/drilled region to a minimum, the penetration of heat from the drill location into the surrounding volume should be kept to a minimum or at least within the region allowed for kerf. Higher light absorption and lower heat conductivity in the top layers of the GaN-on-diamond structure (GHC and GaN/AlGaN layers) facilitate lower heat diffusion and more efficient drilling. Using typical values for thermal diffusivity of CVD diamond grown by microwave deposition technique as α ≈ 4 cm2/s (using κ≈7 W/Kcm) and laser pulse duration τL = 90 ns used in most of this work, we estimate the thermal diffusion length L2D = ατL for CVD diamond relevant to our work to be of the order of tens of micrometers. For picosecond lasers, the heat penetration can be well below a micrometer. Fig. 2 shows an example of a via drilled with 90 ns (this work) and 10 ps [28] Nd:YAG laser. Clearly, for via processing, processing with shorter pulses is preferred because it allows placing the vias in close proximity of the ohmic contacts. The thermal time constant of the drilled volume can be estimated as the ratio of the energy E dissipated within a heated region to the heat flux P flowing out of the heated region into the substrate: τTH ≡E/P (assumes Newtonian cooling). We estimate the thickness of GHC region to be at least 10 to 15 μm. Since the beam diameters used in this experiment are typically double that depth (20–30 μm), it is reasonable to represent the heated volume as a half-sphere with r~ 15 μm submerged in the diamond surface. In the simplest model, the half-sphere is where the power is dissipated and the temperature is high, while the rest of the half-space (into the substrate) is the volume into which the heat is being dissipated. The thermal time constant τTH is then given as τTH =VθTHκ/α, where, V volume of the half-sphere, θTH thermal resistance seen by the half-sphere when cooling into the half-space below it, and κ the thermal conductivity of the surrounding space. The thermal resistance of an iso-thermal halfsphere with radius r immersed in a semi-infinite space is approximately given with θTH = 1/4rκ [29] giving an approximate expression for the thermal time constant: 3ατTH =r2. (Note that this is close to the approximate expressions used in laser interaction with tissue [22]: 4ατTH = d2, where d= depth of absorption.) For CVD diamond and r ≈15 μm we get τTH ≈0.2 μs. Once the drilling progresses into the body of the substrate, the thermal time constant reduces further because the heat conductivity increases (increasing the heat diffusivity), while the heated volume reduces: the laser beam only gets absorbed in the graphitized layer whose thickness depends on the pulse duration and is typically significantly less than 10 μm [23]. Typical thermal diffusivity values for bulk diamond are α≈7 −8 cm2/s [30]. Finally, it is essential
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
677
Fig. 2. Vias in GaN-on-diamond using 1064 nm Nd:YAG Q-switched laser with pulse duration (a) 35 ns and (b) 10 ps.
that the pulse duration should be kept below the thermal time constant of the drilled volume [26,31,32]. 3.3. Cutting from the front versus the back of the wafer The front of processed GaN-on-diamond wafers is epi-smooth, while the back is rough. Laser drilling/scribing may be performed from either side, but the quality of the cut will be different as we describe in this section. Drilling from the back is potentially of interest because the graphite debris formed during the drilling would not coat the completed devices. However, the debris would coat the backside and would have to be cleaned for proper die attachment. When drilling from the uncoateddiamond back, we consistently find that the beam-outgoing (front) surface gets damaged: The beam entering from the back of the wafer refracts on the transparent diamond micro-crystals and is then redirected randomly toward the front of the wafer where it exits in multiple places around the central hole. Upon exiting, the laser beam burns holes in the GHC region due to higher absorption, as shown in Fig. 3(b). We believe laser beam scattering is quite pronounced in these experiments because the size of diamond micro-crystals on the back of the diamond wafer is comparable to the size of the laser beam spot. Fig. 4 illustrates this explanation. One could potentially prevent light beam refraction and penetration into the crystal by coating the back of the wafer with an absorber. We have experimented with TiAu coating as a part of the process (not used specifically to absorb the laser beam), but discovered that it melts and balls up, while neither preventing graphite debris deposition nor light penetration into the wafer. When the beam is incident from the front of the sample, the flat surface, higher absorption, and finer diamond grains easily facilitate graphitization and diamond ablation — results shown in Fig. 3(a). For this reason, the drilling proceeds straight into the sample exiting clearly at one location on the back making it the preferred approach for both via drilling and wafer scribing/dicing. 3.4. Drill/scribe profile All of the drilling and dicing experiments performed in this work exhibit similar drill profiles as shown in Figs. 5 and 9: The width w of the trench as a function of the depth z exhibits convex curvature (d2w/ dz2 N 0). This shape is a result of the Gaussian beam-shape: the middle of
Fig. 3. Photographs of (a) the back side of the wafer when laser-drilled from the front, and (b) the front side when laser-drilled from the back.
