An Assessment of the Lateral Selective Etching, with HCl, of High Versus Low Ge Content SiGe Layers
Authors: Hartmann Jean-Michel , Previtali Bernard , Aussenac Francois , Dezest Pattamon , Glorieux Olivier , Blot-Saby Adrien , Couture Ludovic , Barraud Sylvain 
Published: December 2024

We have evaluated the feasibility of performing, with gaseous HCl, a lateral selective etching of high versus low Ge content SiGe layers. Such a know-how might be useful to fabricate, besides Complementary Field Effect Transistors, other types of devices relying on stacked nanosheets and multiple selective etchings such as 3D Resistive or Oxyde Random Access Memories (for In-Memory Computing).
To that end, we have grown {20 to 25 nm thick Si / ~ 12-15 nm thick Si0.8Ge0.2 or Si0.75Ge0.25 / ~ 11-14 nm thick Si0.5Ge0.5} multilayers on SOI substrates. After a low temperature capping with SiN, such stacks were patterned into regular arrays of square mesas or lines with a stop on the buried oxide, to laterally have access to the layers of interest. The temperature used, in our 300 mm Reduced Pressure - Chemical Vapor Deposition chamber, to grow such stacks was 550°C. Meanwhile, the in-situ HCl etch temperature was always 500°C. The aim of such low process temperatures was to minimize the thermal budget multilayers were submitted to, in order to minimize elastic relaxation and avoid plastic relaxation while still having reasonable growth and etch rates. The overall compressive strain was indeed high in such stacks given the number, individual thicknesses and Ge concentrations of SiGe layers.
Multilayers grown at 550°C, 20 Torr with (i) a SiH2Cl2 + GeH4 chemistry for Si0.5Ge0.5, (ii) a Si2H6 +GeH4 chemistry for Si0.75Ge0.25 or Si0.8Ge0.2 and (iii) pure Si2H6 for Si were in some places defective, with the presence on the surface of islands. Such defects were likely due to Si2H6 mass-flows which were too large, resulting in gas phase nucleation and, in some places, the formation of defects that increased in size as growth happened. We thus proceeded differently. Instead of Si2H6 + GeH4, we have used a SiH4 + GeH4 chemistry for the epitaxy of SiGe 20% or 25% layers. We succeeded in having reasonable SiGe 20% and 25% growth rates at 550°C, 20 Torr, which was not a given with that chemistry. We also showed that a x/(1-x) = 2.81*F(GeH4)/F(SiH4) relationship perfectly accounted for the almost linear increase of the Ge concentration x with the F(GeH4)/F(SiH4) mass-flow ratio. We otherwise reduced the Si2H6 mass-flow and added HCl to minimize gas phase nucleation and suppress the formation of amorphous inclusions in the Si layers. The linear dependency of the 550°C, 20 Torr Si growth rate on the Si2H6 and HCl mass-flows was modelled with the following phenomenological relationship: Si Growth Rate (nm min.-1) = 3260*F(Si2H6)/F(H2) – 1070*F(HCl)/F(H2).
We then determined, at 500°C, 600 Torr, the (001) blanket HCl etch rates of SiGe 30% and 40%: 1.37 and 5.28 nm min.-1. Based on such data, we should have the following SiGe 50%, 25% and 20% etch rates: 20.0, 0.70 and 0.36 nm min.-1, respectively. Meanwhile, the pure Si etch rate should be negligible: 0.025 nm min.-1 only. Should we then define theoretical selectivities as being ratios between (i) SiGe 50% and (ii) SiGe 25%, SiGe 20% or pure Si blanket (001) etch rates, we would obtain promising selectivities (29, 56 and 800) when laterally etching of SiGe 50% versus SiGe 25% SiGe 20% and Si.
We then switched over to patterned wafers. The lateral selective etching of Si0.5Ge0.5 versus Si0.8Ge0.2 (and Si) was feasible. The following features were highlighted: (i) Si0.5Ge0.5 lateral etch rates along the <110> directions were much smaller than in the [001] direction and (ii) because of micro-loading, lateral etch rates were definitely higher for squares than for lines (0.9 - 1.1 nm min.-1 for lines, to be compared with 1.7 – 2.1 nm min.-1 for squares). Meanwhile, the lateral selective etching of Si0.5Ge0.5 versus Si0.75Ge0.25 was far from being perfect, with much higher Si0.5Ge0.5 etch rates and a significant Si0.75Ge0.25 recessing at tunnel entrances (see Figure 1). Si0.75Ge0.25 floors and ceilings were otherwise far more etched and thus tilted than Si0.8Ge0.2 ones. Inserting very thin Si spacers between Si0.5Ge0.5 cores and Si0.75Ge0.25 claddings partly solved those issues, with no recessing and lesser etchings at the entry points of tunnels, then (see Figure 2).

Figure 1