BCB WAFER BONDING COMPATIBLE WITH BULK MICRO MACHINING TAE-JOO HWANG

Size: px

Start display at page:

Download "BCB WAFER BONDING COMPATIBLE WITH BULK MICRO MACHINING TAE-JOO HWANG"

Roderick Gordon
5 years ago
Views:

1 Proceedings of IPACK03 International Electronic Packaging Technical Conference and Exhibition July 6 11, 2003, Maui, Hawaii, USA InterPack WAFER BONDING COMPATIBLE WITH BULK MICRO MACHINING TAE-JOO HWANG Dan O. Popa Jian-Qiang Lu Byoung-Hun Kang Harry E. Stephanou Rensselaer Polytechnic Institute Center for Automation Technologies CII 8015, 110 8th Street, Troy, New York , U.S.A Phone: , FAX: , hwang@cat.rpi.edu ABSTRACT Adhesive wafer bonding is a good substitute in wafer-towafer bonding applications requiring low processing temperatures and electrical potentials, though at the expense of difficulty with chemical, mechanical, and thermal stability over time. In the case of wafer bonding for Micro-Electro-Mechanical System (MEMS) applications, the problem is compounded not just by consideration of bond strength, but also by limitations in the way adhesives are delivered to the interface, since traditional spin-coating methods cannot be directly employed. A typical approach is the formation of microfluidic channels via wafer bonding, where the adhesive layer should only be present on the mesa structures. The introduction of (benzocyclobutene), dry-etchable polymers, makes it possible to pattern the adhesive layer in a similar fashion with the rest of the bulk material. In this paper we present a -based wafer bonding process, which is compatible with bulk micro machining. Depending on applications, can replace the silicon oxide or silicon nitride as a hard mask in bulk micro machining. A process using as both bonding adhesive and bulk-etch mask is a good option for stacking microstructures such as building micro-fluidic circuitry. Keywords: wafer bonding, MEMS packaging, Polymer bonding, Microfluidic channels. INTRODUCTION Wafer-to-wafer bonding as a mature technology in microelectronics is increasingly relevant for high volume packaging of Micro-Electro-Mechanical System (MEMS) devices. Anodic and fusion bonding are known to provide very strong, hermetic wafer bonds, but with very strict requirements for surface preparation and process conditions. In addition, the range of processing temperatures and voltages limits the use of these two bonding techniques in some applications. Wafer bonding using dielectric polymers, such as PI 2610, S1818, and, more, recently, (benzocyclobutene) has been proposed for stacking IC wafers in three-dimensional (3D) electronics [1-3]. The properties of - excellent mechanical strength, very low outgassing, less sensitivity to surface preparation, low cost, make it a very attractive polymer for wafer bonding. Analysis of bonding quality is an important area in wafer bonding research and many tests have been proposed, such as peel testing, crack opening test, double cantilever beam (DCB) test and 4-point bending test [4-8]. In this paper, we refer previous work for determining the adhesion strength of / and /SiO 2 in terms of measured interface fracture energy [9-11]. is dry-etchable by plasma and laser, thus enabling the transfer of micron scale patterns via photolithography [12]. Taking advantage of this property, we propose to use not just as the bonding interface between wafers, but also as masks for bulk micromachining, allowing for fabrication of 3D structures such as microfluidic channels. In the case of MEMS wafers, there are inherent limitations in the way the polymer is delivered to the interface, since traditional spin-coating methods cannot be directly employed [13]. This problem can be addressed by micromachining the layer together with the bulk material. Three-dimensional microstructures fabricated using and DRIE (deep reactive ion etching) have 1 Copyright 2003 by ASME

