SiGe HBT Technology

Introduction

The concept of “bandgap engineering” has been used for many years in compound semiconductors such as gallium arsenide (GaAs) and indium phosphide (InP) to realize a host of novel electronic devices. A bandgap-engineered transistor is compositionally altered, using more than one type of semiconductor, in a manner which improves a specific device metric of interest (e.g., speed). A transistor designer might choose, for instance, to make a bipolar transistor which has a GaAs base and collector region, but which also has a AlGaAs emitter. Such a “heterostructure” device has electrical properties which are inherently superior to what could be achieved using a single semiconductor. In addition to simply combining two different materials (e.g., AlGaAs and GaAs), bandgap engineering often involves compositional grading of materials from point A to point B within a device. For instance, one might choose to vary the Al content in an AlGaAs/GaAs transistor from a mole fraction of 0.4–0.6 across a given distance within the emitter region.

Device designers have long sought to combine such bandgap engineering techniques enjoyed in compound semiconductor technologies with the fabrication maturity, high integration levels, high yield, and hence low cost associated with conventional silicon (Si)-integrated circuit manufacturing. Epitaxial silicon–germanium (SiGe) alloys offer considerable advantages for realizing viable bandgap-engineered transistors in the Si material system, either via compressively strained SiGe layers, tensilely strained Si layers, or some combi- nation of the two. Such Si-based bandgap engineering is an exciting development because it allows Si-based electronic devices to achieve performance levels which were once thought impossible, and thus dramatically extends the number of high-performance applications that can be addressed using low-cost Si-based technology. This chapter reviews the recent progress in both SiGe heterojunction bipolar transistor (HBT) technology.

SiGe Strained Layer Epitaxy

Si and Ge, being chemically compatible elements, can be intermixed to form a stable alloy. Unfortunately, however, the lattice constant of Si is about 4.2% smaller than that of Ge. The difficulties associated with realizing viable SiGe bandgap-engineered transistors can be traced to the problems encountered in growing high-quality, defect-free epitaxial SiGe alloys in the presence of this lattice mismatch. For electronic applications it is essential to obtain a SiGe film which adopts the same lattice constant as the underlying Si substrate with perfect alignment across the growth interface. In this case, the resultant SiGe alloy is under compressive strain. This strained SiGe film is thermodynamically stable only under a narrow range of conditions which depends on the film thickness and the effective strain (determined by the Ge fraction, and typically expressed in % Ge) [1]. The critical thickness below which the grown film is unconditionally stable depends reciprocally on the effective strain (Figure 4.1). Thus, for practical electronic device applications, SiGe alloys must be thin (typically <100–150 nm) and contain only modest amounts of Ge (typically <20–30%). It is essential for electronic devices that the SiGe films remain thermodynamically stable so that conventional Si fabrication techniques such as high-temperature annealing, oxidation, and ion implantation can be employed without generating defects, thereby main- taining complete compatibility with conventional Si manufacturing.

From an electronic device viewpoint, the property of the strained SiGe alloy that is most often exploited in bipolar transistors is the reduction in bandgap with strain and Ge content (~75 meV per 10% Ge) [2]. This band offset appears mostly in the valence band, which is particularly useful for realizing npn SiGe HBTs and p-channel FETs [3]. While these band offsets are modest compared with those that can be achieved in III–V semiconductors, the Ge content can be compositionally graded to produce local electric fields which aid carrier transport. For instance, in a SiGe HBT the Ge content might be graded from 0 to 15% across distances as short as 50–60 nm, producing built-in drift fields as large as 15–20 kV/cm. Such fields can rapidly accelerate the carriers to scattering limited velocity (1 ´ 107 cm/s), thereby improving the transistor frequency response. Another benefit of using SiGe strained layers is the enhancement in carrier mobility. This advantage will be exploited in SiGe channel FETs as discussed below.

Epitaxial SiGe strained layers on Si substrates can be successfully grown today by a number of different techniques, including molecular beam epitaxy (MBE), ultra-high-vacuum/chemical vapor deposition (UHV/CVD), rapid-thermal CVD (RTCVD), and reduced-pressure CVD (RPCVD). Each growth technique has advantages and disadvantages, but it is generally agreed that UHV/CVD [4], has a number of appealing features for the commercialization of SiGe integrated circuits. These features of UHV/CVD which make it particularly suitable for SiGe manufacturing include (1) batch processing on up to 16 wafers

FIGURE 4.1 Silicon–germanium film thickness versus average Ge fraction. The figure shows theoretical stability curves for varying emitter cap thickness (Fischer’s theory) and data points for representative stable (Ge 1 and Ge 2) and metastable (Ge 3) profiles. (From J.D. Cressler, Silicon Heterostructure Handbook: Materials, Fabrication, Devices, Circuits, and Applications of SiGe and Si Strained Layer Epitaxy, Boca Raton, FL: CRC Press, 2006. With permission.)

simultaneously, (2) excellent doping and thickness control on large (e.g., 200 mm) wafers, (3) very low background oxygen and carbon concentrations, (4) compatibility with patterned wafers and hence conventional Si bipolar and CMOS fabrication techniques, and (5) the ability to compositionally grade the Ge content in a highly controllable manner across short distances. The experimental results presented in this chapter are based on the UHV/CVD growth technique as practiced at IBM Corporation, and are representative of the state of the art in SiGe technology.