Lau Wai Shing

Wai Shing Lau (simplified Chinese name: 刘偉成, born 29 July 1955 in Hong Kong) is also known as Lau Wai Shing. Lau is an electrical engineer and also materials scientist. He worked on both Si-based and III-V based microelectronics.

Biography

Lau was born in Hong Kong in 1955 to Lau Pak Chau (1922–2008), a public health inspector and amateur painter, and Tse Kwan Fong (1931–1988). He has 2 older brothers and 2 younger sisters.

He graduated in Electrical Engineering at the University of Hong Kong in 1977. He obtained a master's degree from the Department of Electronics, the Chinese University of Hong Kong in 1980. Subsequently, he published a paper on the analysis of constant-capacitance deep-level transient spectroscopy by negative feedback theory in 1982. For his PhD in Electrical Engineering from the Pennsylvania State University, Pennsylvania, USA in 1987 he worked on transparent conductive thin films like tin oxide, indium oxide and zinc oxide. Then he served as a post-doctoral student in the same laboratory to work on PECVD silicon nitride samples from IBM. New insight was gained regarding hysteresis in the C-V characteristics and how to suppress it in MNS (metal-nitride-silicon) capacitors. (Please see references [1]–[3].)

Lau served as lecturer and then senior lecturer in the National University of Singapore from 1988 to 1997.

Lau worked on gallium arsenide and gallium nitride devices and materials. He developed the "true oxide electron beam induced current" (TOEBIC) technique to study local defects in insulating thin films. TOEBIC can be applied to MOS capacitors. Dr. Lau's theory was that local defects in the oxide and local defects in silicon could be distinguished when electron beam induced current was performed on M/SiO₂/Si capacitor structures. (Please see references [4]–[6].) He proposed to use PECVD silicon nitride / polyimide dual passivation for AlGaN/GaN HEMT (high electron mobility transistor). When PECVD silicon nitride is thick, there is mechanical stress problem. It may also have pinholes. Polyimide can be quite thick without stress problem but it is not a good barrier for moisture. Thick polyimide can cover up the pinholes in the PECVD silicon nitride below. The two combined together with polyimide on top of the nitride can be a practical and better approach. In addition, the dielectric constant of polyimide (about 3.5) is smaller than that of PECVD silicon nitride (about 7), resulting in less parasitic capacitance. (Please see references [7].)

Dr. Lau also worked on Ohmic contact technology for GaN and AlGaN/GaN HEMT. He noticed that the presence of a small quantity of moisture in the rapid thermal annealing chamber is particularly bad for Ohmic contact on AlGaN/GaN HEMT wafers. However, this is less serious for Ohmic contact on GaN only. (Please see references [8].)

Dr. Lau has also published a paper on low frequency noise in polysilicon emitter bipolar transistors in 1992. Basically, the application of a very thin layer of interfacial oxide between the polysilicon emitter and the single-crystalline emitter can help to increase the current gain. However, this approach will very significantly increase low frequency noise probably because of tunneling 1/f noise.

During 1997–1998, he worked on embedded DRAM technology in Chartered Semiconductor Manufacturing, Singapore. He studied the mechanism of under-sensitive test structures and over-sensitive test structures. He pointed out that if this is properly understood, electrical failure analysis can be more easily achieved by electrical testing at the test structure level instead of at the product engineering level. This will make the job of DRAM yield enhancement easier.

Dr. Lau became an associate professor in the Nanyang Technological University since 2001.

He was the first in making a correlation between the leakage current of ultrathin tantalum pentoxide (one of the high-k dielectric) films with defect states detected by zero-bias thermally stimulated current. Most scientists cannot see any relationship between the leakage current in tantalum oxide (tantalum pentoxide) and the defect states detected by thermally stimulated current. For example, Dr. Yasushiro Nishioka (Japan) could not see any relationship between the leakage current and the defect states detected by thermally stimulated current (private communication). Dr. Lau managed to see the relationship between the leakage current and the defect states and published his theory in various papers. He invented "zero temperature gradient zero bias thermally stimulated current" as a method to detect defect states in ultrathin high-k dielectric films. This was patented as US Patent 6909273 in 2005. (Please also see reference [9].) In addition, he has also developed a technique known as "two-scan zero-bias thermally stimulated current" when an insulator or semiconductor has some traps that can be filled at low temperature and some traps that can only be filled at relatively high temperature. (Please see reference [10].)

He also worked on Cu/low-k back-end-of-line (BEOL) technology. (Please see reference [11].)

He also wrote various papers on MOS transistors. For example, he worked on the application of strain engineering to CMOS technology. He pointed out that if tensile stress can increase the on current of n-channel MOS transistors, tensile stress will also increase the off current. Then he proposed a theory why tensile stress can improve n-channel MOS transistors even though there is an increase in the off current. A lady graduate student, Peizhen Yang, was recruited to work on research in this direction. (Please see references [12] & [13].) Similarly, his theory can be applied to the study of p-channel MOS transistors with the conventional <110> channel or the newer <100> channel. (Please see reference [14].) He also studied the reduction of boron lateral channelling (physics) due to switching from the conventional <110> channel to the newer <100> channel.

