Development of advanced plasma process with an optical emission spectroscopy-based end-point technique for etching of AlGaAs over GaAs in manufacture of heterojunction bipolar transistors

 

 

J. W. Lee, Y. J. Son, M. H. Jeon and G. S. Cho

Dept. of Optical Eng., Inje University, Kimhae, Kyoung-nam, 621-749, Korea (ROK).

 

H. C. Yim, S. K. Chang and K. K. Kim

Knowledge*On, Inc., Iksan, Jeon-nam, 570-210, Korea (ROK)

 

M. Devre, D. Johnson and J. N. Sasserath

Unaxis, Inc., St. Petersburg, FL 33716,USA

 

S. J. Pearton

Dept. of Materials Sci. and Eng. University of Florida, Gainesville, FL 32611,USA

 

ABSTRACTS

We demonstrated an advanced plasma etching technology for AlGaAs over GaAs in a BCl3/N2 inductively coupled plasma using optical emission spectroscopy for endpoint detection. The etch rates of GaAs and AlXGa1-XAs (x = 0.2) were equal under our conditions, i.e. selectivity of AlGaAs over GaAs was 1:1. The process also provided very smooth surface morphology, vertical sidewall and residue-free surface. All the results indicated that the process would significantly improve reproducibility and reliability for the plasma process in manufacture of AlGaAs/GaAs-based such as heterojunction bipolar transistors.

 

 

 

INTRODUCTION

             Advanced III-V compound semiconductor devices, such as AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are increasingly required in the rapidly growing tele-communication industry. In manufacturing of HBTs, the GaAs/AlGaAs/GaAs structure is still popular in mass production,[1-2] while other employ a GaAs/InGaP/GaAs configuration.[3] For the first structure, one issue for plasma etching is to have a reliable process for etching of AlGaAs over GaAs. Etching of GaAs over AlGaAs is also important. Recently, Lee et al. published a selective etch process of GaAs over AlGaAs in an inductively coupled plasma.[4] The process is commonly employed for manufacturing of GaAs-based devices. However, etching of AlGaAs over GaAs is still an issue.[5]  There has been no report on a practical plasma chemistry for selective etching of AlGaAs over GaAs. Precise control of the etch-stop process is extremely important for process reproducibility and device reliability. Automatic control of the process is another issue for mass production. In order to overcome these difficulties, it is important to develop reliable in-situ end-point technology during etching of AlGaAs over GaAs. Another advantage of using an end-point detection is to support monitoring the manufacturing process. It can help identify which step is the origin of the issue if there is a problem for process reliability. Therefore, it will be very useful if there is a good technique for in-situ end-point detection during plasma etching. 

          Dry etching with the BCl3/N2 chemistry is an attractive choice for etching of AlGaAs/GaAs-based devices.[6-7]  It is found that interaction of radicals of BCl3 and N2 with photoresist provides excellent sidewall passivation during etching of GaAs-based materials.[4] In HBT manufacturing, realization of vertical sidewalls is important for further processing, such as dielectric passivation of the sidewall.

          Optical emission spectroscopy can be an excellent solution as an in-situ end-point technique for AlGaAs/GaAs plasma etching, if there is a clear difference of intensity of the monitoring peak when the underlying GaAs layer is exposed.[8] Using an OES technique provides convenience of wafer loading and no sacrifice of real estate on the valuable substrate, which are significant differences compared with using laser reflectometry. In this paper, we will report a success of in-situ end-point monitoring with optical emission spectroscopy during AlGaAs/GaAs etching in a BCl3/N2 inductively coupled plasma.

          A Unaxis’ SLR 770 inductively coupled plasma reactor was used for etching of AlGaAs/GaAs. Gas flows of BCl3 and N2 were controlled by electronic mass flow controllers. Chamber pressure was kept at 5 mTorr. Backside He flow was controlled in order to maintain a constant He pressure for wafer cooling during the process. Selectivity of AlGaAs to GaAs etch rate in the BCl3/N2 plasma was measured at 1:1 at a separate experiment at the process condition. A full size of 4 inch (100 mm) epi-wafer was used. Al composition was about 20% in AlGaAs and pattern density of photoresist on the substrates was about 50 %. Optical emission spectroscopy was used for detection of plasma emission light. Ga emission peak (wave length is 417 nm) was monitored during etching. Sidewall anisotropy and surface morphology of the sample was examined with scanning electron microscopy (SEM) after etching.

