Process for producing micro-mechanical structures

Details Process for producing micro-mechanical structures Process for producing micro-mechanical structures

1992-10-14

1994-02-15

Powell, William A.

Adhesive bonding and miscellaneous chemical manufacture

Delaminating processes adapted for specified product

Delaminating in preparation for post processing recycling step

156628, 156633, 156651, 156657, 156662, H01L 21306, B44C 122, C03C 1500, C03C 2506

Patent

active

052863417

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention relates to a process for producing micro-mechanical structures for sensors or actuators in semiconductor wafers.


BACKGROUND

Various processes are already known for producing mechanical structures in semiconductor wafers. A differentiation between bulk processes and surface processes is made in this case. A bulk process is described in the German patent disclosure DE-OS 40 00 496, and corresponding WO 91/03074, published 7 Mar. 1991, for example, where recesses are etched into the surfaces of a semiconductor body with the aid of photographic masking technology, and paddles are exposed by undercutting ridges and etching the back. In the bulk process, the largest portion of a wafer is removed by an isotropic etching process in order to obtain the desired mechanical structure. This has the disadvantage that after etching the wafer has structures which are sensitive to breaking and must be treated accordingly. Furthermore, process-related inner stresses in the structures can lead to mechanical deformations which make further processing much more difficult.
A surface treatment process is described in "Surface Micromachining Process for Electrostatic Microactuator Fabrication" by Theresa A. Lober and Roger T. Howe, IEEE 1988, in which structured auxiliary layers are applied to the surface of a silicon wafer and form pedestal-like protrusions on the surface. Polysilicon is deposited on top of these pedestal-like protrusions, so that the polysilicon layer is partially in direct connection with the surface of the silicon wafer. By subsequent removal of the auxiliary layer, overhanging polysilicon cantilevers are created on the surface of the silicon wafer. With this process, the actually mechanically stressed structures cannot be made of mono-crystalline silicon, which has disadvantageous effects on the expected long-term stability.
A process is also known from the Journal of Electrochemical Society, Extended Abstracts, Vol. 86-1, Masaru Shimbo, Kazuyoshi Furukawa, Kyoshi Funkunda, "A Newly Developed Silicon to Silicon Direct Adhesion Method", by means of which silicon wafers can be bonded to each other without requiring an intermediate layer for this.


THE INVENTION

The process in accordance with the invention has the advantage that the micro-mechanical structures are protected against mechanical stress during the entire manufacturing process, because they are located in the interior of a silicon sandwich structure and are filled up with silicon oxide. This renders the total structure very stable and therefore also easy to process. Here it is also advantageous that possible stresses do not result in an undesirable deformation of the structure during processing, because mechanical deformability is achieved only at the very end, after removal of the oxide. It is of particular advantage that it is possible to produce the structures of mono-crystalline silicon, because of which an increased long-term stability, in comparison to polysilicon structures, can be expected.
Thus, complicated structures with, for example, asymmetrical depth values can be simply realized by producing local oxidations. In this connection, it is also advantageous that the local oxidations can be placed into the wafer from the front as well as the back of the wafer. A further advantage of the process is that silicon wafers can be simply connected with each other by silicon fusion bonding. The thickness of the total structure can be reduced in a particularly advantageous manner by the use of wafers having a doping junction, since the doping junction can be used as an etch stop. For exposing the structure, it is particularly advantageous to etch an opening which leads to the enclosed oxide into one of the wafers. It is then possible to dissolve it out in a particularly advantageous way with hydrofluoric acid (HF).
A particular advantage of the process in accordance with the invention lies in that the entire processing of the sensor or actuator, as well as possibly even the IC production, can be performed a

REFERENCES:
patent: 4236137 (1980-11-01), Kurtz et al.
patent: 4744863 (1988-05-01), Guckel et al.
patent: 4784721 (1988-11-01), Holmen et al.
patent: 4808549 (1989-02-01), Mikkor et al.
patent: 4853669 (1989-08-01), Guckel et al.
patent: 5142781 (1992-09-01), Mettner et al.
VLSI Technology, edited by S. M. Sze (McGraw-Hill, 1983, pp. 466-467).
Halbleiter Elektronik (Semiconductor Electronics), edited by D. Widmann et al. (Springer, 1988, vol. 19, pp. 67-73).
M. Shimbo et al./Toshiba, "A Newly Developed Silicon-to-Silicon Direct Adhesion Method," Journal of Electrochem. Soc. Extended Abstracts, vol. 86-1, Abstract No. 232, p. 337 (1986).
Theresa Lober & Roger Howe, "Surface Micro-Machining Processes For Electrostatic Microactuator Fabrication," IEEE publication THO215-4/83/0000-0059, I.E.E.E., N.Y. 1988.

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