Evaluation of the Si0.8Ge0.2-on-Si Epitaxial Quality by Inline Surface Light Scattering: A Case Study on the Impact of Interfacial Oxygen

Inline light scattering measurements are frequently used to determine wafer quality and cleanliness. In this paper we will show how this technique can be extended to determine the crystalline quality after hetero-epitaxy. Misfits on the surface of the epitaxially grown layer cause increased surface light scattering. The Si_0.8Ge_0.2-on-Si epitaxial quality has been evaluated by surface light scattering and correlated with the controlled variation of the interfacial oxygen between the Si substrate and epitaxially grown Si_0.8Ge_0.2.

SiGe-on-Si hetero-epitaxy is used for strain engineering in transistors. SiGe, Ge and III-V materials are also under consideration as transistor channel in future technology nodes. [1] With the current need for low thermal budget processing in combination with the move to non-planar device structures like FINFETs, oxygen contamination at the Si substrate - epitaxial SiGe interface which is known to degrade the SiGe crystalline quality, is receiving some renewed interest. [2] Dislocations formed during SiGe epitaxy can extend to the surface of the epitaxial layer where they will be present as surface steps. Surface imperfections can be detected by optical differential interference contrast (Nomarski) microscopy. Several other techniques are available and in use to inspect the strained epi layer e.g. HRXRD, TEM and lifetime measurements.

In this paper, we will show how hetero-epitaxial-crystalline quality can be evaluated using surface light scattering. Surface imperfections can also be detected by scanning laser light scattering and quantified using a hazeline algorithm present on the KLA-Tencor Surfscan SP3. Previously, this algorithm has been demonstrated to quantify CMP scratches. [3]

Figure 1a shows a 2D plot of the so-called haze component surface light scattering intensity of an epitaxial 80nm Si-cap/80nm Si_0.8Ge_0.2 bilayer grown on a 300mm Si substrate (ASM Epsilon 3200) measured by a KLA-Tencor Surfscan SP3. The epitaxial layer is grown on a Si substrate in a one-step process after a diluted HF last surface treatment followed by a 2 min H₂ bake at different temperatures. Optimal and non-optimal bake conditions have been used to obtain a controlled variation of the remaining oxide on the Si substrate surface on top of which the epitaxial layer is grown. The light scattering plot clearly shows the presence of defects that follow the crystallographic directions of the Si substrate. The interfacial oxygen correlates well with SiGe epitaxial quality as quantified by the hazeline algorithm. (see Fig. 1b)

In this paper we demonstrate the use of inline light scattering to quantify the epitaxial quality of Si_0.8Ge_0.2 grown on a Si substrate. Its advantages and its limitations will be explained.

 [1] M.L. Lee et al. Appl. Phys. Rev. 97 (2005) 011101

 [2] V. Machkaoutsan, ECS Transactions 50, (2012) 339

[3] B. Pinto et al. Yield Management Solutions (Spring 2007) 28-32

Figure 1: (a) 2D plot of the surface light scattering intensity. The notch of the 300 mm wafer is facing the bottom of the picture; and (b) interfacial oxygen (open symbols) between the epitaxial SiGe and Si substrate and hazeline area (closed symbols) as a function of the H₂ bake temperature.