Y.-L. Chao

A “smarter-cut” approach to low temperature silicon layer transfer

Silicon wafers were first implanted at room temperature by B+ with 5.0×1012 to 5.0×1015 ions/ cm2 at 180 keV, and subsequently implanted by H2+ with 5.0×1016 ions/cm2 at an energy which locates the H-peak concentration in the silicon wafers at the same position as that of the implanted boron peak. Compared to the H-only implanted samples, the temperature for a B+H coimplanted silicon layer to split from its substrate after wafer bonding during a heat treatment for a given time is reduced significantly. Further reduction of the splitting temperature is accomplished by appropriate prebonding annealing of the B+H coimplanted wafers. Combination of these two effects allows the transfer of a silicon layer from a silicon wafer onto a severely thermally mismatched substrate such as quartz at a temperature as low as 200 °C.

FUNDAMENTAL ISSUES IN WAFER BONDING

Semiconductor wafer bonding has increasingly become a technology of choice for materials integration in microelectronics, optoelectronics, and microelectromechanical systems. The present overview concentrates on some basic issues associated with wafer bonding such as the reactions at the bonding interface during hydrophobic and hydrophilic wafer bonding, as well as during ultrahigh vacuum bonding. Mechanisms of hydrogen-implantation induced layer splitting (“smart-cut” and “smarter-cut” approaches) are also considered. Finally, recent developments in the area of so-called “compliant universal substrates” based on twist wafer bonding are discussed.

Preamorphization implantation-assisted boron activation in bulk germanium and germanium-on-insulator

The effect of preamorphization implantation (PAI) on boron activation in germanium was studied. It was found that following PAI, significant dynamic annealing occurred during boron implantation in germanium. For small PAI energy which leads to a thin amorphous layer, recrystallization is completed via dynamic annealing during the boron implantation. As a result, a high-temperature postimplant anneal is required to activate the remaining interstitial boron and to annihilate implantation defects. For high PAI energy, while the thick amorphous layer did not recrystallize during the dynamic annealing, it requires a high-temperature anneal in order to completely recrystallize by solid phase epitaxial regrowth (SPER). The optimized PAI energy needs to be tailored such that the surface amorphous layer not only survives dynamic annealing during boron implantation, but also completes the SPER within the designed thermal budget. Full activation of boron can then be achieved without being limited by its solid solubi...

ONSET OF BLISTERING IN HYDROGEN-IMPLANTED SILICON

The onset of surface blistering in hydrogen-implanted single crystalline silicon was studied. A combination of atomic force microscopy and optical measurements shows that hydrogen-containing platelets grow laterally below silicon surface until they suddenly pop up as surface blisters due to the internal hydrogen pressure after a critical size has been reached. Experimentally and theoretically, the critical size of the onset blisters was found to increase with increasing implantation depth or top layer thickness.

Characteristics of Germanium-on-Insulators Fabricated by Wafer Bonding and Hydrogen-Induced Layer Splitting

There is considerable interest in germanium-on-insulator (GeOI) because of its advantages in terms of device performance and compatibility with silicon processing. In this paper, fabricating GeOI by hydrogen-induced layer splitting and wafer bonding is discussed. Hydrogen in germanium exists in molecular form and is prone to outdiffusion, resulting in a storage-time dependence of blistering. In contrast to the case of silicon, little effect of substrate doping on blistering is observed in germanium. Hydrogen implantation in germanium creates both {100}- and {111}-type microcracks. These two types of platelets are located in the same region for (111)-oriented wafers, but in different zones for (100) samples. This variation in distribution explains the smoother splitting of (111) surfaces than that of (100) surfaces. Hydrogen implantation also introduces a significant concentration of charged vacancies, which affect dopant diffusion in the transferred germanium film. Boron, with a negligible Fermi-level dependence, shows an identical diffusion profile to that of bulk germanium. In contrast, phosphorus diffusion is enhanced in the fabricated GeOI layers. These results also shed light on the understanding of dopant diffusion mechanisms in germanium.

