Adjustable Silicon Corner Rounding Radius by Wet Technique
Authors: Tou Pei-Ting , Liao Hsin-Yi , Huang Hui-Chin , Shih Kai-Yao , Lu Ming-Chen 
Published: February 2020

Shallow trench isolation (STI) and active area (AA) top corner rounding is important to devices performance. AA top corner rounding techniques include STI etch, linear oxidation , H2 annealing and wet oxide pull back (PB). However, conventional STI etch will caused AA facet and need to additional thermal in linear oxidation or H2 annealing processes. Wet oxide PB is easily to control the pad oxide etching amount and reach desired AA top corner top rounding radius.In this study, different pad oxide PB amount by Hydrogen Fluoride (HF) with corner rounding correlation were shown. At high pad oxide PB amount, additional hydrogen peroxide (H2O2) with dilute HF can optimize the AA top corner profile and enlarge the corner radius.
Sharp AA corners profile caused gate oxide formation thickness thinning let strong electrical filed at AA corner. In wafer acceptable test had brought about double hump effect in I-V characteristics and inverse narrow width effect. Pad oxide PB following STI Etch is used by HF to let the silicon top corner be exposed and beneficial for increasing AA top corner rounding radius of liner oxidation process. This study gives some course to improve the silicon corner profile and AA top corner radius.
After STI etch process, different wet etch amount were used for pad oxide PB from pad nitride by 0.49% HF process time adjustment. Different wet oxide PB amount were split for corner radius comparison. As oxide PB amount increased from 0nm to 5nm the corner radius can be improved, and the best radius showed at PB 5nm. When oxide PB amount add to 7.5nm, the corner radius became worse slightly. The radius of PB 10nm is even worse and close to without oxide PB process. This result suspected that AA corner profile might be changed in wet oxide PB process.
When oxide PB amount increase, oxide etch rate might decrease and exposed silicon corner be continually damaged by HF erosion. The silicon damage of PB 5nm is slight and can be recovered after linear oxidation thermal process. At PB 7.5nm the AA corner “stepped” profile was observed. It can explain while the PB amount >5nm the corner rounding radius decrease. Because the “stepped” profile caused the corner split two curvatures then the radius decrease. The “stepped” profile of PB amount >7.5nm still be find after linear oxidation.
To reduce the AA top corner “stepped” profile, the post-HF treatment’s AA silicon exposed in following APM process. The AA top corner and silicon sidewall which with APM treatment is clipping suspect H2O2 oxidize silicon and NH4OH consume the oxide caused. This profile made the region of exposed silicon to be deficient and pointed in AA geometry of silicon corner. The pointed AA top corner profile is unhelpful for corner rounding radius improvement, so the key factor of AA top rounding profile is corner silicon loss controller.
In order to reduce the corner silicon loss amount, using H2O2 to replace the APM and dilute HF concentration to do oxide PB. This optimistic experience will be expected lower the exposed corner silicon consumption and corner rounding radius improvement.
According oxide PB amount 5nm and 7.5nm comparison in 0.49%HF, the corner radius reduced 0.1nm when the PB increased. Nonetheless, HF concentration changed from 0.49% to 0.1% and H2O2 addition, the corner radius can be improved 0.3nm and 0.7nm. This result proved that the corner silicon profile of PB >5nm will be damaged in concentrated HF wet etch. Change to dilute HF concentration will reduce corner silicon loss and enable to recover corner silicon which with “stepped” profile by additional H2O2 treatment.
As the higher oxide PB amount the pad oxide PB rate will become lower cause that without oxide protected corner silicon will be damaged and result in “stepped” profile. Nevertheless, using dilute HF can lower corner silicon loss and reduce the silicon erosion which without oxide protected. Additional H2O2 treatment introduced the rounding and smooth profile.
The relationship between pad oxide PB and AA top corner rounding is established in STI process. The AA top corner radius of oxide PB amount 5nm can be improved 2.6nm compare to without PB process. When oxide PB amount >5nm the corner radius is decreased. The decreasing corner radius come from higher oxide PB amount that without oxide protected corner silicon will be damaged. This phenomenon result in “stepped” profile with difference curvature and corner radius decreasing. Dilute HF with H2O2 treatment is demonstrated that it is beneficial to reduce and recover the corner damaged.

Figure 1