AU660358B2 - Silicon transducer - Google Patents

Description

660 358

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFIT CAT ON FOR A STANDARD PATENT

ORIGINAL

Name of Applicant: AWA MICROELECTRONICS PTY LIMITED, A.C.N. 003 263 170 Actual Inventor: Witold Wiszniewski Address for Service: SHELSTON WATERS Clarence Street SYDNEY NSW 2000 "SILICON TRANSDUCER" Invention Title: Details of Associated Provisional Application No: PL1610 dated March 1992 The following statement is a full description of this invention, including the best method of performing it known to us:- 2 The present invention relates to a method of fabricating silicon microstructures and, more particularly, silicon transducers.

The fabrication of silicon microstructures is well known. In one application, for example an infrared detector, infrared radiation is absorbed by a sensor producing heat which raises the sensor temperature. In order to obtain a maximum temperature rise, the sensor must be thermally insulated from its surroundings.

Sensors made using IC technology employ micromechanical structures such as beams, cantilevers or membranes which provide increased thermal isolation for a sensor located on the structure.

Conventional microelectronic fabrication 15 techniques, which utilise polycrystalline or amorphous silicon deposited on top of a silicon substrate, produce beams of poor mechanical strength. Furthermore, the polycrystalline structure prevents the incorporation of Sactive components in the beam. A monocrystalline 20 microstructure would be highly desirable for its more predictable mechanical properties and because it would allow the inclusion of active components to tailor its response to specific stimuli.

A suitable starting material for the fabrication of monocrystalline microstructures is the so called SIMOX wafer. The term SIMOX stands for Separation by IMplanted OXygen and derives from the known technique of deep implanting oxygen ions into a silicon substrate to form 3 an oxide layer separating the substrate from a top silicon layer, while preserving the monocrystalline form of the silicon layer.

It is an object of the present invention to provide a method of fabricating monocrystalline silicon microstructures which overcomes, or at least ameliorates, the disadvantages of the prior art.

Accordingly, the present invention, in a first aspect, consists in a method for fabricating monocrystalline silicon microstructures, wherein the method includes the steps oi: forming isolation regions in a silicon wafer of the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer, the 15 isolation regions being located in the silicon layer; depositing a silicon nitride layer on the silicon layer of the wafer; opening first contact windows through the silicon S nitride layer to allow electrical connection to the 20 silicon layer; depositing and patterning a first metal layer to provide electrical connection to the silicon layer; etching through both the silicon nitride layer and the silicon layer to provide access to the buried silicon dioxide layer; and etching the buried silicon dioxide layer to leave a self-supporting silicon microstructure composed of monocrystalline silicon and silicon nitride.

4 Preferably, at least one pair of grooves is etched through both the silicon nitride and silicon layers and thereafter the silicon dioxide layer is etched between the or each pair of grooves such that the or each self-supporting silicon microstructure is a beam.

Preferably also, between the steps of forming isolation regions in the silicon layer and depositing a silicon nitride layer, the method includes the step of annealing the wafer.

In a preferred form, the silicon layer and the silicon nitride layer are separated by a thin silicon dioxide layer. More preferably, appropriate doping of S" either or both of the silicon layer and the substrate occurs for facilitating the subsequent formation of 15 active components in either or both of the substrate and the silicon layer. Even more preferably, such components are selected from: p-n junctions; MOS transistors; and, bipolar transistors.

According to a second aspect of the invention there 20 is provided a silicon microstructure formed from a silicon wafer of the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer which includes a cavity, the microstructure including a composite beam spanning said cavity and having adjacent silicon and silicon nitride layers, the beam being self supporting.

Preferably, the beam extends longitudinally across said cavity between two ends and includes a transverse width less than the transverse width of said cavity.

