Wet chemical processes
Wafer cleaning and isotropic thin layer etching
The wet chemical processes are necessary for all semiconductor research tasks of the Institute and include the following standard techniques:
Standard silicon wafer cleaning |
RCA-1-(SC-1)-cleaning (NH4OH/H2O2/H2O) |
Specialized cleaning techniques |
Hydrophilling of Si surfaces (e.g. for silicon wafer bonding) |
Standard silicon oxide etching |
Diluted HF (DHF), (HF/H2O) |
Isotropic silicon etch |
HF/HNO3 solutions with well defined etch rates between 6 and 1000 nm/min |
Standard silicon nitride etching | Hot H3PO4 |
Metal contact layer etching |
H3PO4/HNO3/H2O and H3PO4/HNO3/H2O + TMAH |
Specialized etching techniques for: |
Metals, glasses, poly-Si, etc. |
Anisotropic and selective etching for silicon bulk micromachining
Wet chemical anisotropic etching allows for the precise three-dimensional structuring of large and miniature silicon structures in an IC compatible way. The three main properties that make this technique so widely applicable are the dependence of the etch rate on crystal orientation, dopant concentration and on the applied electrical potential in the case of electrochemical etching.
The crystallographic anisotropy gives the possibility for a very precise lateral and vertical machining of micromechanical devices by proper alignment of structural contours (masks) with either fast or slow etching crystal planes. The dependence on the dopant concentration and on electrical potential allows for the incorporation of well defined etch stop layers by either using a high boron concentration with NB > 5×1019 cm-3, or else exploiting the potential drop across a pn-junction. SiO2 and/or Si3N4 layers can be used as masking layers without problems for creating 3D-structures were very deep patterns are required.
The corresponding doping of etch stop layers is carried out in our Institute by conventional ion implantation, writing focused ion beam (FIB) implantation and subsequent annealing or drive in diffusion.
The main etch parameters for the developed anisotropic etch process are:
Etch solution | KOH/H2O-solution of concentration c = 30 % |
Etch temperature | T = (80±1) °C |
Etch samples | 2", 3", 4" Si wafers One-side processed wafers containing device structures can be etched |
Etch rate R (µm/h) | Anisotropy | Selectivity | Surface roughness r (µm) |
R<100> = 69,8 R<111> = 1,4 RSiO2 = 0,15 RSi3N4 = 0 |
R<100>/R<111> = 49,8 R<100>/R<110> = 0,5-2 |
R<100>/RSiO2 = 465 R<100>p/R<100>p+ = 10...103 R<100>n/R<100>p+ = 103 |
ra = 0,033 rp = 0,11 rv = 0,12 rt = 0,24 |