(Invited) Temporary Bonding with Polydimethylglutarimide for Residue-Free Layer Transfer and 3-D Integration

Residue free temporary bonding with Polydimethylglutarimide (PMGI) was studied for a Layer Transfer Platform. Temporary wafer bonding is one of promising technologies for 3D integration, in particular transferring 2D materials such as graphene. PMGI was traditionally applied for lift off resist (LOR) and used as sacrificial layer for undercutting during lift-off processing. However, it has recently been reported that a sacrificial layer of LOR based PMGI (MicroChem) realized a clean graphene surface and free of residue after protolithic-graphic processing¹. The practicability of LOR as a temporary wafer bond adhesive is investigated by varying bonding parameters and monitoring the size of the bonded area as well as the bond strength.

 Bonding of Si wafers using PMGI as a temporary adhesive was performed. Before bonding, Si wafers were cleaned using Standard Clean 1 (H₂O:NH₄OH:H₂O₂ = 5:1:1). After surface cleaning, Si wafers are spin coated with LOR 5A (MicroChem), which is based on PMGI, and pre-baked to evaporate the solvent in the resist. Finally, the two PMGI surfaces are brought into contact and the wafer pair was introduced into a bonding machine (EV-501 from EVGroup Inc.). In the bonding machine, the two PMGI layers are compressed face to face with a force of 150 N, under vacuum condition of around 10^-3 Pa, and heated to 200-250 ºC. To evaluate these bonded samples, the size of bonded area was observed using an Infrared camera, and the bond strength was evaluated by blade crack test ².

 Under optimized process conditions, the wafers are bonded with a few small voids. The size of bonded area covers over 98% of the wafer surface. Also, the wafers are bonded strongly. The bond strength of the sample under optimized conditions is 4.86 J/m². The surface energy of two Si wafers is 5 J/m². This means the strength of bonded samples is almost as same as the bulk of Si under the optimized conditions.

 For an optimization of spinning speed at coating stage, samples are bonded with the spin speed ranging from 2000 to 5000 rpm. This result shows bonded area is large enough (>98%) at spinning speed is between 2000–4000 rpm. However, some voids appear after bonding when spinning speed is higher than 5000 rpm and the bond strength gradually deceased with higher spin speed. From these results, the strongest bonding as well as largest bonded area is obtained when spin speed is 2000 rpm.

 For an optimization of pre-bake temperature after spinning, samples are bonded with pre-bake temperature from 120 to 220◦C. Bonding quality of the samples baked at 120 ºC was poor in both bond strength and bonded area. However, when the pre-bake temperature is higher than 160◦C, most of the loaded area was bonded and the bond strength is high. One reason for this is that the boiling point of cyclopentanone, the solvent of LOR, is 131◦C. Thus, samples baked at 120 or 140ºC still have residual solvent left in the LOR layer. Therefore, cyclopentanone vapor during bonding prevents successful bonding at the interfaces. Bonding with pre-baking over 160◦C can achieve high quality bonding. Lower process temperature is preferable to reduce thermal damage to bonding materials, so an optimized pre-baking temperature is concluded to be 160◦C.

 For an optimization of bonding temperature samples are bonded under bonding temperature ranging from 200 to 250 ºC under a 750 KPa load for 30 minutes at a vacuum of around 10−3 Pa. With increasing bonding temperature, the bonding across the LOR surfaces improves. When the bonding temperature was at least 250◦C, most of the loaded area was bonded and bond strength was high.

 Moreover, contamination after debonding was studied using atomic force microscopy (AFM). Si wafers bonded with PMGI were debonded using N-Methyl-2-pyrrolidone (NMP)-based solvent called Remover PG (MicroChem). Comparison of the Si wafer surface before processes and after bonding-debonding processes shows RMS of surfaces are 0.75 ± 0.06 nm and 0.87 ± 0.36 nm, respectively. Any residue remaining on the Si wafer surface after LOR removal is negligibly small and any resist residue can be 100% removed with a subsequent oxygen plasma ashing process.

 In conclusion wafer bonding with polydimethylglutarimide (PMGI) has been demonstrated to be suitable for 3-D integration with a temporary bonding process enabling a layer transfer technology.

Reference

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2. W. P. Maszara, G. Goetz, A. Caviglia, and J. B. McKitterick, J. Appl. Phys., 64,4943 (1988).