             Figure 1 shows a Ga peak (light emission wave length is 417 nm) intensity change and a slope of the intensity as a function of time during AlGaAs/GaAs etching. Notice that the intensity was significantly changed when the underlying GaAs was exposed to the plasma. Note also that the slope of the Ga intensity showed its maximum when the GaAs layer was exposed to the plasma, which would make it easier to recognize the end-point signal during the process. With this OES data, it is not difficult to make a sequence for automatic control of etch depth during AlGaAs/GaAs etching, which would significantly improve reproducibility of the process. In-situ monitoring of plasma process is very important to trace not only etch depth during etching but also to understand wafer history in previous steps, such as thickness of AlGaAs epitaxial layer or surface contamination during wafer cleaning. The in-situ end-point technique provides a more reliable process time and also helps compensate for thickness variations in the AlGaAs epitaxial layer. The intensity of the raw Ga peak continuously increased during AlGaAs etching. We speculate that the constant increase of the Ga peak intensity is due to the presence of accumulated residual Ga by-products in the plasma and re-etching of the residuals on the chamber wall during AlGaAs etching rather than actual etch rate change of AlGaAs in the wafer. The actual etch rate of the AlGaAs is generally constant with time during ICP etching with a He backside cooling. The etch rate of AlGaAs was 1800 Å/min. Notice that the slope of the raw signal is quite constant and flat until the GaAs layer was exposed to the plasma.

Figure 2 shows an SEM photo after etching of the AlGaAs/GaAs layer in BCl3/N2 plasma. Note that the etched surface was extremely clean and smooth. Notice also that sidewall of the AlGaAs/GaAs layer was quite vertical. Photoresist was still in place on the AlGaAs layer. Selectivity of etch rate of AlGaAs to the photoresist was about 4:1 at the process condition.

In summary, it was possible to have a clear end-point signal with a change of Ga peak intensity when the underlying GaAs was exposed during AlGaAs/GaAs ICP etching. The OES and SEM results showed that the BCl3/N2 ICP etch chemistry is an excellent choice for AlGaAs over GaAs etching. The results will also accelerate utilization of the OES end-point method in advanced manufacturing of AlGaAs/GaAs-based heterojunction bipolar transistors.

 

ACKNOWLEDGMENTS

             This work is supported by the 2000 Inje University research grant. The work at UF is partially supported by a DOD MURI, monitored by AFOSR (P. Trulove), contract no. F49620-96-1-0026

REFERENCES

1.      see for example Current Trends in heterojunction bipolar transistors, ed. M. F. Chang. World Scientific, MA (1996).

2.      see for example P. M. Asbeck, Chapter 6 in High-Speed Semiconductor Devices, ed. S. M. Sze, John Wiley & Sons, Inc, NY (1990)

3.      P. Leerungnawarat, H. Cho, D. C. Hays, J. W. Lee, M. W. Devre, B. H. Reelfs, D. Johnson, J. N. Sasserath, C. R. Abernathy and S. J. Pearton, “ Selective Dry Etching of InGaP over GaAs in Inductively Coupled Plasmas” J. Electronic Materials, 29 586 (2000).

4.      J. W. Lee, M. W. Devre, B. H. Reelfs, D. Johnson, J. N. Sasserath, F. Clayton, D. C. Hays and S. J. Pearton, “Advanced Selective Dry Etching of GaAs/AlGaAs in High Density Inductively Coupled Plasmas” J. Vac. Sci. Technol. A 18 1220 (2000).

5.      S. Salimian and C. B. Cooper III, “Equal Rate and Anisotropic Reactive Ion Etching of GaAs/AlGaAs Heterostructures in SiCl4 Plasma”, J. Electrochem. Soc. 136 2420 (1989).

6.      J. W. Lee, J. Hong, E. S. Lambers, C. R. Abernathy, S. J. Pearton, W. S. Hobson and F. Ren, “Plasma Etching of III-V Semiconductors in BCl3 Chemistries: Part I: GaAs and Related Compounds” Plasma Chem. Plasma Proc. 17 155 (1997).

7.      T. Maeda, J. W. Lee, R. J. Shul, J. Han, J. Hong, E. S. Lambers, S. J. Pearton, C. R. Abernathy, W. S. Hobson, “Inductively coupled plasma etching of III-V semiconductors in BCl3-based chemistries, I. GaAs, GaN, GaP, GaSb and AlGaAs” Applied Surf. Sci.  143 174 (1999).

8.      P. Collot, T. Diallo and J. Canteloup, “Dry-etch monitoring of III-V heterostructures using laser reflectometry nd optical emission spectroscopy”, J. Vac. Sci. Technol. B 9 2497 (1991).

 

 

 

 

Figure Captions

Figure 1. Trace of Ga peak intensity (417 nm) and its slope during AlGaAs/GaAs etching in BCl3/N2 ICP etching.

Figure 2. An SEM photo of AlGaAs/GaAs layer after inductively coupled BCl3/N2 plasma etching.

 

 

 

 

 

 

 

 

 

 

 

                                                Lee et al. Fig. 1/2

 

 

 

 

 

     

 

 

 

 

 

 

 

 

 

                                                      Lee et al. Fig. 2/2

 Indexing terms

HBT, Plasma etching, GaAs, OES