678
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 4. Sketch explaining the difference between drilling from the back and the front.
the beam drills faster than the edges [26]. Convex drill profile is advantageous for efficient metallization coverage by evaporating or sputtering metal from the front. We have successfully coated vias using TiW sputtering. However, an angled scribe trench results in die with sloped edges. This presents a challenge in die picking; using vacuum to pick die will be preferred. 3.5. Graphite debris formation and removal An ideal diamond laser-machining recipe is the one that leaves no graphite debris. To accomplish this, drilling conditions have to allow sufficient oxygen presence to convert all (or nearly all) of the carbon into volatile products. The rate of oxygen supply at the drilling location is determined by oxygen partial pressure, diffusion of volatile reactants, and the size of the drilling location (determining the efficiency of oxygen diffusion to the burning location). Based on this simple hypothesis, we expect that graphite debris formation will be reduced if drilling is performed under oxygen atmosphere and using average laser powers low enough to allow all of the carbon to convert to volatile products. These conditions would ensure that chemical etching of diamond is reaction limited rather than diffusion limited. In addition, a number of previous reports of laser-assisted etching in oxygen suggest increased etch rates [25,27,33], although they do not address the issue of graphite debris. To illustrate that laser machining under oxygen atmosphere improves the cleanliness, we performed several trench drilling experiments using Nd:YAG 1064 nm laser with 5 kHz pulse repetition frequency (PRF), pulse duration of 35 ns, and average power of 7 W [34]. We laser-scribed GaN-on-diamond wafers (structure shown in Fig. 1) with rectangular
Fig. 5. Typical (a) dicing profile with a through cut (the wafer is 50 μm thick), and (b) thruvias on a 100-μm wafer (the wafer has been cleaved through the vias).
Fig. 6. GaN-on-diamond wafer surface after laser scribing and cleaning.
grid pattern with 400-μm pitch in each direction with 10 scans each line. The laser spot size had a 50-μm diameter. The scribing was performed under the presence of forced nitrogen (N2), forced oxygen (O2), and stationary air (78% N2/21% O2). A ¼″ PVC tubing nozzle was placed about 7 mm from drilling location and the gas flow was adjusted to be about 20,000 sccm from a tank; there was no enclosure. Three samples were scribed. The micrographs of the GaN surface for the three conditions are shown in the far left column of Fig. 6. It is clear that the presence of oxygen has a very beneficial effect on reducing debris. We subsequently investigated ways to clean the debris: using isopropanol or water while agitated in an ultrasonic bath or manual scrubbing did not produce good results. Better results were obtained using 300 MIF® AZ Developer (containing tetramethylammonium hydroxide) and ultrasonic agitation, shown in Fig. 6 under “solvent clean”. We also used a downstream ash tool (Gasonics Aura 1000) for 200 s at 1 kW RF power, and found that it produced the best results as shown in Fig. 6 and labeled “oxygen clean”. Evidently, the combination of oxygen during drilling and atomic oxygen during cleaning fully cleans the samples with no visible residue. 3.6. Debris-free dicing The average power of 7 W, used in the above experiment, clearly results in a diffusion limited laser-assisted chemical etching of diamond. To explore low power laser cutting we used a Q-switched Nd:YAG 1064 nm laser [35][36] with PRF 1.5–5 kHz, pulse duration from 65 ns to 90 ns and average beam powers ranging from 100 mW to 2 W. The laser spot diameter was estimated to be 18 μm. Inasmuch as the laser operates in a pulsed mode, in order to obtain smooth scribes, the
Fig. 7. Etch rate (drill rate) as a function of fluence as determined in this work (in the presence of oxygen overpressure).
