Conexant has had to work hard to identify and eliminate various etch-related defects resulting from a ramp in IC production at its GaAs fab. Jiang Wang, Sam Mony and Nercy Ebrahimi report.

The huge increase in demand for GaAs power amplifiers in mobile communications has led Conexant to expand its efforts to increase its GaAs HBT capacity, achieve high production yields and reduce manufacturing costs. In the last two years Conexant has undertaken a steep production ramp to increase capacity. However, intermittent process defects have emerged that were not obvious at low production volumes, and these defects have lowered yields. This article discusses three process defects that occurred during the photo/etch stages. The defects were classified as emitter mesa protrusions, base-pedestal mouse-bites and collector-contact gross undercut. Conexant has eliminated these defects, which were related to process conditions during higher production volumes, by implementing a resist film post-develop bake and an oxygen plasma cleaning step after the emitter mesa resist strip. This resist film post-develop bake increases resist film adhesion, and reduces wet etch undercut and resist film dissolution. In addition, a post-bake is an essential step before wet etching, even if an adhesion promoter such as HMDS is used. Emitter mesa protrusions shows SEM images of the emitter mesa region. The mesa has a two-layer structure with an emitter metal on the top, and is formed in two etch processes using the same resist film. The first etch is the InGaAs layer wet etch, which is followed by the second anisotropic plasma dry etch. This etches the GaAs layer and results in a vertical profile. The defects were found to be attached to the bottom layer, and protrude out of the emitter mesa. These emitter mesa protrusion defects occurred intermittently in the production line, and were always associated with the bottom layer formed by the plasma etch. (a) shows an SEM of the structure after the InGaAs emitter mesa wet-etch stage and before the dry etch, which shows that part of the emitter mesa photoresist peels and flows onto the substrate. This is a result of resist dissolution in the wet-etch solution. This fallen resist blocks the dry etch, forming the emitter mesa protrusions seen in (b). The photoresist flow occurred only when the wet etch was almost complete. This explains why emitter mesa protrusions are always connected to layers formed in the dry etch but never to layers formed in the wet etch. As a remedy, a resist post-develop bake was introduced into the production process. A post-bake at 115 C for 1 min hardens the resist film, which completely removes resist dissolution and the protrusion defects. No statistical difference was observed for the key electrical parametric data and emitter mesa critical dimension between processes with and without post-bake. This improvement could have profound effects on the yield and reliability, particularly in view of the small spacing between the emitter mesa and the base-contact metal, and a better geometric integrity of the AlGaAs ledge. Base-pedestal mouse-bites The mouse-bite defects occurred during the wet-etch process that forms a base-pedestal structure 500 nm deep. shows the base pedestal with a mouse-bite, which leads to poor geometric integrity after the etch. Base-pedestal mouse-bites always begin at the pedestal edges, as they are caused by poor adhesion at the resist film edges. This enables acid to penetrate beneath the resist film, leading to an etching of the base pedestal. SEM studies showed that poor resist adhesion is caused largely by the resist residues from the previous emitter mesa resist strip. The combined wet and dry etch processes cause the resist to polymerize, making it difficult to strip. Oxygen plasma ashes with different ash rates were tested to remove photoresist residues and enhance adhesion. The AlGaAs ledge remaining after the emitter mesa etch is very sensitive, and critical to the reliability of the HBT since it is unprotected at this step (Canfield). For this reason the effects of different ashes on the device were evaluated through high-temperature operation life tests. Based on parametric data, the operation life tests and statistical analysis, we implemented an oxygen plasma ash with an ash rate of 120 nm/min, and a base-pedestal photoresist post-bake at 120 C for 1 min. The mild oxidation of the AlGaAs layer by the ash improved the adhesion/reaction of HMDS monolayer to the substrate. The post-bake reduces the undercut, as it restores the adhesion weakened by developer penetration along the resist-substrate interface (Dammel). The post-bake alone can reduce mouse-bites, but plasma ash and post-bake can eliminate base-pedestal mouse-bites completely. Collector-contact undercut The collector contacts