5 More preferably, the ends of the beams are respectively electrically accessible by way of contacts..

Preferably also, the beam includes doped regions within the silicon layer for forming at least one active component selected from the following: p-n junction diode; MOS transistor; and, bipolar transistor. More preferably, a voltage is able to be applied to the substrate to modify the electrical properties of said beam.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying diagrams in which: Figure 1 shows the cross-section of a SIMOX wafer; Figure 2 shows in cross-section a simple silicon 15 beam produced by selective removal of SIMOX oxide from "the wafer of Figure 1; Figure 3 shows a cross-section of a composite S: silicon and silicon nitride beam in accordance with the present invention; Figure 4 shows in plan view a thermocouple fabricated according to a first preferred embodiment of the present invention; Figure 5 shows a multiple thermocouple array fabricated according to a first preferred embodiment of the present invention; Figure 6 shows a SIMOX wafer in cross-section following ion implantation to form isolation regions in the silicon layer and deposition of a silicon nitride 6 layer; Figure 7 shows the SIMOX wafer of Figure 6 in cross-section after first contact etch and first metal deposition; Figure 8 shows the SIMOX wafer of Figure 6 in a further cross-section after second contact etch and second and third metal deposition; Figure 9 shows the wafer of Figure 6 in a still further cross-section after the opening of etch grooves; Figure 10 shows the wafer of Figure 6 and Figure 9 after final etch; Figures 11 and 12 show in cross-section a light modulator fabricated according to a second preferred embodiment of the present invention; and S 15 Figure 13 shows the cross-section of a p-n diode infra-red detector fabricated in accordance with the invention; and Figure 14 shows the cross-section of a sensor MOS Stransistor also fabricated in accordance with the invention.

Referring now to Figure 1 there is illustrated a cross-section of a SIMOX wafer with substrate 1, SIMOX silicon dioxide layer 2 and monocrystalline SIMOX silicon layer 3.

A monocrystalline beam 4 can be fabricated in such a wafer by selectively etching the SIMOX dioxide layer 2, as shown in figure 2, but this still lacks sufficient mechanical strength for many applications. In order to 7 increase the strength of the beam 4, an additional silicon nitride layer 5 is deposited on top.of the SIMOX silicon layer 3, as best shown in Figure 3. The resulting composite beam is stronger and does not easily deform. In othar embodiments layers 3 and 5 are separated by a thin intermediate silicon dioxide layer (not shown).

Composite beams can be manufactured with a thickness between 0.1 and 1 pm and a length of at least 100 im. These values are not obtainable by any other manufacturing method. Silicon beams made using previously known methods merely collapse on the substrate when fabricated to half this length. It will be appreciated that beams having a very short span are also 15 able to be manufactured in accordance with the invention.

In a first preferred embodiment of the present invention, that of an infrared thermal detector, the thermocouple 29 shown in Figure 4 is constructed with a SIMOX silicon layer 30 as the first thermocouple leg and 20 an ultra-thin platinum layer 16 as the second leg. The hot junction 10 of the thermocouple is positioned close to the middle of the beam 4 for thermal isolation and the cold junction 11 is located within the body of the .0 silicon adjacent the beam. The platinum 16, in addition to providing the second thermocouple leg, acts as an absorber layer for infrared radiation.

The starting material of the present invention is the SIMOX wafer as shown in Figure 1. In the first 8 steps, p+ and n+ isolation regions (not shown) are created in the SIMOX silicon using ion implantation.

Phosphorous is used as the donor impurity and boron as the acceptor impurity. After a high temperature anneal, a layer of silicon nitride 5 is deposited by low pressure chemical vapour deposition (LPCVD) techniques. A cross-section of the SIMOX wafer at this stage of fabrication is shown in Figure 6. In the Figure, a silicon substrate 1 supports a SIMOX oxide layer 2 and a SIMOX silicon layer 3 divided into p+ regions and n+ regions 3a and 3b respectively. Above layer 3 a silicon nitride layer 5 is deposited.

Fabrication continues with a first contact etch, using plasma etch methods, which opens first contact 15 windows through the silicon nitride layer 5 to provide a •site for first contacts 13. The etch is followed by aluminium deposition and a further plasma etch. These steps create a first metal layer 14 as the output S'"connection of the infrared thermal detector. A cross-section of one of the first contacts 13 appears in Figure 7. A plan view of both first contacts is shown in Figure 4.

