SEMICONDUCTOR DEEP CAVITY DEVICE

Abstract: The invention provides, notably for optical addressability, a semiconductor deep cavity device comprising a relatively rigid optically transparent substrate bearing a semiconductor wafer, having interposed between the wafer and the substrate an apertured sheet of a glass, the aperture of which defines the device cavity. The invention also provides a method of making such a device, comprising placing the apertured sheet and the wafer in their relative positions and applying an electrostatic field between them sufficient to bond them, and (previously, simultaneously or subsequently) placing the substrate and the apertured sheet in their relative positions and applying an electrostatic field between them sufficient to bond them, the bonding being accomplished using field-assisted bonding at an elevated temperature. The wafer may be formed with an annular trench in its surface facing the substrate, with the boundary of the aperture of the interposed glass sheet substantially coinciding with that of the trench.

Description: SEMICONDUCTOR DEEP CAVITY DEVICE ι This invention relates to a semiconductor deep cavity device, and to a method of making a semiconductor deep cavity device. A typical known semiconductor cavity device comprises a diaphragm of semiconductive material (typically silicon) bonded 05 in spaced relationship to a non-conductive substrate (typically glass) through an annular insulating spacer layer of e.g. silicon nitride, the silicon optionally having an annular trench etched in 1t, the better to define the edge of the cavity thus formed between the diaphragm and the substrate and conducing to a more 10 uniform diaphragm movement and reduced bending. The glass substrate may carry a metallic electrode which is disposed between the glass and the silicon within the cavity but arranged for connection to external circuitry. Such an arrangement, with the cavity evacuated and hermetically sealed (and num ular in 15 shape if the aperture of the spacer layer is circular) is common in capacitative pressure sensors, accelero eters, etc. The pressure, or acceleration force as the case may be, depresses the diaphragm of silicon, changing the capacitance of the cavity (the glass being relatively rigid). Capacitative sensing, however, 20 has its limitations and it has been proposed to improve the utility of such semiconductor cavity devices by exploiting their optical addressability. Certain types of optical instrumentation work best with cavity depths 1n such devices substantially greater than the Iμm which may be regarded as standard In 25 capacitative devices. The present invention therefore is aimed at deeper cavities. At present, good (<±2%) dimensional control over the cavity depth is achieved by depositing an annulus of dielectric to a precise thickness on a silicon wafer. An advantage of this 30 method 1s that the silicon surface of the resultant cavity is the original polished surface of the wafer. However, it is difficult to extend this approach to deeper cavities than about lμm, due to poor mechanical integrity of thicker deposits of dielectric and to difficulties in bonding wafers carrying thick dielectric deposits to glass substrates. Cavities of the desired depth could be formed by etching the silicon to a defined depth, but the surface finish would not be as good as that of the unetched surface and dimensional tolerances would be worse (±5%) . Dimensional control could be improved by utilising an epitaxial etch stop process, but this would impose a cost penalty and the surface finish would still not be as good as a polished wafer. A further problem with deep cavities is that (where desired) subsequently etching the annular boundary trench is made more difficult since the deeper the cavity, the more non-planar the surface for photolithography. This could be overcome by instead forming the trench on the backside of the silicon wafer, but apart from the difficulty of double-sided mask alignment, a backside trench is not as effective at reducing bending of the diaphragm inner surface. According to the present invention, a semiconductor deep cavity device comprises a relatively rigid optically transparent substrate bearing a semiconductor wafer, characterised in that between the wafer and the substrate is interposed an apertured sheet of a glass, the aperture of which defines the device cavity between the substrate and the wafer. Optionally, the wafer may have an annular trench in its surface facing the substrate, with the boundary of the aperture of the interposed glass sheet substantially coinciding with that of the trench. A method according to the invention of making such a device comprises placing the apertured sheet and the wafer in their relative positions and applying an electrostatic field between them sufficient to bond them, and (previously, simultaneously or subsequently) placing the substrate and the apertured sheet in their relative positions and applying an electrostatic field between them sufficient to bond them. The bonding step is preferably accomplished using the field-assisted bonding process which is sometimes referred to as anodic bonding, Mallory bonding or electrostatic bonding, at a suitably elevated temperature. With this process, the semiconductive sheet and the substrate are electrostatically pulled together and it has been postulated that both the substrate and insulating layer soften to effect a bond between the insulating layer and the substrate. The "bond" is such that a hermetic seal is provided to the cavity. The substrate may have a different composition from the apertured glass sheet, In which case the glass with the greater alkali content should be treated as the anode in the electrostatic bonding. Divalent cations are so relatively Immobile 1n glass that their content can be ignored, and trivalent cations (aluminium) are even more sessile. Alternatively, one of the glass surfaces can be metallised with a thin coating and this surface can then be treated as the anode for bonding to the other glass component. This method is preferable for high accuracy devices since with fewer thick materials, stress effects will be reduced. Also it would enable the use of Corning 1729 glass which would extend the temperature capability of the devices to >600°C. Preferably the substrate carries a partially reflective coating 1n the cavity, though with some optical instrumentation it is preferable to have a large-gap (>5mm) optical cavity and therefore the partially reflecting coating would then be provided on the outer surface, with an anti-reflection coating on the inner surface. The Invention will now be described by way of example with reference to the accompanying drawing, which is a schematic cross-section of a device according to the invention. A silicon pill 1, 350μm thick and with an annular trench 2 of overall diameter 2500μm (∑^ m), 25μm deep and 250μm across, is mounted on a glass substrate 4 1mm thick provided with a suitable partly reflecting coating 5 and, sandwiched between the two, with a thin (~150μm) glass spacer 7 with an aperture 8 aligned with the trench 2. The structures could be made in wafer form with the spacer 7 being a polished disc or sheet of glass with an array of holes 8 machined in by e.g. ultrasonic drilling, mask etching, sand blasting, water jet drilling or laser cutting, to define cavities. 150μm is a typical thickness for glass cover slips, or thicker glass may be used (for mechanical strength) and then thinned down to 150μm after bonding to the silicon but before bonding to the substrate. The region of silicon within the trench 2 forms a diaphragm 9. The spacer 7 is drilled to form the cavities 8 and 1s then lapped and polished on both sides to produce the required thickness and surface finish. Standard pyrex glass (Corning code 7740) Is a suitable material for the spacer 7. This material has 4.2% alkali content (3.8% Na 2 0, 0.4% K 2 0) whereas 7070 (1.2% L1 2 0, 0.5% K 2 0) would be a suitable material for the substrate 4. With metallised glass, it is possible for the three layer structure to be bonded simultaneously. However, it is usually more convenient to perform the bonding in two stages, for example a silicon/glass bond of the spacer 7 to the silicon pill 1 followed by a glass/glass bond of the spacer to the substrate 4. Glasses of different composition can be bonded if the type with the greater alkali content is treated as the anode. In a typical application, an optical fibre 10 is bonded to the underside of the glass substrate 4. The device may thus be interrogated optically. Reflections (of light sent up the fibre 10) from the partly reflecting coating 5 can be differentiated interferometrically from those from the diaphragm 9 (silicon being highly reflective by itself), revealing the depression (due to pressure or acceleration) of the diaphragm 9. Although the resultant structure has a 150μm deep cavity, one would not utilise all of this for the working or dynamic range of the device. The silicon diaphragm thickness can be controlled to ensure that the maximum acceptable movement equates to the upper working pressure requirement. The trench 2 affords a more uniform diaphragm movement and reduced bending.

