Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015 [Technical Paper] Development of Lift-off Photoresists with Unique Bottom Profile Hirokazu Ito, Kouichi Hasegawa, Tomohiro Matsuki, and Shiro Kusumoto Device Integration Materials Laboratory, Fine Electronic Materials Research Laboratories, Yokkaichi Research Center, JSR Corporation, 100, Kawajiri-cho, Yokkaichi, Mie 510-8552, Japan (Received August 3, 2015; accepted October 29, 2015) Abstract Lift-off method for metal patterning has been widely used in the variety of electronic device fabrication processes such as semiconductor packaging, MEMS, and LED manufacturing. The big advantages of using lift-off method are the cost saving and the process simplification. However there is a serious issue that the deposited metal pattern has unexpected edge crown after photoresist stripping. In order to achieve desired metal patterns, two types of novel lift-off photoresist were developed, one is a single-layer negative tone photoresist and the other is a double-layer positive tone photoresist. After exposure and development processes, both the photoresists show unique and well-controlled undercut profile, which enables to form a targeted metal configuration after stripping. This paper reports the key parameter of photoresist and how to control the undercut profile. Keywords: Lift-off, Photoresist, Undercut, Metal Wire, Sputter 1. Introduction Lift-off method for metal patterning has been widely used in the variety of electronic device fabrication processes such as semiconductor packaging,[1] MEMS,[2] and LED[3] manufacturing. The big advantages of using lift-off method are the cost saving and the process simplification, compared to conventional metal etching method which requires different approach depending on metal species.[4] In the lift-off processes, photoresist with reverse taper profile is patterned on a wafer after coating, exposure, and development processes. And then the designated metal is deposited. In the last step, the photoresist is stripped by a chemical stripper. During the stripping process, and the metal sitting on the top of the photoresist is removed while directly sitting on the wafer remains and becomes a metal pattern, thus inversed metal pattern toward photoresist pattern is fabricated.[5] Generally, e-beam deposition or sputtering is used to form thin metal film, and the sputtering process is often adopted as a deposition method due to several advantages such as good adhesion between metal and substrate and good metal thickness uniformity. After metal sputtered, the metal is deposited not only on the top surface of the photoresist but also directly on the wafer surface without the photoresist. The metal is also deposited on the sidewall of the photoresist because of the isotropic property and it may bridge with the metal sitting on the wafer surface. That makes the photoresist stripping more difficult. Even though the photoresist is successfully able to be stripped, the metal deposited on the substrate sometimes has unexpected edge crown as shown in Fig. 1. To avoid the metal bridge and the edge crown, the photoresist with well-con- Fig. 1 Lift-off process with sputtering against pattern profiles of photoresists. 62 Copyright The Japan Institute of Electronics Packaging
Ito et al.: Development of Lift-off Photoresists with Unique Bottom Profile (2/6) trolled undercut profile is required. In addition, as the photoresist is usually heated up to around 100 C during sputtering process, heat resistance is another key to maintain the unique undercut profile and strippability. To meet such the requirements, two types of novel liftoff photoresists were developed, one is a single layer negative tone, and the other is a double layer positive tone. This paper focuses on the photoresist design concept and key parameters to control the undercut profile. 2. Design Concept 2.1 Single layer negative tone photoresist for lift-off The negative tone photoresist consists of phenolic resin (Mw: 7000, novolak type) as a main component, cross-linkers, photo-acid generator (PAG), additives, and solvent. The design concept is shown in Fig. 2. Phenolic resin has high heat resistance. Irradiation with i-line generates acid from PAG, and the acid works as a catalyst of cross-linking reaction between polymers and cross-linkers. Post exposure bake (PEB) promotes three-dimensional cross-linking network in the exposed area which results in less solubility in alkaline developer. The crosslinking distribution in the depth direction of the photoresist film was optimized and it is the key parameter of the photoresist with undercut profile. 