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
679
Fig. 8. The edge of a through-cut of 100-μm thick GaN-on-diamond wafer (a) in air and (b) under oxygen overpressure. The conditions were PAVE = 150 mW, f = 1.5 kHz.
scribing velocity v (beam spot relative to the wafer motion) has to be selected so that two subsequent pulses overlap. The pulses overlap if v b df/2. Selecting v = 6.25 mm/s (constant in all experiments) meets this condition by a factor of two. The average beam power is given by PAVE = PRF⋅ F ⋅ A, where A [cm2] is the area of the beam spot and F the fluence. Fluence is defined as radiative flux integrated over time (i.e., electromagnetic energy) per unit area delivered by a laser beam (typical units are J/cm2). We use beam diameter as the width of the hole at halfway through a 100 μm substrate since the beam diameter is not clearly defined due to the non-uniformity of the intensity over the beam profile. Above a certain threshold, diamond removal rate monotonically increases with the fluence [23,25,26]. For any fixed beam profile, the fluence is proportional to the average power and the proportionality depends on the beam profile, and pulse duration. The measured etch rate versus the fluence is shown in Fig. 7. The etching threshold was found to be around F ≈ 30 J/cm2, which is in agreement with previously published values for diamond laser drilling [23]. The two main constraints for realizing debris-free laser drilling of diamond are (a) maintaining consistent drilling by keeping the fluence above the threshold (typically 50% above the threshold) and (b) maintaining the average beam power sufficiently low to allow carbon burning rather than ablating. We were successful in scribing a large number of samples under the above constraints, but found that it was not possible to eliminate all of the debris from the neighborhood of the scribe. We performed a number of experiments over a
Fig. 9. Side-view of a scribed and cleaved GaN-on-diamond wafer. One un-cleaved scribe trench with depth of 50 μm is shown. The marks from the multiple passes of the laser beam are visible in the top part of cross section, while the bottom part is the jagged part that was cleaved.
range of average laser powers and concluded that the best results (subjectively established acceptable amount of graphite debris) were obtained when the average laser power was below 1/3 W. Fig. 8 shows an optical micrograph of the effect of oxygen presence in a straight cut through a wafer. We consider debris shown in Fig. 8(a) unacceptable, while the debris shown in Fig. 8(b) barely acceptable. The optimal conditions for scribing were established by reducing the average power to a minimum value for which etching occurs consistently: average power equal to 200 mW, PRF=1.5 kHz, τL =35ns. Under these conditions, the processing time averaged about 2 s/die. Incidentally, the mentioned chemical-etching component makes diamond laser machining significantly more efficient than machining of silicon or silicon carbide, the other two common choices for substrates for GaN growth. If the wafer surface could be protected before scribing, the diamond wafer would have to be scribed in such a way that cleaning could be performed in wafer form rather than chip form. This means that the scribed wafer would have to remain sufficiently rigid to be handled prior to its attachment to dicing tape and cleaving. 4. Cleaving Laser-scribing involves cutting partially through a wafer to facilitate controlled cleaving. CVD diamond used for making the GaN-ondiamond wafers is polycrystalline and it does not have clearly defined cleaving planes as do single crystal silicon or III–V semiconductors: Cleaving CVD diamond wafers does not result in atomically flat surfaces, but in jagged edges and, most importantly, diamond dust. Die separation is typically done by attaching a wafer to a dicing tape, sawing or scribing, and cleaving by applying pressure from below. For this to work well, the adhesion of the wafer (and the chip) to the dicing tape has to be greater than the force necessary to break the wafer. This means that the diamond growth surface (bottom of structure shown in Fig. 1) must adhere well to the dicing tape. We find that, due to diamond back-surface roughness, this is not the case: The adhesion to standard medium-tack dicing tape (“blue tape”) [37] is insufficient and the wafer detaches from the dicing tape during cleaving even when the trench depth is ~90%. Ultra-violet dicing tape has better adhesion, but is still insufficient to hold the wafers in place. To cleave wafers reliably, we resorted to a two-dicing-tape approach: a first blue tape layer is stretched on a dicing hoop with the adhesive up (normal orientation) and the GaN-on-diamond wafers is attached to it with the device side up (back side to adhesive). A second dicing tape piece is stretched and
680
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 10. View of a 380 μm × 400 μm die (a) before scribing, and (b) after scribing, cleaving, and die separation.
