6"-etch-cell for macropores on p-type-silicon

Introduction

Pore Etching in 150 mm p-type Wafers Most applications envisioned call for the etch-ing of standard size Si wafers, at least 100 mm, better yet 150 mm or 200 mm. While any etch-ing recipe that produces satisfying pores with small samples of typically (1 – 4) cm2 will of course also work for larger wafers if the proper conditions can be maintained everywhere on the surface, there are a number of specific technical problems to large area etching which are not easy to solve. Macropores on p-type Si – either in aqu. or org. electrolytes – do not require illumination which makes cell design easier (but with the added complexity of sever restrictions in usable materials). A design suitable for 150 mm wafers is now operative. The cell has been constructed from PVC for the trial and error period which presently limits the use of organic electrolytes. So far, mostly p-macropores(aqu) were etched (p-macropores (org) have to await a reconstruction with TeflonTM for the crucial parts). Fig. 3 shows a etched p-type wafer. Additional SEM micrographs from other parts of the wafer demonstrated that homogeneous pore etching was achieved everywhere on the wafer. Etching micro- and mesopores of just a few micrometers in depth on large areas is a com-paratively easier task. The ELTRAN (TM) process by Canom (TM) obviously has mastered this; but little is known about the cell design except that an electrolyte backside contact is used and several wafers are processed at the same time. The ELTRAN (TM) process is an industrial level SOI wafer production with the use of a mesoporous silicon sacrificial layer.

Figure 3 - First full etched 6" p-type Wafer

Applications of macropores Macropores offer a number of attractive fea-tures for many kinds of applications. A listing includes:

Extremely large aspect ratios (length/diameter) of > 600 for diameters in the 1 µm region.
Defined geometries (arrays) are possible – periodic, periodic with defects, or non-periodic – the only limitation is that the average pore density should be constant.
Large porosity and thus large volume to surface ratios are possible.
Large areas can be homogeneously etched due to the supreme homogeneity of Si wafers.
Full compatibility with Si technology.

There are, however, also some difficulties in employing macropores for products. These may be listed as follows:

Si technology is often needed, i.e. a clean-room with some standard processes like lithography, oxidation, Si3N4 deposition – in other words an expensive infrastructure for research and development.
Etching macropores on large areas (e.g. an 200 mm wafer), while certainly possible, is neither easy nor (initially) cheap.

Publications:

Formation and application of porous silicon
H. Föll, M. Christophersen, J. Carstensen, and G. Hasse Materials Science and Engineering: R: Reports, 39 (4), 93-141, 2002

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Last modified: 08/16/02
URL: http://www.marc-christophersen.de