2.2 Double layer positive tone photoresist Double layer positive tone photoresist is composed of two coating layers. The upper layer is positive tone photoresist containing phenolic resin as a main polymer and naphtoquinone diazide (NQD) as a photo-active compound (PAC). NQD acts as a dissolution inhibitor for alkaline soluble phenolic resin, and promotes dissolution in the exposed area. NQD is transformed to indene carboxylic acid by i-line exposure as shown in Fig. 3. The upper layer shows adequate alkaline solubility contrast between exposed/un-exposed area. The under layer contains an acrylic polymer as a main component which has higher dissolution rate than upperlayer to form undercut profile with alkaline developer. The under layer also contains a small amount of NQD to enable undercut width adjustment by developing time. The solvent used for the upper layer does not dissolve the under layer polymer, otherwise, the desired undercut profile is not obtained due to the intermixing between the two layers. 3. Experimental Section The conditions of standard process are summarized in Table 1. Each photoresist either single layer or double layer process was coated on a Si wafer to target the optimum coating film thickness, and then pre-baked. For double layer photoresist, the second layer was also coated and pre-baked again. Exposure was conducted with an i-line stepper. Before development, the negative tone photoresist was baked at 95 C for 120 sec (PEB; post exposure bake). Finally, the photoresist film was developed with 2.38 wt% TMAH aqueous solution. Cu sputtering evalua- Table 1 Standard process conditions. Fig. 2 Cross-linking system of single layer negative tone photoresist. Fig. 3 Alkali solubility of positive tone photoresist with NQD compound. Item Single layer negative tone 1 st layer thickness 2.5 μm 1 st soft bake 95 C 90 sec Double layer positive tone 1.0 μm (Under layer) 120 C 180 sec (Under layer) 2 nd layer thickness 2.0 μm (Upper layer) 2 nd soft bake 110 C 180 sec (Upper layer) Exposure i-line srepper (N.A = 0.63, σ = 0.54) PEB 95 C 120 sec Development Rinse Sputtering Stripping 60 sec/2.38%tmah 30 sec/di water 23 C 300 sec/nmp 63
Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015 Fig. 4 Undercut width and height of single layer negative tone photoresist (left) and double layer positive tone photoresist(right). tion was conducted with batch-type sputtering system SX-200, and photoresist was stripped with NMP at 23 C for 5 min. Pattern profile was observed by using FE-SEM SU-8030. The averages (n = 3) of undercut width and height are defined in Fig. 4. 4. Results and Discussion 4.1 Single layer negative tone photoresist As described in the introduction, the density control of crosslink is the key of lift-off photoresist, Three studies were discussed: (1) cross-linker loading amount, (2) absorbance at 365 nm, and (3) process condition such as exposure dose, developing time, and bake temperature. 4.1.1 Study for loading amount of cross-linker Comparison of pattern profile variation with different loading amount of cross-linker (Fig. 5) was studied. Exposure dose was adjusted to obtain comparable pattern profile with almost the same undercut width. Sample 1 with lowest loading amount of cross-linker showed gradual slope profile with reverse taper, and sample 2 and 3 both with higher loading amount showed steep slope profile at the top, and deep undercut at the bottom. The heat resistance and strippability were evaluated as well because the amount of cross-linker affects these properties. The sample 3 with highest amount of cross-linker showed the highest heat resistance up to 100 C without compromising strippability. The results explain that adequate cross-linking density is necessary to obtain suitable undercut profile and excellent heat resistance as well. 4.1.2 Study for absorbance at 365nm Exposed i-line on the top of photoresist goes through the bottom of that while being absorbed by the film. Thus, the absorption at 365 nm makes the distribution of exposure intensity in the depth direction, resulting in the distribution of crosslinking density. Figure 6 shows the pattern profile of sample 4-7 with different absorption at 365 nm. Exposure dose was adjusted to obtain comparable pattern profile with almost the same undercut width. As absorption at 365 nm increases with higher exposure dose, the slope at the top of pattern profile becomes steeper. This Fig. 5 Cross-section SEM observation of samples 1-3 with different loading amount of cross-linker. Fig. 6 Cross-section SEM