Effect of Dissolved Ozone and In-Situ Wafer Cleaning on Pre-Epitaxial Deposition for Next Generation Semiconductor Devices

INTRODUCTION

The standard approach for Si surface cleaning prior to epitaxial growth processes is a high temperature, usually greater than 1050 ^oC, gas phase method to dissolve the native oxide along with any other contaminants on the surface of the wafer in order to prevent formation of any defects [1]. However, in the case that an advanced device with sensitive structures requires lower thermal budget treatments, a low-temperature pre-epitaxial cleaning process is needed. However, lowering the process temperature in turn causes an issue by lowering the desorption rate of SiO₂. This issue can be resolved by an HF-last process which converts the surface of the silicon wafer to a hydrogen-terminated surface, which when accomplished properly can yield a hydrophobic surface with the least defects [1,4,5,6].The pre-epitaxial cleaning of Si wafers for deposition can be approached in a wet bench at much lower temperatures than the gas phase method. The process chemicals, sequence and number of cleaning steps are becoming more critical in determining the desired end results [2,3]. The following study provides the data and process of proving that a one-step dilute in-situ-HF in the dryer is more effective than a traditional multi-tank HF-last process in a wet bench [7].

EXPERIMENTAL

All experiments were conducted on Akrion Technologies’ GAMA^TM automated wet station which is capable of performing both a multi-tank sequence and single tank in-situ process. The silicon wafers are processed in the tool for the pre-epitaxial cleaning prior to the epitaxial growth step. Bare silicon wafers were processed with dummy oxide wafers, alternated or sandwiched, in order to simulate a situation with patterned wafers. The contamination levels from the oxide wafers on the bare wafers would be large due to the etch by-products from the oxide wafers depositing onto the bare wafers during processing. Multiple cleaning techniques were used in order to counteract the high level of contamination caused by the etching process and were compared with the conventional multi-tank method. The materials used were: a GAMA^TM wet bench equipped with a LuCID^TM dryer (HF controlled injection), KLA-Tencor SurfScan (inspected at  ≥ 0.12µm), bare Si wafers with low particle counts and thermal oxide wafers. Concentrations and parameters: 100:1 HF (23 ^oC), 400:1 dHF (23 ^oC), 1:2:50 dSC1 (50^oC and 800W megasonic), DIO₃ rinse (~5-10 ppm at 23 ^oC).

RESULTS AND DISCUSSION

The typical standard process is to use a high temperature H₂ pre-bake to desorb the native oxide on the wafers to prepare the surface for an epitaxial layer deposition. However, lower temperatures are required to ensure isothermal processing for these advanced next-generation devices [7-10]. In IC manufacturing, wafers are typically mixed with oxide wafers or the wafers are patterned, and exposed silicon is typically adjacent to oxide or nitride areas. When the wafers are exposed to HF solutions, the by-products of the etched wafers will be removed from the hydrophilic surface and be deposited on the hydrophobic surface. This deposition results in high particle counts on the exposed silicon surface. The process contained herein was created to overcome this issue.

Before proceeding with the experimental procedures, tests to ensure particle neutrality within the GAMA^TM wet bench were performed. The results of a conventional HF/Rinse/SC1/Rinse/Dry process yielded low particle addition even in the presence of oxide wafers; i.e. an average particle addition of – 6 (1 s Stdev = 11, Figure 1). When only using bare silicon wafers, the conventional HF-last process yielded low particle addition as well; i.e. the average particle addition was less than 40 particles at 0.12 mm (Figure 3). In addition, post epitaxial defects were also low (~ 1.26 defects/cm²), as shown in Figure 2 as a best case scenario.

Silicon wafers were sandwiched between oxide filler wafers in order to simulate patterned wafers in a typical manufacturing environment. A conventional HF-last process (SC1/Rinse/HF/Rinse/Dry) resulted in high particle counts at 0.12µm (> 1,000). The high pre-epitaxial particle counts also caused high post-epitaxial defects (> 30,000). The particulate defects are normally considered as nucleation sites of epitaxial defects during the epitaxial deposition process. Conventional methods of wafer transfer between tanks plays a significant role in increasing the deposition of silicate particles onto the silicon surface due to wafers crossing the liquid-to-air interface. To counteract the silicate deposition, two different approaches were tested.

An in-situ process was thus developed in order to prevent the wafers from crossing the liquid-to-air interface in which the contaminants reside and deposit onto the wafer surface. HF chemical injection was used in the dryer to perform the in-situ process which yielded much lower particle deposition due to wafers not crossing the liquid-to-air interface. Figures 3 and 4 show the results that the average particle adders of less than 50 particles.

An important note is that the use of ozonated rinse after HF and before going to the SC1 step is critical for eliminating any potential of metal-induced pitting on the hydrophobic surface [11,12]. As reported by Knotter [10], Fe in the SC1 can induce pitting on hydrophobic wafer surface. The oxide chemically grown in the ozonated cascade rinse (OCR) is stable and thick enough (7-10 Å, as shown in Figure 5) to protect the silicon surface from any effects of metal roughening. The post epitaxial cleaning results for the in-situ method are shown in Figure 6, and the average LPD density per wafer is about 0.89 defects/cm². Figure 6 also indicates that the lower the HF-last defects are, the lower the post epitaxial deposition defects will be.

The results of each of the different cleaning recipes are summarized previously [7]. The results from testing showed that the most critical step in order to achieve extremely low post epitaxial deposition defects is the in-situ process which requires no wafer transfer between steps. In order to characterize the background oxide thickness, measurements were also taken as an indicator of the oxygen content on the wafer surface. It is equally important to notice that the amount of oxygen content on the wafer surface could significantly increase the number of post-epitaxial defects on the wafer. The lower the oxygen content on an H-passivated surface, the lower the amount of post-epitaxial defects on the wafer surface, as shown in Figure 7.

CONCLUSIONS

An in-situ HF-last cleaning process in the dryer was developed and used for pre-epitaxial growth. Results for the in-situ cleaning showed a significant improvement over the standard HF-last process. The reason for the in-situ process having a great impact on the elimination of particle deposition is due to the Si wafers not crossing the liquid-to-air surface between the HF-etch/rinse/dry process. The experiments conducted in the study proved that the use of dilute chemicals and the in-situ HF-etch/rinse/dry process yield lower defects than that of the standard multi-tank HF-last process. The defects after the in-situ clean are directly correlated to the post epitaxial growth defects. The lower the oxygen content and particle defects after cleaning, the lower the post epitaxial deposition defects.

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