Novel Wet Etchant for Highly Selective Etching of Si1-XGex-Film to Si-Film with Sodium Periodate Oxidant
Authors: Lee Changjin , Lee Ji-Eun , Jang Eun-Woo , Kim ji Hoon , Park Jea-gun 
Published: February 2024

As the technology node of complementary-metal-oxide-semiconductor (CMOS) transistor has been scaled down to 3 nm, CMOS transistor’s structure has been changed from fin field-effect transistors to gate-all-around field-effect transistors (GAAFETs) to improve the short-channel effects by enhancing gate controllability. In addition, dynamic random-access memory (DRAM) also has been faced the limitation in scaling-down of a capacitor having high aspect ratio and in migrating from 6 to 4F2 cell increasing floating body effects. To overcome the limitation, 3-dimensional (D) DRAM structure having GAA structure has been researched as an alternative structure to the conventional 6F2 cell structure. In fabricating GAAFETs and 3-D DRAM, two major fabrication processes are needed, i) epitaxial growth of Si1-xGex/Si multi-stacks, and ii) selective etching of Si1-xGex films to Si films. For process i), low Ge concentration (<30%) is desirable for reducing stacking faults and defects at the interfaces between Si1-xGex and Si. In addition, for process ii), considerably high etch rate (>30nm/min) and etch selectivity of Si1-xGex films to Si films (>300:1) are required. In particular, for etching Si1-xGex grown on Si layer of a sacrificial layer, a high selectivity, high etch rate and isotropy of wet chemical etching are strongly required. In general, among the components of wet chemical etchant consisting of oxidant, etching accelerator (i.e., HF), selectivity enhancing agent (i.e., acetic acid), and deionized water, the higher standard reduction potential (SRP) of oxidant leads the higher selectivity and higher etch rate of Si1-xGex films to Si films due to the increase in the potential differences of SRP and Si1-xGex valence band edge. For the case of a conventional wet etchant using peracetic acid (PAA) or hydrogen peroxide (H2O2) as an oxidant, their SRP are ~1.75 and ~0.69 eV, respectively. However, its selectivity and etch rate of Si1-xGex films to Si films are insufficient at low Ge concentration of Si1-xGex despite the high SRP value of PAA and H2O2 because the bandgap energy of Si1-xGex increase with decreasing Ge concentration.
In our research, therefore, we suggested a novel oxidant, sodium periodate (NaIO4) to enhance the etch rate and the selectivity of wet etchant based on the concept of radical generation. We investigated the dependencies of Si-, Si0.7Ge0.3-film etch rate and selectivity on oxidant species, and dependencies of lateral etch rate of Si0.7Ge0.3/Si multi-stack structures on oxidant species. This was motivated by that NaIO4 generates radicals in acidic medium, such as iodyl (IO3
•) and reactive oxygen species (i.e., the superoxide radical (O2
•–) and hydroxyl (•OH) radical), as shown in Fig. 1. SRP of NaIO4 itself has relatively low (~1.60 eV) compared to PAA (~1.75 eV), IO3
•, O2
•– and •OH radicals generated from NaIO4 have higher SRP (O2
•–: ~2.80 eV and •OH: ~2.42 eV), which resulted in a higher etch rate at lower Ge concentration less than 30%.For the experiments, the Si0.7Ge0.3 layer grown on Si and 3-layer Si0.7Ge0.3/Si multi-stack wafers were grown by using ultra-high-vacuum chemical vapor deposition (EUREKA, JUSUNG ENGINEERING). Coupon wafers with 1.5 × 1.5 cm2 of Si0.7Ge0.3 and Si substrate (reference group), and patterned wafers of 3-layer Si0.7Ge0.3/Si multi-stack layers with a line width of 10 μm were prepared and then all samples were dipped in the etchant solutions for 1 min at room temperature. The thickness of the samples was characterized by using spectroscopic ellipsometry (V-VASE, J.A. Woollam). In addition, after etching of the 3-layer Si0.7Ge0.3/Si multi-stack structures, a lateral etch rate were characterized by using field-emissive scanning electron microscope (Helios 5 CX, Thermofisher).
It was confirmed that our novel etchant using NaIO4 as an oxidant demonstrated extremely high etch rate for Si0.7Ge0.3 film (140 nm/min), whereas conventional etchant using oxidant PAA and H2O2 demonstrated the etch rate of 5.7 and 2.4 nm/min respectively. In addition, the selectivity between the Si0.7Ge0.3- and Si films etched by using the NaIO4 oxidant was much higher (318:1) than that etched by PAA and H2O2 oxidant (29:1 and 6:1), as shown in Fig. 2, which sufficiently satisfied the requirement of etch rate (>30nm/min) and selectivity (300:1) for fabricating GAAFETs. The lateral etch rate of 3-layer Si0.7Ge0.3/Si multi-stack, mechanism on radical generation of NaIO4, and the characterization method to quantify radicals will be presented in detail.

Acknowledgement

This work was supported by the Technology Innovation Program (20022467, Development of Wet Etchant and Core Materials for Next Generation Semiconductor Manufacturing Process below 10 nm) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea)

Figure 1