observation of samples 4-7 with different absorbance at 365 nm. Fig. 7 SEM Cross-section observation of sample 6 at various exposure dose. result indicates that desired undercut profile can be obtained by adjusting the value of absorption and exposure dose. 4.1.3 Study for process condition Figure 7 shows the pattern profile of sample 6 at various exposure doses. Increased exposure dose make the undercut width and height smaller because of the proceeding of cross-link reaction. Higher dose exposure results in pattern profile gentle slope and reverse taper, rather than steep slope at the top and deep undercut at the bottom. Figure 8 shows the pattern profile of the sample 6 at various developing time. As developing time becomes longer, the undercut width becomes larger without changing the undercut height. These results indicate that pattern profile with desired undercut width and height was obtained by selecting appropriate exposure dose and 64
Ito et al.: Development of Lift-off Photoresists with Unique Bottom Profile (4/6) Fig. 8 Cross-section SEM observation of sample 6 at various developing time. Fig. 10 SEM Cross-section observation of patterning resolution of sample 6. Fig. 11 Heat resistance and strippability of sample 6. Fig. 9 Pre-bake (left) and PEB (right) temperature margin of sample 6. developing time. Figure 9 shows the undercut width and height of the sample 6 at different temperature of pre-bake and post exposure bake (PEB). These graphs show that the sample 6 has enough pre-bake and PEB temperature margin for practical use as a lift-off photoresist. The following sections describe single layer negative tone photoresist (sample 6) performance data such as fine pitch application, heat resistance, strippability, and lift-off result. 4.1.4 Fine pitch application As shown in Fig. 6, at a standard condition with 250 mj/ cm 2, the undercut profile is 2.9 μm width which cannot result in 6 μm line width. In order to form higher resolution pattern, the exposure dose higher than 250 mj/cm 2 was attempted. Figure 10 shows pattern profile of the sample 6 at 420 mj/cm 2. L/S = 1.5 μm/0.5 μm pattern is formed with simple reverse taper profile. 4.1.5 Heat resistance and strippability Figure 11 shows pattern profile and strippability of the sample 6 before and after post bake for one hour on a hot plate. After baking at 100 C, the sample 6 showed no change in undercut profile and strippability. However, after baking at 120 C the photoresist is not strippable with NMP at 23 C. These results show that heating up to 100 C during sputtering does not change lift-off performance. Fig. 12 SEM micrographs before Cu sputtering (left) and after Cu sputtering followed by stripping (right), with sample 6 as a lift-off photoresist. 4.1.6 Lift-off performance for sputtering Deposition of Cu by sputtering was tested with single layer negative tone lift-off photoresist. Figure 12 shows SEM micrographs before and after Cu sputtering with the sample 6. After stripping, the photoresist and Cu sit on photoresist were stripped, and only the Cu sit on the wafer remained. These results indicate that the single layer negative tone photoresist we developed is useful to carry out high quality metal deposition by sputtering. 4.2 Double layer positive tone photoresist Under layer dissolution rate is the key of double layer positive tone lift-off photoresist, several key factors have been studied: (1) NQD loading amount, (2) process condition such as exposure dose, developing time, and bake temperature. 4.2.1 Study for polymer solubility in TMAH soution The pattern profile and solubility in TMAH solution of sample 8 (upper layer) and sample 9-11 (under layer) have been studied as shown in Fig. 13. The sample 9 without NQD compound has been exhibited with high solubility in 65
Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015 Fig. 13 SEM Cross-section observation of sample 8 (upper layer) and samples 9-11 (under layer). Fig. 16 1 st (left) and 2 nd (right) pre-bake temperature margin of sample 8 (upper layer) and 10 (under layer). Fig. 14 SEM observation of sample 8 (upper layer) and 10 (under layer) at different exposure dose. Fig. 17 SEM Cross-section observation of patterning resolution of sample 8 (upper layer) and 10 (under layer). Fig. 15 SEM observation of sample 8 (upper layer) and 10 (under layer) at different developing time. Fig. 18 Heat resistance and strippability of sample 8 (upper layer) and 10 (under layer). TMAH solution. With small change of developing time gave a drastic effect on undercut width. The sample 10 and 11 both with NQD compounds showed a wide developing time margin because of alkaline solubility inhibiting effect derived by NQD. Especially the sample 10, comparing with the sample 11, contains a small amount of NQD compounds and showed a straight profile of under layer, thus the sample 10 is expected to form undercut profile without peeling during fine pitch patterning. 