Ammonium hydroxide effect on low-temperature wafer bonding energy enhancement

A wafer prebonding treatment by ammonium hydroxide (NH 4 OH) leading to a high bonding strength at low temperatures is presented in three material systems. After 200°C annealing, a surface energy of about 700 mJ/m 2 for thermal silicon-oxide bonding and of 1300 mJ/m 2 for plasma-enhanced chemical vapor deposition oxide bonding is realized. It is suggested that the lower ability of ammonia, the by-product of a polymerization reaction, to break the siloxane (Si-O-Si) bridging bonds appears to be responsible for the increase in surface energy in both silicon oxide bonding cases. NH 4 OH treatment is also effective on bare germanium/ silicon-oxide bonding with a surface energy of 800 mJ/m 2 . A highly hydrophilic germanium surface obtained by this treatment accounts for the high bonding energy.

Germanium

This paper shows that germanium n+/p shallow junction formation often results in poor leakage current control. It is due to the counteraction between the fast diffusion of phosphorus and the high-temperature annealing requirement for dopant activation and defect annihilation. When the dopant concentration is above a threshold value, the concentration-dependent diffusion enhances phosphorus diffusion and results in a box profile, leading to an electrical concentration lower than its solid solubility limit. A refrained thermal budget may increase the active concentration, but it is not sufficient to repair the implantation-damaged lattice. Moreover, any plasma-involved fabrication processes after rapid thermal annealing may introduce additional field-assisted defects into the depletion region when the junction is near the surface. Thus, several tradeoffs must be considered between high P activation, low junction leakage, and a shallow junction in order to obtain functional negative-channel metal-insulator-semiconductor field-effect transistors.

\hbox{n}^{+}/\hbox{p}

A reduction of parasitic resistance is presented with incorporation of preamorphization implantation (PAI) and self-aligned Cu3Ge in the source/drain region for germanium p-MOSFETs. Full activation of boron in the amorphous layer can be obtained during solid-phase epitaxial growth, and a concentration as high as 4 x 1020/cm3 is achieved. This nonthermal equilibrium concentration is maintained during the subsequent Cu3Ge formation. Cu3Ge is adopted as a contact metal in germanium p-MOSFETs for the first time, due to its superior electrical properties (6.8 muOmegaldrcm for resistivity and ~1 x 10-7 Omega cm2 on p-type germanium for specific contact resistance). The fabricated p+/n diode yields a five order of magnitude between forward and reverse currents, which can be attributed to the reduction in parasitic resistance. The low reverse current mitigates concerns of possible deep-level traps introduced by copper. It also confirms the nonexistence of extended defects created by PAI as a result of the unique role of vacancies in germanium. With high dopant concentrations achieved by PAI and low resistance of Cu3Ge, excellent MOSFET characteristics are demonstrated in self-aligned Cu3Ge p-MOSFETs. A 15% mobility enhancement over Si universal mobility and a 60% parasitic resistance reduction are achieved.

Diodes: A Dilemma Between Shallow Junction Formation and Reverse Leakage Current Control

The material and electrical characteristics of /spl epsiv/-Cu/sub 3/Ge as a contact metal were investigated. The samples were prepared by direct copper deposition on germanium wafers, followed by rapid thermal annealing. The /spl epsiv/-Cu/sub 3/Ge formed at 400 /spl deg/C has a resistivity of 6.8 /spl mu//spl Omega//spl middot/cm, which is lower than typical silicides for silicon CMOS. Cross-sectional transmission electron microscopy showed smooth germanide/germanium interface, with a series of nanovoids aligning close to the top surface. These voids are believed to be the results of Kirkendall effect arising from the different diffusion fluxes of copper and germanium. The specific contact resistivity of Cu/sub 3/Ge, obtained from four-terminal Kelvin structures, was found to be as low as 8/spl times/10/sup -8/ /spl Omega//spl middot/cm/sup 2/ for p-type germanium substrate. This low resistivity makes Cu/sub 3/Ge a promising candidate for future contact materials.

Source/Drain Engineering for Parasitic Resistance Reduction for Germanium p-MOSFETs

In this paper, a novel Schottky tunneling source MOSFET utilizing the concept of gate controlled Schottky barrier tunneling has been examined and successfully demonstrated. Much better short channel immunity in terms of smaller DIBL, reduced threshold roll-off and increased output resistance has been confirmed. Excellent ROUT and gain as compared to conventional SOI-FET are demonstrated. To further improve the performance, smaller Schottky barrier height junction is preferred. Toward this end germanium based transistor is very attractive. Germanium channel devices are also explored for their enhanced transconductance performance. The development of GeOI substrates for control of SCE and reduction of parasitic resistance for improvement of ION is presented. These two devices are promising candidates for the system-on-chip applications