Claims: CLAIMS 1. A semiconductor deep cavity device comprising a relatively rigid optically transparent substrate bearing a semiconductor wafer, characterised in that between the wafer and the substrate 1s interposed an apertured sheet of a glass, the aperture of which defines the device cavity between the substrate and the wafer. 2. A device according to Claim 1, wherein the wafer has an annular trench in Its surface facing the substrate, and the boundary of the aperture of the interposed glass sheet substantially coincides with that of the trench. 3. A device according to Claim 1 or Claim 2, wherein the substrate carries a partially reflective coating in the cavity. 4. A method of making the device of any of Claims 1 to 3, comprising placing the apertured sheet and the wafer in their relative positions and applying an electrostatic field between them sufficient to bond them, and (previously, simultaneously or subsequently) placing the substrate and the apertured sheet in their relative positions and applying an electrostatic field between them sufficient to bond them. 5. A method according to Claim 4, wherein the bonding step is accomplished using field-assisted bonding (electrostatic bonding), at an elevated temperature. 6. A method according to Claim 5, wherein the substrate is glass of a different composition from the apertured glass sheet, and the glass with the greater alkali content is treated as the anode in the electrostatic bonding. 7. A method according to Claim 5, wherein one of the glass surfaces is metallised with a thin coating and this surface is the anode in the electrostatic bonding.

Inventors: ROGERS TONY WILLIAM JAMES, ROGERS, TONY, WILLIAM, JAMES
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1990
Main classification: G01L9
IPC codes: G01L9