2 been previously presented in [14], and hermeticity of cavities for housing RF MEMS were presented in [18]. And in [4] microchannels are formed via wafer bonding using different polymers through the use of the stamping method. This method allows for the transfer of adhesive from a flat surface to the mesa structures of a patterned wafer, but, in this case, the adhesion between polymer and the cannot be precisely controlled. is highly resistant to etchants such as KOH and TMAH (tetra methyl ammonium hydroxide). In this paper we combine the processing of with wet etching of bulk s. A number of inorganic etch-stop layers and etchants are investigated and we demonstrated that a hardcured layer could be used both as a hard mask and a bonding inter-layer. is introduced in the process before photolithography, and it is fully cured after the patterned wafers are aligned and bonded. RIE processing conditions for soft cured film were studied with a range of CF 4 :O 2 ratios. For the use of the layer as a wet etchmask, we present results rating compatibility with several bulk-etching processes, including KOH, HF-KOH, TMAH and HF-TMAH etching. Although is stable in most organic solvents and aqueous acids, soft-cured showed layer delamination during Si wet-etch in KOH and TMAH solutions with very shallow sidewall definition. A much better sidewall definition is obtained with inorganic layers such as silicon dioxide, or silicon nitride. We concluded that the success of bulk micro machining with depends on adhesion at the interface between substrate and layer. In addition, the cured state of, and adhesion promoters are important factors influencing the resistance to the wet etchants. FABRICATION The fabrication process for bulk micro machined structures with a layer is depicted in Fig A layer of oxide or nitride (if needed) is deposited on the wafer prior to the layer. is then spin-coated and cured (partially or completely). Patterning of is done using conventional photolithography and RIE plasma etching. The oxide and nitride layers are then opened for wet etching of the bulk wafer. A list of process steps using silicon nitride is listed below: 1. Deposition of low stress silicon nitride using PECVD; 2. Spin-coat and bake adhesion promoter; 3. Spin-coat with dynamic dispensing & spreading; 4. Pre-bake ; 5. Soft-curing or hard-curing of on a hot plate with nitrogen shower along with the curing schedule [15]; 6. Spin-coat and bake adhesion promoter; 7. Spin-coat and pre-bake photoresist (PR); 8. Photolithography process to develop the patterns; 9. Post-bake the photoresist; 10. RIE etch the patterns; 11. Etch silicon substrate using KOH or TMAH; 12. Align and bond wafers. In the case of silicon dioxide used as an intermediate layer below, an extra buffer oxide-etching step should be added to the steps prior to Si etching. During the process development a number of critical factors were identified and addressed via targeted experiments: Whether there is adequate adhesion of to bare silicon, or to nitride and oxide layers. We found that satisfactory adhesion is obtained by pre and post baking of adhesion promoters. In our experiments we used adhesion promoters at each interface. It has been previously reported that after wafer bonding, the weakest interface is not the to cross-link, but the adhesion between and hard mask (SiO 2 or ) [9,10]. Whether PR can be removed from the layer. We found that PR on the layer can be patterned and developed using conventional photolithography. Whether the layer can be used as a hard mask during wet etching of the bulk material. According to published data we expected that the selectivity of, SiO 2 and Si to KOH etching to be roughly (4000:400:1), which seems to suggest that the layer can be used as a hard mask. During our experiments, the hard-cured layer on bare Si remained in good shape. We also found that the delamination under TMAH etching is dependent on cured state. Whether partially cured can be plasma-etched, and what etch-rates can be obtained. To answer this question we performed RIE experiments using different gas concentrations, and level of curing in order to determine the etch selectivity of to photoresist. Experiments show that selectivity depends on the gas composition, and an optimum composition can be selected to maximize the overall etch rate while the etch selectivity to PR is as high as necessary. What layer thickness is necessary to ensure enough is left at the end of the process for an adequate bond. Considering that the layer will be thinned by both the dry and wet etching processes, we found that the layer thickness shown in Fig. 1 is adequate for bonding wafers with channels at least 100 µm deep Unit: µm PR 1813 "Si" substrate Adhesion promoter, ~ µm 100µ m "Si" substrate Figure 1. Layer thickness prior and after processing. 2 Copyright 2003 by ASME

