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Hydrogen Plasma Utilization in Advanced Logic Technology
H2-based plasma process has been controversial for a long time as it’s reported the high H2-based plasma could result in device instability [1]. In this work, we investigated various H2 related etch processes in advanced logic technology from the point of view of selectivity, defect, reliability and yield performance. Due to the chemi-sorption and recombination of H atom at Si [2], high temperature(>250℃) H2 remote plasma can be used to minimize the material loss for process where oxygen is forbidden, while and low temperature(<60℃) H2-based plasma process can be used in Si/oxide related etch processes with the improved selectivity to either mask or stop layer. Nevertheless, the low temperature H2 plasma directly contacting the exposed metal might result in plasma damage such as PID (plasma induced damage).
High temperature H2 based plasma process
Both shallow junction and HKMG have been integrated into the advanced logic process. This leads to the introduction of forming gas (4% H2 in N2/H2 mixture) to replace the traditional O2-based ashing process for metal gate EOT concern. Compared with conventional forming gas ash process, high H2(>10% H2 in N2/H2 mixture) ash process delivers superior photo resist removal capability, much less Si loss and higher throughput at high-dose implant strip, thus yield enhancement as shown in Fig.1.
Besides, Si-C bond after p-MOS Si recess etch could inhibit SiGe epitaxy and result in defect. We proved that high H2 ash process could effectively remove the Si-C bond at EPI surface and greatly reduce the SiGe EPI defect count [3]. In Static Random Access Memory vehicle, high H2 process delivers >10% Vmin yield enhancement as shown in Fig 2.
Furthermore, after Dummy Poly Gate Remove, H2 based ash process provided more hydrogen mobiles to passivate dangling bond at Si/SiO2 interface and delivered 7 times better Negative Bias Temperature Instability performance than forming gas ash process as shown in Fig 3.
Low temperature H2 based plasma process
Low temperature H2 can be used to etch Si or used as post etch treatment to remove the poly-si residue on ICP etcher. In this work, we demonstrated the H2 based DPGR process could deliver >50% gate leakage(Ig) improvement than conventional halogen gas etch process because of less HK capping layer loss as shown in Fig4.
At same time, low temperature H2 based plasma was evaluated as post etch Treatment at Ultra Low K(ULK) etch to remove etch by-product. The hot H radicals can make the hydrophobic CHn group to be more reactive CHn-1 group. Si which loses the CHn group will absorb the H2O during the exposure to ambient and result in ULK degradation (Fig5).When we implemented same H2 PET process at Pad etch process, severe Plasma Induce Damage issue was detected (Fig6). This phenomenon also proved that low temperature H2 plasma dissociated more high energy ions because of small mass. Pad acted as large area antenna which collected more charges. The reversed electron shading effect results in PID.
In ULK Metal Hard mask approach, we introduced H2 in via etch to achieve self-aligned Via. CxFy and H2 gas plasma delivered high selectivity between ULK and metal hard mask because H atom can decrease the F concentration in plasma. The via-via distance can be doubled with H2 implementation (Fig7). and this will benefit the time-dependent dielectric breakdown reliability performance.
Conclusions
In brief, H2 based plasma process plays important role in advanced logic technology, especially in HKMG technology. Low temperature H2 based plasma process needs to be evaluated prudently because H radical is more reactive.