A cleaning process for polysilicon CMP applications
By Eugene Zhao, Diane Hymes, Jackie Zhang, Willy Krusell
Polycrystalline silicon (polysilicon or poly-Si) has been an important material in integrated circuit (IC) technology for nearly two decades. It is widely used in MOS devices and bipolar ICs. Some of the recent applications of poly-Si include trench isolation, thin film transistors and micromachining. In many applications, chemical mechanical polishing (CMP) of polysilicon material is utilized to achieve surface smoothing or global planarization.
After the CMP process, the surface of polysilicon is predominantly H-terminated and hydrophobic. In order to successfully remove slurry particles from the polished poly-Si surface, the surface has to be converted to hydrophilic. Currently, the cleaning process for poly-Si CMP1a consists of a two-step, multi-tool process: (1) megasonic tank with SC1 to grow a thin layer of chemical oxide; and (2) mechanical brush scrubbing to remove particles and surface metal contamination. This article discusses a cleaning process that combines SC1 with brush scrubbing into a single tool. This approach eliminates the need for a separate megasonic tank and enables high throughput process integration of polishing and cleaning.
The results of the polishing and cleaning experiments on polysilicon film will also be presented in this article. Atomic force microscopy (AFM) analysis shows that the surface roughness of poly-Si film decreases by a factor of 10 after the CMP process. By using SC1 in the first brush box and dilute acid in the second brush box of an OnTrak Systems, Inc. scrubber, low particle counts (<1 particle/square centimeters at 0.2 µm) and low surface metal concentrations (~1E10 atoms/square centimeters per element) are observed on a polished poly-Si surface.
Chemical mechanical planarization (CMP) has become an integral process step in the manufacturing of sub-0.35 micron devices with three or more levels of metal interconnect. Inherently, CMP produces an extremely high level of surface contamination, thus the CMP cleaning step is a critical part of the overall process. More recently, new applications in CMP have emerged, one of them being a cleaning requirement for polysilicon.
Polysilicon has been an important material in IC technology for nearly two decades. It was first used as the gate material in MOS devices because of its compatibility with subsequent high temperature processing and its ability to form self-aligned gates. Polysilicon is included in virtually all modern bipolar ICs to form base contacts and polysilicon emitters. It is also used early in the fabrication process for trench isolation and in DRAM capacitors. Recently, the advantages of using polysilicon as the active layer of thin film transistors have led to its replacing amorphous silicon in advanced active-matrix displays and its application for fabrication of the high density, static random-access memory (SRAM). In addition to its IC applications, polysilicon is becoming vital to the emerging field of microsensors and microactuators. In these devices, polysilicon is used as a mechanical material. In many applications, CMP of polysilicon material is utilized to achieve surface smoothing or global planarization.1
The research that follows describes the development of a single-tool cleaning process for polysilicon CMP. By combining SC1 and dilute acid with mechanical brush scrubbing to achieve low particles and low surface metallic contamination, the method provides a high throughput process integration of CMP and cleaning.
Polysilicon CMP applications can be divided into two categories: polysilicon surface smoothing and inlaid polysilicon structure formation. In thin film transistor applications such as active matrix LCDs and high density SRAMs, a smooth polysilicon surface is required to reduce the defect state density within the energy band gap. CMP following poly-Si deposition provides a smooth surface for the fabrication of transistor devices. Surface smoothing is also important in the fabrication of polysilicon structures used in microelectromechanical system (MEMS) devices. In the trench fill applications used in DRAM fabrication or MEMS structures, the purpose of CMP is to polish away the excess poly-Si material in order to form the in-laid polysilicon structures.
Depending on the application, the requirements of polysilicon CMP varies significantly. However, as far as CMP cleaning is concerned, the requirements are expected to be similar. These requirements include slurry particle removal and surface metal contamination reduction. After the CMP process, the surface of polysilicon is predominantly H-terminated and hydrophobic.1 In order to successfully remove slurry particles from the polished poly-Si surface, the surface has to be converted to hydrophilic. Currently, the cleaning process for polysilicon CMP consists of two steps1: (1) megasonic tank with SC1 (ammonia:hydrogen peroxide:DI water mixture) to grow a thin layer of chemical oxide; and (2) mechanical brush scrubbing to remove particles and surface metal contamination. Recently, a cleaning process that combines SC1 chemical with mechanical brush scrubbing in a single tool set was developed. This approach eliminates the need for a separate megasonic tank and enables a high throughput process integration of polishing and cleaning.
The following is a highlight of the SC1 scrub cleaning process for polysilicon CMP applications:
Polyvinyl alcohol (PVA) brush material is chemically and mech anically compatible with SC1.
SC1 delivered in brush box #1 of a scrubber converts hy drophobic poly-Si surfaces into hydrophilic surfaces.
Defect level of poly-Si wafers depends on both polishing and cleaning conditions. Poly-Si wafers polished on a commercial polisher and cleaned with the SC1 scrub process had average light point defect (LPD) counts on a 200 mm wafer of 75 @ 0.2 µm and 30 @ 0.3 µm with 5 mm edge exclusion on Tencor 6420.