In the next step, second contacts 15 are opened in a similar manner. The second contacts form the hot and cold junctions 10 and 11 respectively of the thermocouple shown in Figure 4, this time however, platinum 16 is deposited as the second metal layer using lift-off techniques. As the platinum layer is very thin, about 9 angstroms, a third metal layer 17 is required in and around the second contact on top of the second metal layer. This ensures proper metal continuity over the contact edges. Titanium is used for the third metal layer 17 and it is also patterned by a lift-off process.

The cross-section of one of the second contacts 15 from Figure 4 is shown in Figure 8.

The next step is the opening of etch grooves 18 by plasma etch. Figure 9 shows a cross-section of a composite silicon beam fabricated in accordance with the preceding steps and before the buried SIMOX oxide layer 2 is removed.

The last step of the process is therefore the final etch which locally removes at least substantially all the SIMOX oxide layer from beneath the beam to leave a self supporting structure. The final etch is performed in a liquid solution of hydrofluoric acid. A partial cross-section of the finished thermal detector is shown in Figure A thermal detector may consist of one or more thermocouples. Figure 5 shows a multi-thermocouple detector or thermopile. Each beam 4 of the thermopile includes two thermocouples 19a, 19b, one at each end of the beam. Once more, the hot junctions 31 of each thermocouple are located in the middle of the beam. The thermocouples are connected in series to produce an array. It will be appreciated that this array can include more or less of the thermocouples, as required.

10 A second preferred embodiment of the present invention is that of a light modulator 60 in which the composite beam 62, having a monocrystalline silicon layer 63 and a silicon nitride layer 64, is supported by an etched SIMOX silicon dioxide layer 61 and adapted for electrostatic deflection to vary the beam's separation from the substrate 65. Such a modulator is best illustrated in Figures 11 and 12.

It is well known that the varying thicknesses of silicon dioxide on top of a silicon wafer create interference patterns which cause light to be reflected as different colours. The apparent colouration of areas of 'blank' silicon is often observed, most clearly through a microscope. This effect is not to be confused 15 with diffraction of the light by the regular patterns of small structures, also familiar to those skilled in the art.

The first mentioned effect is caused by the constructive interference of some wavelengths of light 20 and the destructive interference of others as incident light passes through the silicon dioxide and is reflected from the underlying silicon surface.

The same effect is used to advantage in the second preferred embodiment of the present invention, such that incident light passes through the substantially transparent composite silicon/silicon nitride beam 62, and the cavity 66 beneath, to be reflected back by the substrate. If the beam is made to deflect, its 11 separation from the substrate and the light wavelength at which the desired interference occurs can be varied.

Further, when the incident light is monochromatic and of suitable wavelength, the invention in this embodiment may be used to modulate the intensity of the light reflected and to function as an optical switch.

Figures 11 and 12 show such a light modulator with the beam in an undeflected and a deflected state, respectively. The silicon substrate 65 is provided with an aluminium back electrode 67 to allow a potential to be developed between the substrate and the silicon beam.

Once again, a silicon nitride layer 64 provides structural strength. Contact holes 21 and aluminium S" interconnects 22 allow a modulating voltage to be applied 15 as shown in Figure 12.

The method of fabricaticn corresponds to t''at of the previously described embodiment, except that aluminium alone is used as a metal layer.

It will be appreciated that the beam 62 n not take the form of a rectangular prism, as shown in Figures 4 and 5, indeed it may be more appropriate to vary the beam cross-section so that, in the second preferred embodiment for example, interference effects are apparent over a greater area.

Referring to Figure 13 a cross-section of a third preferred embodiment of the invention is illustrated and includes an infra-red detector 33 based on a thermally isolated p-n diode 34. Detector 33 includes a base 12 formed by substrate 35 and adO': onal SIMOX silicon dioxide layer 36 and monocrystalline SIMOX silicon layer 37. A silicon nitride layer 38 is formed on layer 37, and as described above, etching of layer 36 occurs to provide a self supporting beam comprised by layers 37 and 38.