3 Wafer cleaning Spin-coat AP3000 & Curing UV Developing patterns PECVD Spin-coat PR O CF 2 4 RIE etching at the same time for 2 minutes and the film thickness was measured using an Alpha step surface profiler. We varied the relative ratio of gas components between oxygen and CF 4 because the selectivity and etch rate are highly dependent on this ratio [16]. The rest of the processing parameters were the same as recommended by the manufacturer. Figures 3, 4, and 5 show the experiment diagram and the etch rate and selectivity results. The results indicate that a higher selectivity can be achieved for higher fluorine gas composition, but a higher etch rate is achieved at higher oxygen concentrations. In our experiments the initial layer thickness was 2 µm for PR and 2.4 µm for, therefore we need higher selectivity than that the one given by the high etch rate with 9:1 (O 2 :CF 4 ) composition. A ratio of 5:5 gas composition was chosen to minimize the loss of layer, and also provide a reasonably fast etch rate. a ( h ) PR init hpr = a, = RPR (1) t where t is the process time, R PR is the photoresist etch rate. SiO 2 HF 10:1 Buffered Oxide etch (only in case of SiO2) KOH or TMAH Si Nitride silicon etching htotal {( htotal) init a} = R (2) t where R is the etch rate, and the etch selectivity is given by: R = S (3) PR R PR Figure 2 Process flow for building a microstructure with a layer PR S1813 (h total ) init PR S1813 (h PR ) init EXPERIMENTAL INVESTIGATION FOR SELECTING DRY ETCHING PARAMETERS RIE etch for 2 min. RF power: 300W Pressure: 200mTorr After the photoresist pattern is developed, we can transfer the pattern to the layer by RIE etching. Because both and PR will be etched together during this step, the process parameters such as power, fluorine gas component ratio and pressure need to be selected in order to match the etch rates. Inappropriate settings for these parameters can lead to severe layer loss and decrease in bond strength. To determine appropriate parameters for etching, :PR etch selectivity experiments were performed. Soft masking using AZP 4620 photoresist was previously investigated by Berry et. al. [12]. In our case, we performed selectivity experiments on Shipley 1813 photoresist and cyclotene We prepared two sets of samples for measuring the relative step size between the photoresist and layers. One sample had only PR on the substrate and the other had both PR and partially or fully cured processed according to our recipe. The two samples were RIE-etched h total h PR Figure 3 Selectivity and etch rate measurement scheme Etch rate (micron/min) Etch rate for and 10:90 20:80 30:70 40:60 50:50 90:10 (O2/CF4) gas composition Figure 4. Etch rates of and photoresist. R(PR) R() 3 Copyright 2003 by ASME

Ratio 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Selectivity(:PR) 1 2 3 4 5 6 (O2/CF4) gas composition Figure 5. Etch selectivity of to PR.

Yes Hard +Si TMAH H Yes Hard Si TMAH Table 1 Sample preparation for etching MASKING FOR BULK SILICON ETCHING Figure 6. Sample A prior to and after KOH etching.

After the layer is patterned, the nitride/oxide layers are opened using dry-etch/boe, and the bulk can be etched using KOH or TMAH.

