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Use of SIMS in SiGe process control
Applied Surface Science 231–232 (2004) 713–715
Use of SIMS in SiGe process control J.L. Maula,*, Pei-Fen Choub, Y.H. Lub a
FEI Company, Bruckmannring 40, Oberschleissheim, Munich D-85764, Germany TSMC, 121 Park Avenue, Science-Based Industrial Park, Hsin-Chu, Taiwan, ROC
b
Available online 25 May 2004
Abstract Epitaxial processes are increasingly used in semiconductor technology to cope with the requirement of ultra thin layers and ultra shallow junctions. A variety of MBE and CVD tools are used for process development and in production. Monitoring of the processed EPI-wafers is required. SIMS is a method of choice for this task. Quantifiability and even more repeatability delivered by the SIMS tool are of key importance. This paper reports the use of the FEI SIMS 4500 with a special optical conductivity enhancement (OCE) device to monitor a SiGe process in industrial routine. # 2004 Elsevier B.V. All rights reserved. Keywords: FEI SIMS; SiGe; Process monitoring
1. Introduction
2. Instrument and method
SiGe-based HBT processes are making their way into mainstream at many semiconductor corporations. The SiGe-based hetero bipolar device is often embedded in the CMOS process. It has the advantage of higher unity gain cut-off frequencies compared with the Si-based Bi-CMOS process and can be integrated within the monolithic Si process. SiGe technology has become a major application for dynamic SIMS during the process development stage and beyond. Subjects of process control, among others, are quantitative Ge and B depth profiles (Fig. 1), because of their correlation to the bipolar performance figures such as Ft (cut-off frequency), Fmax (maximum oscillation frequency at unity gain), BVceo (collector–emitter breakdown voltage) [1].
The quantitative Ge and B depth profiling can be done with normal incidence oxygen primary ions and low energy analysing beam energy. [2]. These measuring conditions are best for high depth resolution profiling of the B co-doping. And, under these conditions the Ge concentration is directly proportional to the Ge secondary ion intensity in the entire concentration range of interest [2]. No Ge concentration dependent calibration factors need to be applied to convert the measured Ge intensity to Ge fraction percentage, as it is required when using Cs primary ions. In other words, the relative sensitivity factor Ge intensity/Si intensity in the bulk is constant. This has been verified for this paper using 1 keV and 500 eV normal incidence oxygen primary ions. Three different samples with four and five layers of RBS quantified Ge concentration levels have been measured with three different SIMS tools. The relative sensitivity factors shown in Fig. 2 are constant with a relative standard deviation (R.S.D.) of 1.45%. Hence, the
*
Corresponding author. Tel.: þ49-89-315-891-23; fax: þ49-89-315-59-21. E-mail address: [email protected] (J.L. Maul).
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.199
714
J.L. Maul et al. / Applied Surface Science 231–232 (2004) 713–715
Fig. 1. Subjects of SiGe process control.
SiGe: RSF Ge / Si-bulk FEI SIMS4550
rel. RSF
1.10 1.05
TSMC SIMS4500
1.00 0.95
FEI SIMS4500
0.90 0
8
16
24
Ge %
Fig. 2. Normal incidence oxygen analysing beam delivers Ge concentration independent RSFs.
measured Ge intensities are directly proportional to the Ge fraction. Likewise, this holds for the Si intensity/concentration relation [2]. The Ge fraction equals (100%—Si fraction percentage). However, under these measuring conditions the sample surface is fully oxidised and a surface potential change may occur in the course of depth profiling which would make accurate and precise measurements impossible. To avoid such artefacts an optical conductivity enhancement (OCE) device (Fig. 3) has been developed by FEI-Atomika [2]. Absence of charging can easily be verified by monitoring the primary ion beam current measured directly on the sample. The linear scale overlay of three Ge profiles from the same sample in Fig. 3 demonstrates the precision obtained with a red colour laser OCE.