4.2.2 Study for process condition Figure 14 shows the pattern profiles of the sample 8 (upper layer) and 10 (under layer) at various exposure doses. As exposure dose increases, the undercut width became larger with same undercut height. Figure 15 shows pattern profile at various developing time. As developing time becomes longer, the undercut width became larger. Figure 16 shows the undercut width at various temperature of 1 st and 2 nd pre-bake. The graph indicates that double layer lift-off photoresist with the sample 8 (upper layer) and 10 (under layer) has enough pre-bake temperature margin for practical use. The following sections describe double layer positive tone photoresists (sample 8 and 10) performance data such as fine pitch application, heat resistance, strippability, and lift-off result. 4.2.3 Fine pitch application Figure 17 shows fine pitch patterning profile of the sample 8 (upper layer) and 10 (under layer). The L/S = 1.5 μm/0.5 μm patterns were formed by optimizing lithographic conditions. 4.2.4 Heat resistance and strippability Figure 18 shows pattern profile and strippability of double layer photoresist using the sample 8 (upper layer) and 10 (under layer) after post bake for one hour on a hot plate. After baking at 100 C, the undercut profile and strippability were maintained while baking at 120 C caused upper layer pattern profile deformation. The results indicate that thermal treatment up to 100 C during sputtering does not change lift-off performance. 4.2.5 Lift-off performance for sputtering Cu deposition by sputtering process was tested with 66
Ito et al.: Development of Lift-off Photoresists with Unique Bottom Profile (6/6) Fig. 19 SEM micrographs before Cu sputtering (left) and after Cu sputtering followed by stripping (right), with sample 8 (upper layer) and 10 (under layer) as a lift-off photoresist. double layer positive tone lift-off photoresist. Figure 19 shows SEM micrographs before and after Cu sputtering with the sample 8 (upper layer) and 10 (under layer). After stripping, the photoresist was stripped with the Cu sitting on the photoresist. The results indicate that the double layer positive tone photoresist is useful to carry out high quality metal deposition by sputtering. 5. Summary and Conclusion Two types of novel lift-off photoresist were developed, one is a single-layer negative tone photoresist and the other is a double-layer positive tone photoresist. Both the photoresists show unique and well-controlled undercut profile and enable to form a designated metal configuration after stripping. Desired pattern profiles of the photoresists were obtained by selecting appropriate exposure doses and developing time. Especially single-layer negative tone type shows higher heat resistance, and doublelayer positive tone type shows better strippability. The newly developed photoresists are expected to contribute to the progress of metal patterning in the variety of electronic device fabrication processes. References [1] C. S. Premachandran, N. Rangnathan, S. Mohanraj, C. S. Choong, and M. K. Iyer, A Vertical Wafer Level Packaging Using Trough Hole Filled Via Interconnect by Lift Off Polymer Method for MEMS and 3D Stacking Applications, Proceedings - Electronic Components & Technology Conference, Vol. 55, Issue 2, pp. 1094 1098, 2005. [2] X.-Y. Wang, C.-Y. Lee, C.-J. Peng, P.-Y. Chen, and P.-Z. Chang, A micrometer scale and low temperature PZT thick film MEMS process utilizing an aerosol deposition method, Sensors and Actuators A, Vol. 143, pp. 469 474, 2008. [3] D. Steigerwald, J. Bhat, D. Collins, R. Flecher, M. Holcomb, M. Ludowise, P. Martin, and S. Rudaz, Illumination with solid state lighting technology, IEEE J. Sel. Top. Quant., Vol. 8, pp. 310 320, 2002. [4] Y. Fu, L.-L. Ye, and J. Liu, Thick film patterning by lift-off process using double-coated single photoresists, Materials Letters, Vol. 76, pp. 117 119, 2012. [5] A. Voigt, M. Heinrich, K. Hauck, R. Mientus, G. Gruetzner, M. Töpper, and O. Ehrmann, A single layer negative tone lift-off photo resist for patterning a magnetron sputtered Ti/Pt/Au contact system and for solder bumps, Microelectronic Engineering, Vol. 78 79, pp. 503 508, 2005. Hirokazu Ito Kouichi Hasegawa Tomohiro Matsuki Shiro Kusumoto 67