2. Strain Engineering

The influence of strain on the intrinsic mobility of Si was first investigated in the early 1950's [Hall51,Smith54]. While this effect was not exploited initially, the idea was revived in the early 1990's [Fitzgerald91]. In 1992 it was first demonstrated that n-channel MOSFETs on a strained Si substrate exhibit a 70% higher effective mobility ( $\mu_\mathrm{eff}$) than those on unstrained substrates [Welser92,Welser94]. Ever since semiconductor industry has adopted several different technologies to introduce strain in the channel of a MOSFET.

Strain technologies are based on mechanically stretching and/or compressing the Si crystal lattice by various means. Innovative techniques introducing stress in the Si channel have been developed, which require only small modifications of some process steps, thus keeping the additional costs small. At the same time the integrability of strained Si in the CMOS manufacturing process flow is retained.

A classification of strain techniques into two categories is possible. Strain is introduced across the entire substrate in global strain techniques, whereas local techniques induce strain in selected regions of the wafer. Some of the most prominent strain technologies that are currently used in industry are given in Figure 2.1. A key challenge of all technologies is their ability to be integrated into the CMOS manufacturing processes and to avoid significant increase in processing costs. In this chapter several strain technologies and the current progress in high-mobility strained MOSFETs are reviewed.

Figure 2.1: Classification of strain technologies.

Subsections

• 2.1 Global Strain Techniques
• 2.2 Local Strain Techniques
  • 2.2.1 Contact Etch Stop Liner Technique
  • 2.2.2 Stress Memorization
  • 2.2.3 Selective Epitaxial Growth Technique
  • 2.2.4 Stress from Shallow Trench Isolation
  
  
• 2.3 Strain Technologies used in High Volume Production
  • 2.3.1 Hybrid Orientation Technology

E. Ungersboeck: Advanced Modelling Aspects of Modern Strained CMOS Technology