3. Results Data of both blank and patterned B-doped SiGe wafers are presented in this work. LP CVD has been
Fig. 3. With the OCE device a distortion of depth profiles is avoided: (a) two distorted depth profile sets measured without OCE and an undistorted one measured with OCE; (b) linear scale overlay of Ge profiles measured with OCE.
used to grow the SiGe epi on Si substrates. The layer structure consists of a near 12% SiGe layer deposited on the Si substrate, followed by a B-doped, graded SiGe layer topped by a B-doped Si cap. The wafers have been analysed with a FEI-Atomika SIMS 4500, installed at TSMC using 1 keVoxygen beam at normal incidence. Figs. 4–7 show the repeatability of Ge plateau concentration, B-peak concentration and B-peak depth
J.L. Maul et al. / Applied Surface Science 231–232 (2004) 713–715 Relative Ge Concentration 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Relative B-peak Concentration 1.2 1.0 0.8 0.6 0.4 0.2 0.0
test 1 test 2 RSD: 2.8% 1
5
3
9 7 sample #
11
13
RSD: 4.5%
1
15
2
3
4
5
6
7
8
9
10 11
sample #
Fig. 4. Same day repeatability of germanium plateau concentration. Note: R.S.D. 2.8% equals to <0.5% of Ge atoms.
Fig. 6. Day-to-day repeatability of boron peak concentration.
Boron Peak Position [arbu]
Relative Ge Concentration 1.2 1.0 0.8 0.6 0.4 0.2 0.0
715
20 16 12 8
RSD: 4.6%
RSD: 0.4%
4 0 1
2
3
4
5 6 7 sample #
8
9
10 11
1
2
3
4
5
6
7
8
9
10 11
sample #
Fig. 5. Day-to-day repeatability of germanium plateau concentration.
Fig. 7. Day-to-day repeatability of boron peak depth position. Note: R.S.D. 0.4% is <0.2 nm.
position, respectively. Note: R.S.D. includes R.S.D. of sample composition and R.S.D. of measurement.
SIMS with normal incidence oxygen low energy primary ion beam in combination with OCE can reliably monitor an important step in the SiGe production process.
4. Conclusions It has been shown that in this process control application the Ge concentration is measured with a precision of better 0.5% atom. The repeatability of the B-peak concentration is better 5%. The B-peak depth position precision is 0.2 nm. It is fair to conclude that
References [1] B.S. Meyerson, IBM J. Res. Dev. 44 (2000) 3. [2] M.Dowsett, et al., in: Proceedings of the SIMS XIII, Appl. Surf. Sci. 500 (2003) 203–204.
Use of SIMS in SiGe process control J.L. Maula,*, Pei-Fen Choub, Y.H. Lub a
FEI Company, Bruckmannring 40, Oberschleissheim, Munich D-85764, Germany TSMC, 121 Park Avenue, Science-Based Industrial Park, Hsin-Chu, Taiwan, ROC
b
Available online 25 May 2004
Abstract Epitaxial processes are increasingly used in semiconductor technology to cope with the requirement of ultra thin layers and ultra shallow junctions. A variety of MBE and CVD tools are used for process development and in production. Monitoring of the processed EPI-wafers is required. SIMS is a method of choice for this task. Quantifiability and even more repeatability delivered by the SIMS tool are of key importance. This paper reports the use of the FEI SIMS 4500 with a special optical conductivity enhancement (OCE) device to monitor a SiGe process in industrial routine. # 2004 Elsevier B.V. All rights reserved. Keywords: FEI SIMS; SiGe; Process monitoring
1. Introduction
2. Instrument and method
SiGe-based HBT processes are making their way into mainstream at many semiconductor corporations. The SiGe-based hetero bipolar device is often embedded in the CMOS process. It has the advantage of higher unity gain cut-off frequencies compared with the Si-based Bi-CMOS process and can be integrated within the monolithic Si process. SiGe technology has become a major application for dynamic SIMS during the process development stage and beyond. Subjects of process control, among others, are quantitative Ge and B depth profiles (Fig. 1), because of their correlation to the bipolar performance figures such as Ft (cut-off frequency), Fmax (maximum oscillation frequency at unity gain), BVceo (collector–emitter breakdown voltage) [1].