attached from the top to cover the device wafer. The side of the second dicing tape that had no adhesive is adjacent to the device wafer. Pressure onto the die to be separated is applied using a cleaving wedge from below, while additional pressure is applied on adjacent dies from the top. The top tape generally touches only edge of the chip and does not damage the circuitry on the chip. Using this method, we were able to reliably cleave die as small as 400 × 400 μm2 that has been scribed at least 40% (die width to remaining thickness ratio ~7×). The ability to cleave a wafer using the cleaving wedge depends on the difference in torque that can be exerted on two adjacent dies on the wafer. Our experience is that the ratio between the die size and the remaining uncut thickness of the diamond should be at least 15:1 for an efficient cleave. Higher ratios are preferred, but the bottom of the cut should still not approach the rough back side of the diamond. If the cut enters the rough side of the substrate, the wafer may break apart (cleave) while it is being laser-scribed and the alignment may be lost. Cleaving scribed diamond wafers produces dust. Unless precautions are taken to minimize it or protect the chip surface, the diamond micro-crystals, which form an excellent abrasive, will scratch the chip surface during cleaving and handling. One way to avoid this is to cut through the entire wafer and avoid cleaving altogether. Although this is possible by dicing the wafer while it is mounted on a dicing tape, it requires precise cutting depth to avoid damaging the dicing tape. We prefer to maintain the ratio between the die size and the remaining uncut thickness of the diamond at least 15:1, which minimizes the dust generation and makes the cleaving easy. Fig. 10 shows optical micrographs of a completed high-electron mobility transistor (a) prior to scribing and (b) after cleaving and die separation. The scribing was performed at average laser power equal to 200 mW, f = 1.5 kHz, τL = 65 ns, and the cleaving using two-dicing tape approach described above. The scribing trench depth was ~40% of the wafer thickness (die size to remaining thickness ratio ~7). The first noteworthy feature of this example is that the short pulse duration and low power resulted in minimal heat damage. The transistors were distanced by 50 μm and 150 μm in the two directions, and the dicing trench was almost 50-μm wide, while there is no evidence of heat damage (gold metallization remained unchanged) and there was no graphite debris correlated to the die edge. The second noteworthy feature is that diamond dust is present at random places (the dust does not track the edge of the die, but is distributed unevenly) and that it has produced some surface damage during cleaving.
5. Discussion Via drilling and dicing steps generally do not occur at the same stage in the device manufacturing: via drilling is followed by via coating (and associated lithography) and backside processing, while dicing occurs at the end of the wafer processing after which the die is separated. As we showed in Section 2, cleaning of the graphite debris is possible and straightforward on the wafer level. Dicing, on the other hand, requires a high degree of cleanliness to facilitate efficient die attach and visually clean shippable devices. The approaches for accomplishing this are either (a) cleaning individual die or (b) scribing/dicing under conditions that minimize debris. We have successfully employed (b) in this work and the previous work [12]. The difficulty, however, with this latter approach is the long processing time resulting from using low average laser power. In laser machining, cost reduction is accomplished primarily by increasing the throughput. Operating at reduced laser powers makes the scribing take longer time and increases the die cost. One alternative to meeting both requirements (faster processing and clean final die) is to protect the devices on the wafer, drill the vias and scribe at the same stage in the process, and clean the debris by dissolving the protect layer while the wafers are not yet cleaved. For this to be successful, the scribe depth has to be carefully managed to maintain the wafers sufficiently rigid to allow cleaning and further limited processing (e.g., lithography), while still allowing it to be efficiently cleaved. For example, a 100-um-thick diamond wafer could be scribed to around 50% still enabling an efficient final cleave step. We find that patterning of the protective coating (on the devices) and leaving scribe alleys and via locations unprotected, is necessary to prevent laser-charred protect-layer residue which can be very difficult to remove (typically, we would use thick photoresist). Using laser-assisted etching in oxygen as described in Section 2 and subsequent cleaning in atomic oxygen produced best results. Finally, we explored laser dicing using 355 nm tripled Nd:YAG laser [38]. We found that the etch rate is not very wavelength dependent, and confirmed that shorter pulses and reduced average laser powers lead to cleaner scribing. This can be seen in the example shown in Fig. 11 where a 600 μm × 600 μm die was laser-scribed at PAVE = 330 mW, τL = 35 ns, and PRF = 5 kHz. 6. Conclusion Dicing and via drilling of GaN-on-diamond wafers to obtain clean shippable die present a number of opposing constraints owing to
D.I. Babić et al. / Diamond & Related Materials 20 (2011) 675–681
Fig. 11. Optical micrograph of an array of 600 × 600 μm2 dies after laser scribing using 355 nm Q-switched Nd:YAG laser with pulse duration 35 ns, f = 5 kHz, and average power equal to 330 mW [38].