Dilute acid delivered into brush box #2 significantly reduces surface metal contamination on poly-Si and TEOS surfaces. Surface metal concentrations are typically below 1E10 atoms/square centimeters per element.
Particle removal and surface conversion performance of the SC1 scrub process is equivalent to that of the SC1 megasonic tank and DI water scrub process combined.
The SC1 scrub process achieves high throughput (60+ wafers per hour).
Polysilicon blanket films were deposited on 200 mm wafers using LPCVD; TEOS films were deposited using a PE CVD process. Two commercially available polysilicon slurries from Rodel (SDE3000) and Fujimi (PL6101) were used in the polishing and cleaning experiments. CMP was done on an experimental rotary polisher and a commercial CMP tool. All the brush scrubbing experiments were done using the Synergy cleaning system from OnTrak Systems, Inc., (San Jose, CA).
Figure 1 shows typical AFM scans of the polysilicon film before and after the CMP process. The surface roughness of poly-Si film decreases by a factor of 7 after the CMP process.
After the CMP process, the polysilicon surface is hydrophobic, as reflected by the high contact angle between a water droplet and the wafer surface. SC1 used during scrubbing or in a wet bench grows a thin layer of chemical oxide, and converts the surface to hydrophilic, as shown in Table 1. The surface conversion performance of the SC1 scrub process is equivalent to the SC1 wet bench process.
Under the same set of CMP conditions, the particle removal performance of the SC1 scrub process is compared to that of the combined SC1 megasonic tank and DI water scrub process. Table 2 shows that these two processes achieve comparable particle removal performance.
The final defect counts on the wafer surface after CMP and clean not only depend on the cleaning process, but also the polishing parameters. Table 3 shows the defect count results from two different slurries and two different CMP tools.
During the CMP process and the SC1 scrub process, the wafer surface may become contaminated with metal oxide due to the high pH environment. Surface metal contamination is significantly reduced by applying dilute HCl in the second brush station (Table 4). Most of the metal elements are below total x-ray reflection fluorescence (TXRF) detection limits after the HCl process. This process combines chemicals with brush scrubbing without compromising the throughput of the cleaning tool.
In order to better understand the surface metal removal process, surface SIMS is used to measure the depth profiles of three metal elements (Al, Ca, K) in the polished polysilicon film (Figure 2). Most of the metal elements are found very near the surface, and the concentrations decrease exponentially going into the film. Dilute HCl removes most of the Al and Ca contaminants because HCl dissolves the metal oxide deposited on the wafer surface.
The new cleaning process for poly-Si provides a single-tool solution for cleaning after polysilicon CMP processing. SC1 delivered in brush box #1 during scrubbing converts hydrophobic polysilicon film after CMP to hydrophilic. Diluted HCl used in the brush scrubber in brush box #2 significantly reduces surface metallic contamination on polished polysilicon and TEOS surfaces. This single-tool SC1 scrub process achieves the same level of cleaning performance as the combined tool set of an SC1 wet bench and brush scrub process. This new process provides an overall higher throughput CMP process without compromising final defect performance.
Eugene Y. Zhao, Ph.D., received his bachelor`s degree in chemistry from Fudan University in Shanghai and his doctorate in physical chemistry from the University of California at Berkeley. Since 1996, he has been a senior process development engineer at OnTrak Systems (San Jose, CA), a wholly owned subsidiary of Lam Research Corp. His responsibilities include CMP defect characterization and post-CMP cleaning process development for oxide, tungsten and copper CMP processes. Zhao has authored and co-authored a number of technical papers for publications in the areas of semiconductor clusters and semiconductor processing.
Diane J. Hymes, Ph.D., is director of cleaning process technology at OnTrak Systems (Milpitas, CA) and has been a member of the company`s technical team for more than two years. Before joining OnTrak, she spent six years as an applications research scientist at MEMC Electronic Materials, a silicon wafer manufacturer. She received her master`s degree and doctorate in materials science & engineering from Brown University in 1984 and 1987, respectively.
Wilbur C. Krusell, Ph.D., is an executive vice president and chief technical officer at OnTrak Systems. An expert in the areas of wafer cleaning, polishing and CVD technologies, Krusell sets and manages the company`s technological strategies. During his career, Krusell has held management positions at MEMC Electronic Materials, Advantage Production Technology and Watkins-Johnson Corp. Krusell holds a doctorate in inorganic chemistry from Massachusetts Institute of Technology and a master`s and bachelor`s degree in chemistry from the University of Michigan. He has published more than 40 technical papers and holds patents in the area of chemical processing of semiconductor wafers.
Jackie Zhang is a lab manager at OnTrak Systems, working on cleaning technology development. Prior to joining OnTrak in 1993, she worked at the Vacuum Technology Research Institute in Beijing, China, specializing in sputtering technology and vapor deposition equipment. She received her bachelor`s degree in mechanical engineering from the Northeast University in Liaoning, China, and her master`s degree in ME from San Jose State University.
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2. G. J. Pietsch et al., Appl. Phys. Letts., 64 (1994) 3115.
At right: Typical ATM scans of polysilicon film before and after the CMP process.
Below: Surface SIMS is used to measure the depth profiles of three metal elements (A1, Ca, K) in the polished polysilicon film.