A limitation to the use of direct wafer bonding methods for micromachining and thin film device manufacturing has been the necessity for high temperature anneals to strengthen the bonded interface. Obviously, strong interface strength is needed to withstand backthinning processes and the rigors of device fabrication. Unfortunately, the elevated temperature exposure has a detrimental effect on implanted or diffused etch stop layers via diffusive broadening. Additionally, for many micromachined applications wafer bonding could be used as a final assembly step, replacing epoxies. However, the sensitive components of the device must be protected from thermal effects. This paper describes the use of oxygen plasmas to develop chemical free, room temperature, wafer to wafer bonding methods. The bond developed between plasma-activated silicon wafers is virtually at full strength upon contact bonding and does not require further thermal strengthening. The results for silicon dioxide bonding show that full strength material is achieved with anneals below 300~ This process has been applied to a number of wafer materials including sapphire, silicon dioxide, silicon nitride, and gallium arsenide. The data presented are the results of strength tests, interracial defect etching, transmission electron microscopy analysis, initial interface reaction kinetics, and mechanisms studies. We also show preliminary results from a suggested model to explain the observed increases in kinetics compared to conventional aqueous solution processing of samples.

Articles you may be interested inAtomic layer deposition TiO2-Al2O3 stack: An improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors Al 2 O 3 and TiO 2 deposited by atomic layer deposition are evaluated as etch masks for dry etch processes in an inductively coupled plasma reactor using the Bosch process. In the inductively coupled plasma chamber during deep silicon etching, because of the chemical nature of the etch process and the inert nature of Al 2 O 3 , the result is exceptional selectivity for silicon over as-deposited Al 2 O 3 , particularly at relatively low bias and high pressures used for through-wafer etching. TiO 2 is less resistant and appears to suffer more from chemical attack. In both cases, etch rate increases slowly with increasing rf bias. However, there is a sharp discontinuity in the etch rate of Al 2 O 3 when the bias power is operated in a pulsed low-frequency mode. This is thought to be due to increased sputtering from heavier ions. Preliminary studies indicate the etching conditions for Al 2 O 3 may be extended into a dielectric etch regime requiring more study.

Elastic constants c11, c12, and c44 of degenerately doped silicon are studied experimentally as a function of the doping level and temperature. First-and second-order temperature coefficients of the elastic constants are extracted from measured resonance frequencies of a set of MEMS resonators fabricated on seven different wafers doped with phosphorus (carrier concentrations 4.1, 4.7, and 7.5 x 10(19) cm(-3)), arsenic (1.7 and 2.5 x 10(19) cm(-3)), or boron (0.6 and 3 × 10(19) cm(-3)). Measurements cover a temperature range from -40°C to +85°C. It is found that the linear temperature coefficient of the shear elastic parameter c11 - c12 is zero at n-type doping level of n ~ 2 x 10(19) cm(-3), and that it increases to more than 40 ppm/K with increasing doping. This observation implies that the frequency of many types of resonance modes, including extensional bulk modes and flexural modes, can be temperature compensated to first order. The second-order temperature coefficient of c11 - c12 is found to decrease by 40% in magnitude when n-type doping is increased from 4.1 to 7.5 × 10(19) cm(-3). Results of this study enable calculation of the frequency drift of an arbitrary silicon resonator design with an accuracy of ±25 ppm between the calculated and real(ized) values over T = -40°C to +85°C at the doping levels covered in this work. Absolute frequency can be estimated with an accuracy of ±1000 ppm.

Erratum: "On the kinetics of growth of highly defective GaN epilayers and the origin of the deep trap responsible for yellow-band luminescence" [Appl.

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