Dr. Lau also studied the theory of electron mobility in Si-based MOSFET. In 2005, Dr. Lau pointed out as Lau's hypothesis that "remote Coulombic scattering" is only important in the subthreshold region and in the region slightly above threshold. (Please see reference [15].)

Dr. Lau also explained the observation of anomalous narrow channel effect in very short p-channel MOSFET. This effect is relatively weak in n-channel MOSFET. An explanation why this effect is strong in PMOS but weak in NMOS has been proposed by Dr. Lau; p-type dopants like boron tends to segregate into the STI oxide while n-type dopants tend to accumulate at the STI oxide/Si interface. (Please see references [16] & [17].)

Dr. Lau is also interested in quasi-ballistic transport in MOS transistors and has published his theory on it. (Please see reference [18].) Instead of a carrier saturation velocity according to convention velocity saturation theory, an effective saturation velocity can be defined. It turns out that the value of effective saturation velocity in real-life MOS transistors is similar to established values of the carrier saturation velocity. However, Dr. Lau pointed out that the effective saturation velocity can be a function of the gate voltage.

Besides semiconductor materials and devices research, Dr. Lau also works privately on the research of traditional Chinese medicine, foot or hand reflexology, "unrefreshing sleep", chronic fatigue syndrome, irritable bowel syndrome, toothache, etc. For example, he noticed that there are two toothache mechanisms according to his personal experience: (A) Mechanism according to Western medicine and (B) Mechanism according to traditional Chinese medicine. He had direct experience of toothache that can be easily explained by Western medicine and also a kind of toothache that cannot be explained by Western dental theory but can be explained by traditional Chinese medicine. Thus the two mechanisms are not contradictory. They are actually complementary. The solution of the second kind of toothache is explained in various books on traditional Chinese medicine. However, those solutions are too complicated or inconvenient for an ordinary person living in the Western World; Dr. Lau has privately developed a much simpler and convenient solution (unpublished). Dr. Lau is also interested in the application of Oldenlandia diffusa (a Chinese herb) to treat some medical problems.

He has also worked on the application of infrared spectroscopy to semiconductor materials. He has published a book Infrared Characterization for Microelectronics, World Scientific, Singapore, 1999. He has also developed some new insight regarding the application of Raman spectroscopy to health problems. He has also done some work on control theory and applied it to his research. The application of negative feedback control to a chaotic system is highly challenging. He has developed some insight on the practical feedback control of chaotic systems.

Dr. Lau has developed a theory regarding the symmetry of the I-V characteristics of high-k dielectric capacitor structures [19]. In addition, he has proposed an "extended unified Schottky-Poole-Frenkel theory" [20]. Furthermore, he observed Poole-Frenkel saturation experimentally [21].

Personal

Dr. Lau married Sin Shuk Ying in 1989 and they had one daughter, Florence Lau Pui Yan born in 1991. (Note: Sin is the family name of Mrs. Lau.)

References

[1] W. S. Lau, "The identification and suppression of defects responsible for electrical hysteresis in metal-nitride-silicon capacitors", Japanese Journal of Applied Physics, Part 2, Letters, 29, no. 5 (May 1990), pp. L690–L693.

[2] W. S. Lau and C. H. Goo, "Confirmation of the correlation between the electrical hysteresis and silicon dangling bond density in silicon nitride by UV irradiation of nearly hysteresis free metal-nitride-silicon capacitors", Japanese Journal of Applied Physics, Part 2, Letters, 30, no. 12A (1 December 1991), pp. L1996–L1997.

[3] W. S. Lau, "The preparation and characterization of nearly hysteresis-free metal-nitride-silicon capacitors on both p- and n-type silicon substrates", Journal of Applied Physics, 71, no. 1 (1 January 1992), pp. 489–493.

[4] W. S. Lau, D. S. H. Chan, J. C. H. Phang, K. W. Chow, K. S. Pey, Y. P. Lim and B. Cronquist, "True oxide electron beam induced current for low-voltage imaging of local defects in very thin silicon dioxide films", Applied Physics Letters, vol. 63, no. 16 (18 October 1993), pp. 2240–2242.

[5] W. S. Lau, D. S. H. Chan, J. C. H. Phang, K. W. Chow, K. S. Pey, Y. P. Lim, V. Sane and B. Cronquist, "Quantitative imaging of local defects in very thin silicon dioxide films at low bias voltage by true oxide electron beam induced current", Journal of Applied Physics, vol. 77, no. 2 (15 January 1995), pp. 739–746.

[6] W. S. Lau, V. Sane, K. S. Pey and B. Cronquist, "Two types of local oxide/substrate defects in very thin silicon dioxide films on silicon", Applied Physics Letters, vol. 67, no. 19 (6 November 1995), pp. 2854–2856.