The quantitative Ge and B depth profiling can be done with normal incidence oxygen primary ions and low energy analysing beam energy. [2]. These measuring conditions are best for high depth resolution profiling of the B co-doping. And, under these conditions the Ge concentration is directly proportional to the Ge secondary ion intensity in the entire concentration range of interest [2]. No Ge concentration dependent calibration factors need to be applied to convert the measured Ge intensity to Ge fraction percentage, as it is required when using Cs primary ions. In other words, the relative sensitivity factor Ge intensity/Si intensity in the bulk is constant. This has been verified for this paper using 1 keV and 500 eV normal incidence oxygen primary ions. Three different samples with four and five layers of RBS quantified Ge concentration levels have been measured with three different SIMS tools. The relative sensitivity factors shown in Fig. 2 are constant with a relative standard deviation (R.S.D.) of 1.45%. Hence, the
*
Corresponding author. Tel.: þ49-89-315-891-23; fax: þ49-89-315-59-21. E-mail address: [email protected] (J.L. Maul).
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.199
714
J.L. Maul et al. / Applied Surface Science 231–232 (2004) 713–715
Fig. 1. Subjects of SiGe process control.
SiGe: RSF Ge / Si-bulk FEI SIMS4550
rel. RSF
1.10 1.05
TSMC SIMS4500
1.00 0.95
FEI SIMS4500
0.90 0
8
16
24
Ge %
Fig. 2. Normal incidence oxygen analysing beam delivers Ge concentration independent RSFs.
measured Ge intensities are directly proportional to the Ge fraction. Likewise, this holds for the Si intensity/concentration relation [2]. The Ge fraction equals (100%—Si fraction percentage). However, under these measuring conditions the sample surface is fully oxidised and a surface potential change may occur in the course of depth profiling which would make accurate and precise measurements impossible. To avoid such artefacts an optical conductivity enhancement (OCE) device (Fig. 3) has been developed by FEI-Atomika [2]. Absence of charging can easily be verified by monitoring the primary ion beam current measured directly on the sample. The linear scale overlay of three Ge profiles from the same sample in Fig. 3 demonstrates the precision obtained with a red colour laser OCE.
3. Results Data of both blank and patterned B-doped SiGe wafers are presented in this work. LP CVD has been
Fig. 3. With the OCE device a distortion of depth profiles is avoided: (a) two distorted depth profile sets measured without OCE and an undistorted one measured with OCE; (b) linear scale overlay of Ge profiles measured with OCE.
used to grow the SiGe epi on Si substrates. The layer structure consists of a near 12% SiGe layer deposited on the Si substrate, followed by a B-doped, graded SiGe layer topped by a B-doped Si cap. The wafers have been analysed with a FEI-Atomika SIMS 4500, installed at TSMC using 1 keVoxygen beam at normal incidence. Figs. 4–7 show the repeatability of Ge plateau concentration, B-peak concentration and B-peak depth
J.L. Maul et al. / Applied Surface Science 231–232 (2004) 713–715 Relative Ge Concentration 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Relative B-peak Concentration 1.2 1.0 0.8 0.6 0.4 0.2 0.0
test 1 test 2 RSD: 2.8% 1
5
3
9 7 sample #
11
13
RSD: 4.5%
1
15
2
3
4
5
6
7
8
9
10 11
sample #
Fig. 4. Same day repeatability of germanium plateau concentration. Note: R.S.D. 2.8% equals to <0.5% of Ge atoms.
Fig. 6. Day-to-day repeatability of boron peak concentration.
Boron Peak Position [arbu]
Relative Ge Concentration 1.2 1.0 0.8 0.6 0.4 0.2 0.0
715
20 16 12 8
RSD: 4.6%
RSD: 0.4%
4 0 1
2
3
4
5 6 7 sample #
8
9
10 11
1
2
3
4
5
6
7
8
9
10 11
sample #
Fig. 5. Day-to-day repeatability of germanium plateau concentration.
Fig. 7. Day-to-day repeatability of boron peak depth position. Note: R.S.D. 0.4% is <0.2 nm.
position, respectively. Note: R.S.D. includes R.S.D. of sample composition and R.S.D. of measurement.
SIMS with normal incidence oxygen low energy primary ion beam in combination with OCE can reliably monitor an important step in the SiGe production process.
4. Conclusions It has been shown that in this process control application the Ge concentration is measured with a precision of better 0.5% atom. The repeatability of the B-peak concentration is better 5%. The B-peak depth position precision is 0.2 nm. It is fair to conclude that
References [1] B.S. Meyerson, IBM J. Res. Dev. 44 (2000) 3. [2] M.Dowsett, et al., in: Proceedings of the SIMS XIII, Appl. Surf. Sci. 500 (2003) 203–204.