Implementation of a low temperature wafer bonding process for acceleration sensors

Abstract

The paper describes a low temperature bond process based on an oxygen plasma pretreatment followed by 200°C and 400°C annealing which was to be integrated in our technological process flow to produce micromechanical devices in bulk and surface micromachining like acceleration sensors, gyroscopes and mirror arrays [1]. The results of infrared transmission and the measured bond strengths of the prepared test wafers will be presented in dependence on various pre-treatments and annealing times as well as temperatures. First die separation tests as well as additional detailed investigations showed that the bonding process has the potential to replace an anodic bonding process. During the development of the low temperature bond process it was shown that it is possible to reach a bond strength between 1.5 J/m2 and 2.8 J/m2 depending on the annealing conditions. To optimize the necessary size of the bond frame and to quantify the bond strength limits of the process a test pattern was designed with different arrangements of sensor structures like bond frames, spaces and chevron notch structures. The investigation showed the achieved bond yield in dependence on different sensor structures and bond conditions.

A Review of Hydrophilic Silicon Wafer Bonding

“…The validity of the equation was then tested by controlling the thickness of the debonding blade and the thicknesses of the wafer samples under test. [13][14][15][16][17][18][19][20][21][22][23][24][25] …”

“…The validity of the equation was then tested by controlling the thickness of the debonding blade and the thicknesses of the wafer samples under test. [13][14][15][16][17][18][19][20][21][22][23][24][25] γ = 3E * u 2 w 3 8l 4 [1] Where E * is the effective Young's modulus which varies with the orientation of the moment of bending to the crystal lattice. It should be noted that a second, common form of this equation is as follows:…”

“…There are many other papers concerning plasma activated wafer bonding, typically reporting on a new plasma generator source or an optimized set of parameters [18][19][20][21]41,50,52,64,65,67,[87][88][89][90][91][92][93][94][95][96][97][98][99][100] that allow for high strength bonding at low annealing temperatures and with minimal annealing voids. A significant number of these results are applicable only to the precise experimental setup of the reporting authors or are simply unrepeatable.…”

Reactive Bonding and Low Temperature Bonding of Heterogeneous Materials

“…Thereby the water molecules diffuse out either along the interface to the outside or they diffuse through the native oxide to the bulk material, the wafers are moving closer together and form hydrogen bonds between the Si-OH groups. With increasing temperature the opposing silanol groups can react with each other to form covalent siloxane bonds [18][19][20][21].…”

Wafer Bonding Technologies in Industrial MEMS Processing - Potentials and Challenges

“…Plasma-assisted low-temperature bonding processes are CMOS compatible and do not influence processed wafers in any way. Because similar materials are bonded, the mechanical stress in the bonded stack is, in-principal, low 6 and alignment accuracies better than 2 µm can be achieved. From the economic point of view, direct bonding has many advantages.…”