phenomena specific to diamond processing: graphite debris deposition around the cut, cleaning of the debris, appearance of diamond dust during cleaving, and finally the speed of wafer processing. The conditions and approaches for optimal laser micromachining will eventually be defined when these processes are integrated into a standard GaN HEMT fabrication sequence. The fact that the amount of graphite debris can be controlled with the average laser power and oxygen concentration, and that it can be efficiently cleaned, clearly favors performing wafer scribing (and subsequent processing) on the wafer level. Protecting the devices during via and scribing steps is essential. Acknowledgment This work was supported in part by generous SBIR grants from MDA and AFRL (John Blevins). Other parts were also funded by SBIR grants from NASA (Lihua Li and Arnold Silva). The authors gratefully acknowledge James Gillespie for providing some of the processed wafers that were diced during this work (Fig. 10), Brian Price, Terry Pothoven, Grey Brooks for assistance with laser micromachining, Lumera Laser for performing test via-drilling, and Alex Schreiber for the design of the mechanical tools used in this work. References [1] J.G. Felbinger, et al., IEEE Electron Device Lett. 28 (2007) 948. [2] P.R. Hageman, J.J. Schermer, P.K. Larsen, Thin Solid Films 443 (2003) 9–13. [3] P.W. Maya, H.Y. Tsai, W.N. Wang, J.A. Smith, Diam. Rel. Mat. 15 (2006) 526–530.
681
[4] M. Oba, T. Sugino, Diam. Rel. Mat. 10 (2001) 1343. [5] G.W.G. van Dreumel, J.G. Buijnsters, T. Bohnen, J.J. ter Meulen, P.R. Hageman, W.J.P. van Enckevort, E. Vlieg, Diam. Rel. Mat. 18 (2009) 1043. [6] J.W. Zimmer, Mater. Res. Soc. Symp. Proc. 956 (2007). [7] E. Piner, J.W. Zimmer, J.C. Roberts, G. Chandler, R.A. Sadler, Epi-inverted N-face GaN/Diamond for AlGaN/GaN/AlGaN FETs, 33rd WOCSDICE, Málaga, Spain (May 17–20, 2009), 2009. [8] A. Dussaigne, M. Maliverni, D. Martin, A. Castiglia, N. Grandjean, (0001) GaN grown on (111) single crystal diamond substrate for high power electronics applications, Proc. 18th European Workshop on Heterostructure Technology (HETECH 2009), 2–4 November 2009, Ulm, Germany, 2009, p. 33. [9] D. Francis, F. Faili, D. Babić, F. Ejeckam, A. Nurmikko, H. Maris, Diam. Rel. Mat. 19 (2010) 229–233. [10] D. Francis, J. Wasserbauer, F. Faili, D. Babic, F. Ejeckam, W. Hong, P. Specht, E.R. Weber, GaN HEMT epilayers on diamond substrates: recent progress, CS MANTECH, Austin, TX (May 14–17, 2007), 2007. [11] Q. Diduck, J. Felbinger, L.F. Eastman, D. Francis, J. Wasserbauer, F. Faili, D.I. Babić, F. Ejeckam, Electron. Lett 45 (2009) 758–759. [12] D.I. Babić, Q. Diduck, P. Yenigalla, A. Schreiber, D. Francis, F. Faili, F. Ejeckam, J.G. Felbinger, L.F. Eastman, GaN-on-diamond field-effect transistors: from wafers to amplifier modules, Proc. of the Symposium on Microelectronics, Electronics, and Electronic Technologies (MEET), MIPRO, Opatija, Croatia, May 20–24, 2010. [13] R.J. Trew, J.