are visible as rectangular windows in the optical micrograph in . The windows are opened by wet etching to depths ranging from 580 to 880 nm. The etch is conducted at 1.5 C with a high etch rate, and usually causes a large contact undercut. All the edges of the collector windows close to the base-metal pad are rough, while those far from it are smooth with good geometric definition. The undercut has three unique features: it happens on the edges closer to the big base metal pads, it never occurs where the base-metal pad is small or no base metal exists, and the defect shows up intermittently. The asymmetric appearance suggests that this undercut must form under the influence of the base-metal pads, and that it is not caused by the wet etch as this is isotropic in nature. is a schematic of the collector-contact layer showing the structure of the base-metal pad, SiN film, resist film and two collector-contact openings before the etch process. The resist film experiences a shear force (F) that results from the shrinkage of the film toward the base metal. This reduces resist adhesion at edges close to the base-metal pad. The resist film covering the gold metal pad shrinks more due to the weak interactions of the HMDS monolayer with the gold surfaces. The degree of resist film dehydration and the higher tension at the gold surface could be two other factors causing resist shrinkage. The resist film shrinks more if dehydration is incomplete, and shrinks to surfaces with higher surface tension (Somorjai). Post-develop baking To minimize collector-contact undercut a post-develop, 1 min bake at 120 C was implemented. The post-bake completely eliminated the gross asymmetric undercut, and reduced the normal undercut formed during isotropic wet etching. Process steps such as collector-metal lift-off were also evaluated and no difference was found between processes with and without the post-bake. The post-bake process has several advantages. At such high post-bake temperatures, reflow occurs on the top layer of the film. However, at this temperature the resist is still stable and the critical dimension of the collector opening does not change. A mild resist reflow and profile change is not sensitive to the wet etch since it is highly selective. The bake removes most of the absorbed water and solvent, and produces a melting effect. This can lead to less shrinkage due to the maximization of the contact surface between the resist and the substrate. Also the high bake temperature may induce interdiffusion between a silylated surface primer and the overlying resist that improves adhesion (Parisi et al.). Other groups have noted that the melting of the resin in the resist can act as an adhesive to seal the interface between the resist and substrate, and reduce edge lifting during wet etching (Andrasi). For the emitter-mesa, base-pedestal and collector-contact layers, the adhesion promoter HMDS is used to treat the substrate before the photoresist coat, and at least one monolayer of HMDS is formed on the substrate. HMDS is less effective at promoting resist adhesion during GaAs processing than it is in Si processing, because GaAs processing leads to surfaces with fewer surface hydroxyl (OH) groups such as SiN. This is especially true of inert gold surfaces. Due to the deficiency of OH groups HMDS cannot form sufficient covalent bonds, so HMDS adhesion to the substrate is weak (Martin). As a result the adhesion of resist film to the substrate will be weak, even though the adhesion between the HMDS layer and resist film is strong. Conclusions Emitter mesa protrusions, base-pedestal mouse-bites and collector-contact gross undercut defects have been observed intermittently under high-volume production conditions. These defects are caused by poor resist adhesion and resist dissolution during the wet etch. However, a resist post-bake in the resist develop process can completely remove emitter mesa protrusions and collector-contact undercut defects. Also the post-bake and a plasma clean eliminate the pedestal mouse-bite defect. The bake time and temperature were optimized to ensure that there was no drift in the key electrical parameters and critical dimensions. In addition, a resist post-bake and surface clean enhance the adhesion of the photoresist. The resist post-bake is an essential addition to wet etching if gross undercut and defects caused by resist dissolution are to be removed. Further reading M Andrasi 1980 Thin Solid Films 67 229. P Canfield 2000 GaAs Mantech Digest R Dammel 1993 Diazonaphthoquinone-base resist SPIE Optical Engineering Press. R Martin The Applied Chemistry of Shipley Microposit Products for Semiconductor Manufacture. G Parisi et al. 1977 J. Electrochem. Soc. 124 917. G A Somorjai 1990 Chemistry in Two Dimensions: Surface Fifth Edition Wiley-Interscience.