Materials Science in Semiconductor Processing 4 (2001) 229–231

Process control of Si/SiGe heterostructures by X-raydiffraction

Tom Ryan

Philips Analytical, 12 Michigan Drive, Natick, MA 01760, USA

Abstract

The introduction of a silicon–germanium epitaxial layer in the base of a bipolar transistor brings about significantgains in speed. SiGe heterojunction bipolar transistors (HBTs) are now challenging GaAs in its traditional strongholdof wireless communications. But this new process step introduces new requirements for control of the Ge content and

profile. X-ray diffraction (XRD) has been extensively used to characterize SiGe during it’s research and developmentphase but, until now, it was considered too difficult and time consuming for use in a production environment. Recentadvances in analytical software, however, allow fast and reliable extraction of layer thickness, germanium content andgrading profile. XRD process control tools are currently being installed in a number of SiGe fab lines. # 2001

Published by Elsevier Science Ltd.

1. Introduction

Si/SiGe heterojunction bipolar transistors (HBTs) arenow challenging GaAs ICs in their traditional strong-

hold of wireless and high-speed data communicationsapplications. Commercial products are now availablefrom, amongst others, IBM and Temic (Atmel). More

than 30 commercial sites in North America are activelyinvolved in development or incorporation of SiGeprocesses. SiGe is the first heteroepitaxial process to beincorporated in a high-volume Si Fab environment and,

as such it introduces new characterization challenges.Heteroepitaxial structures are common in the III–Vindustry and X-ray diffraction (XRD) characterization

techniques are well established for these materials. Inprinciple, the transfer of those techniques to a siliconenvironment should be quite straightforward. However,

SiGe HBTs present additional challenges. The SiGe baseregion is thin, less than 40 nm, and can be graded incomposition. Typically, the Ge concentration can vary

from 0% at the emitter–base junction to 15% at thebase–collector junction. The base–collector regions arealso B-doped. Tolerances on thickness and compositionare tight and the epitaxial growth process is tricky.

There is a requirement for a characterization method

that is accurate (i.e. gives an absolute measure of both

composition and thickness), precise, suitable for use onpatterned product wafers and fast. No single techniquecan fulfill all of the above requirements } and a

combination of analytical tools is required for fullcharacterization. XRD plays an important role }

providing an absolute and unambiguous measure of

the SiGe grading profile.

2. XRD applied to SiGe heterostructures

XRD is unique amongst laboratory (or fab) scale

analytical techniques in that it provides a highlyaccurate measure of the lattice parameter (or d-spacing)of the crystal being probed. The fundamental relation-ship of XRD, Bragg’s Law nl ¼ 2d sin y, shows that thelattice parameter (d) can be obtained by measuring y,the angle through which an X-ray beam is diffracted. Inthe case of a non-graded, binary-compound SiGe

heterostructure there is a direct and unambiguousrelationship between the lattice parameter measured byXRD and the composition of the epitaxial layer. An

XRD measurement (or rocking curve) from an un-graded (box) structure is shown in Fig. 1a.

1369-8001/01/$ - see front matter # 2001 Published by Elsevier Science Ltd.

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The rocking curve is a measure of the diffractedintensity (I) against diffraction angle (y). The non-

graded structure shows two distinct features, the sharp,intense Bragg peak from the Si substrate and a broad,weak diffraction peak from the SiGe epitaxial layer.

Since the lattice parameter (d) of silicon is well known,the silicon peak acts as an internal reference. Differencesin lattice parameter (Dd) are obtained by measuring thedifference from the substrate peak (Dy). The separation

of the peaks (Dy) is related to the germaniumconcentration of the layer while the shape of the peakcontains information on the layer thickness.

The rocking curve in Fig. 1b is from a full SiGe HBTstructure consisting of a SiGe graded layer and cappedby a layer of silicon. The spectrum is more complicated.

The layer peak is asymmetric and there is a visiblemodulation of the intensity (a result of interference of X-rays diffracted by the silicon cap layer). Quantitative

interpretation is not straightforward. However, a newgeneration of analysis programs has recently emerged(e.g. Philips ‘‘Auto-fit’’ dynamical scattering simulationand fitting software), enabling an unambiguous mea-

surement of layer composition and thickness fromcomplex structures.

A comparison between a measured and simulated(fitted) rocking curve is shown in Fig. 2. The structureconsists of a graded layer where the composition varies

linearly from 26 to 0% over a thickness of 46 nm. Thereis a Si cap of thickness 82.5 nm. The time to acquire therocking curve was 5min and the fitting time was also inthe region of 5min. All of the major features of the

experimental rocking curve are reproduced in thesimulation } indicating that the model is, indeedaccurate. The simulation is shifted vertically for clarity.

The precision of the XRD analysis was tested bymaking 20 measurements at the center of a wafer,unloading and loading between measurements (Table 1).

The data were fitted using the ‘‘Auto-fit’’ program with aconsistent set of starting parameters.XRD, then, offers a very accurate, precise method of

composition and thickness analysis for graded SiGelayers. However, XRD is unable to measure the borondoping profile and, with an illuminated area in theregion of a few mm2, XRD is unsuitable for measure-

Fig. 1. (a) Rocking curve of single, non-graded SiGe epitaxial layer, (b) rocking curve of SiGe graded-layer structure.

T. Ryan / Materials Science in Semiconductor Processing 4 (2001) 229–231230

ment on patterned wafers. Table 2 gives an overview ofsome of the most important characteristics of several ofthe relevant analytical techniques.

3. Conclusion

XRD’s strengths lie in its ability to provide anabsolute, calibration free, accurate and precise measureof the germanium profile. Data acquisition and inter-

pretation can be fully automated and the tool is suitablefor use by low-skilled operators. The weaknesses ofXRD lie in its inability to measure the boron-doping

profile and its large illuminated area } making itunsuitable for work on patterned wafers. Although thereare X-ray optical configurations available giving sub 500

micron spot sizes, there remain unresolved trade-offsbetween spot-size, sensitivity and data acquisition time.Spectroscopic ellipsometry has emerged as the mostpromising technique for on-wafer measurement }

although it’s ability to give information on the grade isvery limited. SIMS is the standard technique forobtaining a full composition profile } but it is slow,

destructive and expensive. XRD can replace SIMS as anin-fab tool for accurate measurement of the Ge profile ingraded structures.

Fig. 2. Rocking curve (blue) and fitted simulation (green).

Table 1

Accuracy and precision of XRD analysis

Composition (top of grade) (%) SiGe thickness (%) Cap thickness (%)

Accuracy 1 3 3

Precision 3s (%) 0.2 1 0.5

Table 2

Comparison of some of the most important attributes of SiGe characterization tools

XRD XTEM Spectroscopic ellipsometry SIMS

Absolute method Yes Yes No No

Measure grading profile Yes No No Yes

Thickness precision 3s 0.5% 2% 1% 10%

Composition precision 3s 0.4% } 6% 10%

Probed area 2mm2 } 100mm 100mmThroughput (wafers/h, 5 sites/wafer) 2wafers/h } 5wafers/h }

Boron profile No No No Yes

Uniformity mapping Yes No Yes No

T. Ryan / Materials Science in Semiconductor Processing 4 (2001) 229–231 231