-B. Yan, P.M. Mock, IEEE Proc. 79 (1991) 598. [14] A. Aleksov, M. Kubovic, M. Kasu, P. Schmid, D. Grobe, S. Erttl, M. Schreck, B. Strizker, E. Kohn, Diam. Rel. Mat. 13 (2004) 233–240. [15] O. Dorsch, M. Werner, E. Obermeier, R.E. Harper, C. Johnton, I.M. Buckle-Golder, Diam. Rel. Mat. 1 (1992) 277–280. [16] M.P. Hiscocks, C.J. Kaalund, F. Ladouceur, S.T. Huntington, B.C. Gibson, S. Trpowski, D. Simpson, E. Ampen-Lassen, S. Prawer, J.E. Butler, Diam. Rel. Mat. 17 (2008) 1831–1834. [17] R. Otterbach, U. Hilleringmann, Diam. Rel. Mat. 11 (2002) 841–844. [18] Y. Ando, Y. Nishibayashi, K. Kobashi, T. Hirao, K. Oura, Diam. Rel. Mat. 11 (2002) 824–827. [19] H. Verhoeven, A. Flőter, H. Reiβ, R. Zachai, Appl. Phys. Lett. 71 (1997) 1329–1331. [20] J.E. Graebner, S. Jin, G.W. Kammlott, J.A. Herb, C.F. Cardinier, Appl. Phys. Lett. 60 (1992) 1576–1578. [21] J.E. Graebner, Diam. Rel. Mat. 4 (1995) 1196–1199. [22] N.B. Dahotre, S.P. Harimkar, Laser Fabrication and Machining of Materials, Springer Science + Business Media, New York, USA, 2008. [23] V.V. Kononenko, T.V. Kononenko, S.M. Pimenov, M.N. Sinyavkii, V.I. Konov, F. Dausinger, Quantum Electron. 35 (2005) 252–256. [24] T.V. Kononenko, V.G. Ralchenko, I.I. Vlasov, S.V. Garnov, V.I. Konov, Diam. Rel. Mat. 7 (1998) 1623–1627. [25] V.G. Ralchenko, K.G. Korotushenko, A.A. Smolin, E.N. Loubnin, Diam. Rel. Mat. 4 (1995) 893–896. [26] T.V. Kononenko, V.G. Ralchenko, I.I. Vlasov, S.V. Garnov, V.I. Konov, Diam. Rel. Mat. 7 (1998) 1623–1627. [27] S. Gloor, S.M. Pimenov, E.D. Obratsova, W. Lűthy, H.P. Weber, Diam. Rel. Mat. 7 (1998) 607–611. [28] Lumera laser GmbHKaiserslautern, Germany, www.lumeralaser.com. [29] S.S. Kutateladze, Fundamentals of Heat Transfer, Academic Press, New York, 1963. [30] S. Albin, W.P. Winfree, B.S. Crews, J. Electrochem. Soc. 137 (1990) 1973–1976. [31] M.D. Shirk, P.A. Molian, A.P. Malshe, J. Laser Appl. 10 (1998) 64–708 still need to get it. [32] V.V. Kononenko, M.S. Komlenok, S.M. Pimenov, V.I. Konov,“Photoinduced laser etching of a diamond surface”, Quantum Electron. 37 (2007) 1043–1046. [33] V.V. Migulin, V.G. Ralchenko, Y.-J. Baik, Proc. SPIE 3483 (1998) 175. [34] Laser Marks, Inc., Santa Clara, CA; www.lasermarks.com. [35] Laserod, Inc., Torrance, CA; www.laserod.com. [36] Lee Laser, Inc., Orlando, FL; model 712THP. [37] Medium-tack dicing tape, www.semicorp.com8 p/n 18551–7.50. [38] Coherent, Inc, Santa Clara, CA; Q-switched tripled Nd:YAG model AVIA 355.