[7] W. S. Lau, S. Gunawan, J. B. H. Tan and B. P. Singh, "The application of polyimide/silicon nitride dual passivation to AlGaN/GaN high electron mobility transistors", Microelectronics Reliability, vol. 48, no. 2 (Feb. 2008), pp. 187–192.

[8] W. S. Lau, W. T. Wong, J. B. H. Tan and B. P. Singh, "Effect of a trace of water vapor on Ohmic contact formation for AlGaN/GaN epitaxial wafers", Microelectronics Reliability, vol. 48, no. 5 (May 2008), pp. 794–797.

[9] W.S. Lau, K.F. Wong, T. Han and N.P. Sandler, "Application of zero-temperature-gradient zero-bias thermally stimulated current spectroscopy to ultrathin high-dielectric-constant insulator film characterization”, Applied Physics Letters, 88, no. 17 (24 April 2006): article number 172906 (USA).

[10] W.S. Lau, "Similarity between the first ionized state of the oxygen vacancy double donor in tantalum oxide and the first ionized state of the cadmium vacancy double acceptor in cadmium sulfide", Applied Physics Letters, vol. 90, article number 222904, 2007.

[11] W. S. Lau, H. J. Tan, Z. Chen and C. Y. Li, "A comparison of various dielectric/metal sidewall diffusion barriers for Cu/porous ultra-low-K interconnect technology in terms of leakage current and breakdown voltage”, Vacuum, vol. 81, no. 9 (May 2007), pp. 1040–1046.

[12] P. Yang, W. S. Lau, V. Ho, C. H. Loh, S. Y. Siah and L. Chan, "Effect of tensile stress on the various components of the off current of n-channel metal-oxide-semiconductor transistors”, Applied Physics Letters, vol. 91, no. 7 (13 August 2007), pp. 073514–1 to 073514-3.

[13] P. Yang, W. S. Lau, T. L. Ng, V. Ho, C. H. Loh, S. Y. Siah and L. Chan, "Improvement of n-channel MOS transistors by tensile stress despite increase in both on and subthreshold off currents”, Journal of Applied Physics, vol. 103, no. 9 (1 May 2008), pp. 094518–1 to 094518-12.

[14] P. Yang, W. S. Lau, V. Ho, B. K. Lim, S. Y. Siah and L. Chan, "Switching from <110> to <100> orientation increases both the on current and off current of p-channel metal-oxide-semiconductor transistors”, Applied Physics Letters, vol. 93, no. 3 (21 July 2008), pp. 033501–1 to 033501-3.

[15] C. W. Eng, W. S. Lau, D. Vigar, S. S. Tan and L. Chan, "Effective channel length measurement of MOS transistors with pocket implant using the sub-threshold current-voltage characteristics based on remote Coulomb scattering", Applied Physics Letters, vol. 87, no. 15 (10 October 2005) pp. 153510–1 to 153510-3.

[16] W. S. Lau, K. S. See, C. W. Eng, W. K. Aw, K. H. Jo, K. C. Tee, J. Y. M. Lee, E. K. B. Quek, H. S. Kim, S. T. H. Chan and L. Chan, "Anomalous narrow width effect in NMOS and PMOS surface channel transistors using shallow trench isolation", Proc. IEEE EDSSC 2005, pp. 773–776.

[17] W. S. Lau, K. S. See, C. W. Eng, W. K. Aw, K. H. Jo, K. C. Tee, J. Y. M. Lee, E. K. B. Quek, H. S. Kim, S. T. H. Chan and L. Chan, "Anomalous narrow width effect in p-channel metal-oxide-semiconductor surface channel transistors using shallow trench isolation technology", Microelectronics Reliability, vol. 48, no. 6 (June 2008), pp. 919–922.

[18] W. S. Lau, P. Yang, V. Ho, C. H. Loh, S. Y. Siah and L. Chan, "An explanation of the dependence of the effective saturation velocity on gate voltage in sub-0.1 um metal-oxide-semiconductor transistors by quasi-ballistic transport theory”, Microelectronics Reliability, vol. 48, no. 10 (Oct. 2008), pp. 1641–1648.

[19] W.S. Lau, "A new mechanism of symmetry of current-voltage characteristics for high-k dielectric capacitor structures", ECS Transactions, vol. 45, no. 3 (2012), pp. 151–158.

[20] W.S. Lau, "An extended unified Schottky-Poole-Frenkel theory to explain the current-voltage characteristics of thin film metal-insulator-metal capacitors with examples for various high-k dielectric materials", ECS Journal of Solid State Science and Technology, vol. 1, no. 6 (2012), pp. N139-N148.

[21] W.S. Lau, "Experimental observation of Poole-Frenkel saturation in an ultrathin tantalum oxide capacitor structure", ECS Transactions, vol. 53, no. 1 (2013), pp. 361–368.