4 Ratio Selectivity(:PR) (O2/CF4) gas composition Figure 5. Etch selectivity of to PR. Selectivity Sample Adhesion Promoter % curing Substrate Etchant A No Soft Si KOH B Yes Soft Si KOH C Yes Soft SiO 2 +Si KOH D Yes Soft SiO 2 +Si TMAH E Yes Pre-baked +Si TMAH F Yes Soft +Si TMAH G Yes Hard +Si TMAH H Yes Hard Si TMAH Table 1 Sample preparation for etching MASKING FOR BULK SILICON ETCHING Figure 6. Sample A prior to and after KOH etching. The layer is completely gone in 40 minutes. Figure 7. KOH etched sample B. Some is still present on the wafer surface. Silicon sidewall Figure 8. Microscopic view of sample B. After the layer is patterned, the nitride/oxide layers are opened using dry-etch/boe, and the bulk can be etched using KOH or TMAH. In this section we describe the experiments undertaken to determine whether the layer can survive this process step. While is highly resistant to chemical attack by most organic solvents, bases, and aqueous acids, the adhesion layer between and substrate is particularly vulnerable. These adhesion problems were studied by comparing adhesion between and Si with that between and nitride or oxide. We also studied loss (or delamination) in KOH and TMAH as a function of the level of curing prior to the wetetch step. The adhesion strength might also be dependent on the type of adhesion promoter used, however, only one type promoter (AP 3000) is currently available. In order to compare the etch resistance of the film, eight different types of samples (labeled A-H) were fabricated and tested, as shown in Table 1. Sample A, B, and C were dipped into 45% KOH solution for 40 minutes and then inspected, while samples D, E, F, G, and H were dipped into (25% TMAH solution) for up to 100 minutes. Figures 6,7, and 8 show that inadequate adhesion between and the substrate causes delamination of film in the KOH solution. While the film etches much slower in KOH than the Si substrate, the failure of film occurred at the interface, and a thin film residue is still present in solution after 40 minutes. We conclude that the higher etch rate for the substrate material, and the weaker the adhesion of to the substrate, the shallower the sidewall definition will be. The observed delamination mechanism is shown in Fig. 9. We expected that the penetration of KOH under the film would be retarded if we used a substrate having a slower etch rate. The experiments were thus repeated with silicon dioxide (Fig. 10). The experimental results show that the sidewall definition improved even further if TMAH is used for etching (sample D), and if a silicon nitride hard mask is used (sample F). 4 Copyright 2003 by ASME

KOH attacks interface btw. and Si wafer surface in Table 2, and Fig. 11, 12, and 13. In Fig.

We conclude that hard-cured layers combined with bulkmicromachining in TMAH solution, are capable of transferring of a adhesive pattern on mesa structures at least 100 µm high, and with a feature

3 mm 20 min Soft-cured ~ 400 micron 20 min Hard-cured Not visually measurable with 5 x magnification microscope 100 min KOH starts to etch surface and layer starts to delaminate Table 2.

5 KOH attacks interface btw. and Si wafer surface in Table 2, and Fig. 11, 12, and 13. In Fig. 14, a 15 µm strip of was removed by mechanical means, but a 5x magnification image does not allow the measurement of any etch region after 100 minutes of etching in TMAH. We conclude that hard-cured layers combined with bulkmicromachining in TMAH solution, are capable of transferring of a adhesive pattern on mesa structures at least 100 µm high, and with a feature resolution better than 1 µm. OH- surface is exposed to KOH % of curing Distance from to the TMAH etch wafer edge time Pre-baking ~1.3 mm 20 min Soft-cured ~ 400 micron 20 min Hard-cured Not visually measurable with 5 x magnification microscope 100 min KOH starts to etch surface and layer starts to delaminate Table 2. Etch conditions and the results for studying the effect of curing on the etch rate. residue Si surface is etched along with a gentle angle. ~1.3mm Figure 9. Delamination mechanism in KOH solution. Figure 11. Pre-baked sample after TMAH etching ~0.4mm Figure 10. KOH etched sample C. Most of the film remains on the mesa structures after 40 minutes. The etch selectivity SiO 2 :Si is in the range :1 in TMAH and :1 in KOH, depending on concentration. Both fully cured and etch rates in KOH and TMAH are too small to be determined. After performing the previous experiments, we concluded that TMAH/KOH etching with a silicon nitride bottom layer for provides the best result. For samples E, F, and G, results show that the partially cured film didn t survive the wet-etch while a fully cured layer did. Samples E and F were etched for 20 minutes in TMAH, while sample G was etched for 100 minutes. We measured the distance between the film and the edge of silicon nitride to show the film loss. The results are shown Figure 12. Soft-cured sample after TMAH etching 5 Copyright 2003 by ASME

$A special instrument, such a XPS (X-ray photoelectron spectroscopy), is necessary in order to identify the fractured layer. However, this is consistent with a result provided by R. J.$ $Maidenberg in [9], where the interface fracture energy, "Gc", was measured by double cantilever beam tests and ranged from 10 to 60 J/m² for the interface of and with epoxy underfill. In addition, J.$ $M. Snodgrass and D. Pantelidisthe measured the interface fracture energy for /SiO 2 with AP3000 to be below 21 J/m², by double cantilever beam tests [10].$

