Method for making a machined silicon micro-sensor

Details Method for making a machined silicon micro-sensor Method for making a machined silicon micro-sensor

1999-11-23

2001-06-26

Picardat, Kevin M. (Department: 2823)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

C438S024000, C438S026000, C438S048000, C438S051000, C438S053000

Reexamination Certificate

active

06251698

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the production of microsensors machined in silicon, in particular accelerometers for applications of assisting with navigation in aircraft, and pressure sensors.
2. Discussion of the Background
Various embodiments of such sensors have already been proposed in the art, and in particular sensors produced by superposition of three silicon plates assembled together by bonding or welding, the plates being machined partly before assembly and partly after assembly. The central plate has directly active mechanical elements, that is to say on which the acceleration is exerted in the case of an accelerometer, the pressure in the case of a pressure sensor; it also has electrical elements needed for its operation. The upper and lower plates frame the central plate and, where appropriate, carry the electrical elements needed for its operation, in addition to the electrical elements of the central plate (in particular electrical connections).
One of the main problems which are posed in the fabrication of these sensors is the etching of certain elements of the central plate, and most particularly elements which require high micromachining precision because of their role in the sensor.
This is the case, in particular, with the moving suspension elements of the microsensor, that is to say the mechanical elements which undergo movements or stresses as a function of the parameters which it is desired to measure (accelerations, pressure etc.).
Typically, in an accelerometer having a sensitive mass attached to a fixed framework by suspension elements and a resonant beam, the problem resides in the etching of the suspension elements and of the resonant beam. Similarly, in a pressure sensor having a thin plate experiencing a pressure and transmitting its deformations to the ends of a resonant beam, the problem resides above all in the etching of the resonant beam and of the elements through which it is connected to the thin plate.
In order to obtain sufficient etching precision in terms of the widths and thicknesses of these beams, it is currently necessary to resort to complex silicon-plate structures, with silicon oxide layers integrated in the thickness of the plate (structures with multiple epitaxy or structures with implanted oxide layers), which is expensive. Otherwise, the etching is not precise enough, although the performance of the sensor does in fact depend on the etching precision. It has been found, for example, that the precision with which the beam is etched is not sufficient when the beam is etched by chemical attack on its two faces through a mask deposited on one face.
Lastly, in general, the machining of the elements of the central plate is made particularly difficult because, further to the etching of the active elements, it is necessary to provide other etches, including very deep etches which may extend as far as full separation of an active central area from a peripheral frame, these deep etches having to be carried out without compromising the partial etches of the active elements. The result of this is that the fabrication process has to take a large number of constraints into account, and the object of the present invention is to propose to carry out certain fabrication steps in an advantageous way while being compatible with the many other steps which need to be performed in order to obtain an operational sensor.
SUMMARY OF THE INVENTION
In order to find an effective compromise between the problems of precision, of compatibility with the various micromachining operations, and of fabrication cost, the present invention provides a process for micromachining a beam of a microsensor having at least two plates, at least the first of which is in micromachined silicon, this process being characterized in that:
a beam having a thickness equal to the desired final thickness, and a width greater than the desired final width, is produced by micromachining the silicon on the first plate, the beam being covered on its upper face by a mask defining the desired final width;
the two plates are assembled together;
the two faces of the beam are oxidized in order to cover them with a thin protective layer;
this thin protective layer on the upper face is removed, by vertical directional etching, without removing the mask already present;
the silicon in the area exposed by the preceding operation is attacked by means of an etch through the upper face, until the entire part of the beam not protected by the mask is eliminated, and the beam having the desired width is thus formed.
At this stage, it is possible to remove the protective layer and the preceding mask, which is moreover in silicon oxide like the protective layer.
Among the advantages of the process according to the invention, it may be noted that, in the case when the etching of the protective layer needs to be carried out in the presence of a mask (and this is the case when it is desired for elements other than the suspension elements to be protected during the attack on the silicon), it is easier to put this mask on the assembled plates than on a single plate which has been weakened by significant thickness reductions.
Furthermore, in the case when the active elements etched on the first plate are surrounded by a spacer frame which is physically separated from the active elements but is machined from the same plate, temporary attachments are kept between the frame and the active elements, until the two plates are assembled together, and these temporary attachments are not eliminated until after assembly, during the operation of directional attack on the silicon.
In practice, before the plates are assembled together, machining of the beam comprises the following operations:
depositing a layer which protects against etching of the silicon over a width greater than the width of the beam desired, this layer being thicker over a width corresponding to the final width desired for the beam;
attacking the silicon through both the upper face and the lower face in one or more steps, until the beam is given the desired final thickness;
removing the protective layer on the edges of the beam on either side of the desired final width, while keeping a portion of protective layer having the desired final width.
In the case of an accelerometer having a proof mass connected to a base not only by the beam but also by suspension arms located in the median plane of the thickness of the first plate, a first step of attacking the silicon through the upper face and the lower face is firstly carried out on the first plate alone, this first step ending when the suspension arms machined both from the top and from the bottom have reached their desired thickness; these suspension arms are then oxidized in order to protect them during the rest of the silicon attack.


REFERENCES:
patent: 5305643 (1994-04-01), Thomas et al.
patent: 5510276 (1996-04-01), Diem et al.

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