6 15 µm Figure 13. Hard-cured sample after TMAH etching Si substrate suggest that the weakest interface is the adhesion promoter layer (Fig. 16). A special instrument, such a XPS (X-ray photoelectron spectroscopy), is necessary in order to identify the fractured layer. However, this is consistent with a result provided by R. J. Hohlfelder and D. A. Maidenberg in [9], where the interface fracture energy, "Gc", was measured by double cantilever beam tests and ranged from 10 to 60 J/m² for the interface of and with epoxy underfill. In addition, J.M. Snodgrass and D. Pantelidisthe measured the interface fracture energy for /SiO 2 with AP3000 to be below 21 J/m², by double cantilever beam tests [10]. Our experiments shown in Table 3 using a four point bending test, indicate that the bond strength after the proposed recipe will be almost as good as the bond strength between flat wafers processed using the standard recipe. 1.18mm Figure 14. Fluidic circuitry fabricated by TMAH etching using SiNx and as a wet-etch mask. BONDING AFTER BULK MICROMACHINING Using pre-baked, flat wafers can be aligned and bonded at temperatures as low as 180 C, and pressures greater than 1.7 bars [1,5]. In our case, we must bond wafers with a fully cured and patterned layer because uncured film is not able to survive during bulk micromachining. A preliminary experiment failed to bond two wafers with fully cured patterned at such a low temperature. This can be explained by the fact that uncured provides more conformity and activation to the mated surface due to a lower glass transition temperature than that of fully cured. The glass transition temperature for uncured is very low and is strongly dependent on curing percentage [17]. Therefore, it was expected that relatively high temperature (but not higher than 350 C) and relatively high pressure was necessary to successful bonding of wafers with fully cured mesa structures. Preliminary bonding experiments were performed with "/AP3000/Si" and "/AP3000/SiNx/Si" samples processed in similar conditions to the G, and H samples. In addition, we bonded another pair of wafers with pre-baked on SiNx, which is not exposed to TMAH, for the purpose of comparison. The bonding was performed using the standard recipe for bonding. In all cases, a simple razor blade debonding test showed one of the mated surfaces appeared to have no, apparently as shown in Fig. 15. This seems to Figure 15. Mixed-mode delaminating beam sample C Fully cured Fully cured AP3000 D A B Possible fracture path & the weakest link in the system Figure 16. Schematic diagram of bonded layers 3 Samples Critical adhesion energy, G c (J/m 2 ) Standard deviation Hard-cured bonding Normal bonding Table 3 Comparison of critical adhesion energy 6 Copyright 2003 by ASME

7 CONCLUSION In this paper we presented a wafer bonding process for patterned wafers. The wafer bonding using a layer is taken through a normal wet etching of the bulk wafer leading to the formation of deep lithographically defined structures. We showed that the hard-cured layer survives bulk micromachining with TMAH. After TMAH etching, we found that the bonding strength is similar to the normal strength of flat wafers bonded with. Moreover, it was observed that the bonding strength between hard cured layers was higher than that of the and silicon nitride. Due to the very good chemical stability of, our bonding method provides a low cost alternative to fusion bonding for MEMS wafers, and to the traditional stamping and screen deposition of adhesives on mesa structures. This method is simple, yet very useful for the wafer-level encapsulation of micro-fluidics, micro-photonics, and MEMS. Further on-going experiments are aimed at a quantitative characterization of the -etched cavities, and the bonding energy for patterned MEMS wafers. REFERENCES [1] Niklaus, F., Enoksson, P., Kalvesten, E., and Stemme, G., 2001, Low-temperature full wafer adhesive bonding, Journal of Micromechanics and Microengineering 11, no. 2 [2] Niklaus, F., Enoksson, P., Kalvesten, E., and Stemme, G., 2000, Void-Free Full Wafer Adhesive Bonding, 13th IEEE Int. Conference on MicroElectroMechanical Sytems (MEMS'00) Miyazahci, Japan, January 23-27, 2000, pp [3] Niklaus, F., Enoksson, P., Griss, P., Kalvesten, E., and Stemme, G., 2001, Low temperature Wafer-Level Transfer Bonding, J. of Microelectromechanical systems, Vol. 10, NO. 4, pp [4] Den Besten, C., van Hal, R.E.G., Munoz, J., and Bergveld, P., 1992, Polymer bonding of micro-machined silicon structures, Proceedings. IEEE Micro Electro Mechanical Systems. An Investigation of Micro Structures, Sensors, Actuators, Machines and Robots, New York, NY, USA : IEEE, xv+237, pp [5] Lu, J.-Q., Kwon, Y., Rajagopalan, G., Gupta, M., McMahon, J., Lee, K-W., Kraft, P.R., McDonald, J. F., Cale, T.S., Gutmann, R.J., Xu, B., Eisenbraun, E., Castracane, J., and Kaloyeros, A., 2002,"A Wafer-Scale 3D IC Technology Platform using Dielectric Bonding Glues and Copper Damascene Patterned Inter-Wafer Interconnects", Proceedings of 2002 IEEE International Interconnect Technology Conference (IITC), San Francisco, CA, June 3-5, 2002, pp [6] Blom, M.T., Tas, N.R., Pandraud, G., Chmela, E., Gardeniers, J.G.E., Tijssen, R., Elwenspoek, M., and van den Berg, A., 2001, Failure mechanisms of pressurized microchannels: model and experiments, Journal of Microelectromechanical Systems 10, no. 1, pp [7] Tong, Q., Y., and Gosele, U., 1999, "Semiconductor wafer bonding: science and technology", John Wiley & sons, New York. [8] Satoh, A., 1999, Water-glass Bonding, Sensors and Actuators, A72, pp [9] Hohlfelder, R. J., Maidenberg, D. A., and Dauskardt, R. H., 2001, Adhesion of Benzocyclobutene-passivated silicon in epoxy layered structures, J. Master. Res., Vol. 16, No. 1, pp. 243~255. [10] Snodgrass, J.M., Pantelidis, D., Jenkins, M.L., Bravman, J.C., and Dauskardt, R.H., 2002, "Subcritical debonding of polymer/silica interfaces under monotonic and cyclic loading", Acta Materialia 50, no. 9, (24 May 2002), pp [11] Dauskardt, R.H., Lane, M., Ma, Q., and Krishna N., 1998, "Adhesion and debonding of multi-layer thin film structures", Engineering Fracture Mechanics 61, 1998, pp [12] Berry, M. J., Garrou, P., Rogers, B., and Turlik, I., 1994, "Soft Mask for Via Patterning in Dielectric", Int. J. Microcircuits & Electronic Packaging, Vol. 17, 1994, pp [13] Matsumoto, T., Satoh, M., et., al., 1998, New Three- Dimensional Wafer Bonding Technology Using the Adhesive Injection Method, Jpn. Journal of Applied Physics, Vol. 37, pp [14] Chou, T. K., and Najafi, K., 2001, 3D MEMS Fabrication Using Low-Temperature Wafer Bonding With Benzocyclobutene (), Transducers 2001, Eurosensors XV, pp [15] The Dow chemical website, [16] The Dow chemical website, [17] Bair, H. E., and Pryde, C. A., 1991, "Curing & Glass Transition Behavior of a Benzocyclobutene Polymer", Proceedings Society of Plastics Engineers, ANTEC, Montreal, 1991, pp [18] Joudain, A., et., al., investigations of the hermeticity of -sealed cavities for housing RF MEMS devices, Proc. MEMS 2002, Las Vegas, NV, pp Copyright 2003 by ASME

Thin Wafer Handling Challenges and Emerging Solutions

1 Thin Wafer Handling Challenges and Emerging Solutions Dr. Shari Farrens, Mr. Pete Bisson, Mr. Sumant Sood and Mr. James Hermanowski SUSS MicroTec, 228 Suss Drive, Waterbury Center, VT 05655, USA 2 Thin