Additionally, to achieve the p-n junction, layer 37 is appropriately doped to form a p+ silicon region 40, an n+ silicon region 41 and an intermediate n silicon region 42. Both region 40 and 41 can be electrically connected with other componentry via metalisations 43 and 44 which are established through layer 38. A thin platinum layer is deposited on layer 38 and acts as an infra-red see** absorber.

15 This form of infra-red detector is more sensitive than the thermocouple described above due to lightly doped n region 42. That is, the thermoelectrical effect is much greater than for the p+ and metal junction as used in the thermocouple. Furthermore, direct absorption 20 of infra-red radiation in the depletion 7,ayer of the p-n junction allows detector 33 to operate as both a thermal and quantum detector.

The detector is advantageously located such that the p-n junction is located in the centre of the beam in order to provide a maximum available thermal isolation.

Reference is now made to Figure 14 where there is illustrated a cross-sectional view of a sensor MOS transistor 47 constructed in accordance with the 13 invention and which is hereinafter referred to as an SMOS transistor. Transistor 47 includes a silicon substrate 48, an etched SIMOX silicon dioxide layer 49, a doped SIMOX silicon layer 50 having two spaced apart p+ regions 51 and an intermediate n region 52, and a silicon nitride layer 53. Transistor 47 is fabricated in accordance with the invention and layer 50 and silicon nitride layer 53 form a composite beam which is physically spaced from the underlying substrate by the unetched portions of layer 49.

Metalisations 54 and 55 define a source and drain contact respectively, while an aluminium metalisation 56 on substrate 48 provides a gate contact.

Transistor 47 is used for selectively sensing one of a number of parameters such as vibration, temperature, 15 infra-red radiation, acceleration or applied voltage.

This is due to the cavity 57 between substrate 48 and layer 50 being analogous to the silicon dioxide layer in a prior art MOS transistor. That is, the substrate voltage is able to modulate the conductivity of layer The SMOS transistor is able to be either n-channel or p-channel, as required.

Although the above embodiments have been described with the inclusion of a single active component, a plurality of such components can be included on a single beam, if required.

Where the thermal isolating properties of the beam are paramount the cavity in the etched oxide layer is sealed under vacuum to minimise the thermal conductivity 14 of the cavity.

Although reference has been made to specific examples of silicon transducers, it will be understood by those skilled 5.n the art that the invention may be embodied in many other fozms.

a.

a 0* a 0 a. 0 0 0*

C

C.

a a a. 0 a.

Claims (28)

1. A method for fabricating monocrystalline silicon microstructures, wherein the method includes the steps of: forming isolation regions in a silicon wafer of the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer, the isolation regions being located in the silicon layer; depositing a silicon nitride layer on the silicon layer of the wafer; opening first contact windows through the silicon nitride layer to allow electrical connection to the silicon layer; depositing and patterning a first metal layer to provide electrical connection to the silicon layer; etching through both the silicon nitride layer and 15 the silicon layer to provide access to the buried silicon dioxide layer; and etching the buried silicon dioxide layer to leave a OS self supporting silicon microstructure composed of S monocrystalline silicon and silicon nitride.

2. A method according to claim 1 wherein at least one pair of longitudinally extending transversely spaced apart grooves are etched through both said silicon nitride and said silicon layers and thereafter the silicon dioxide layer is etched between said grooves such that the or each self supporting silicon microstructure is a beam.

3. A method according to claim 1 or claim 2 wherein 16 between the steps of forming isolation regions in tne silicon layer and depositing a silicon nitride layer, the method includes a step of annealing the wafer.

4. A method according to any one of the preceding claims wherein the silicon layer and the silicon nitride layer are separated by a thin silicon dioxide layer.

A method according to any one cf the preceding claims wherein either or both of the substrate and the silicon layer are doped for facilitating the formation of at least one active component.

6. A method according to claim 5 wherein said at least one active component is selected from: p-n junction diode; MOS transistor; and bipolar transistor.

7. A method according to claim 2 wherein said beam is at least 50m long.

8. A method according to claim 7 wherein said beam is at least 100l m long.

9. A silicon microstructure formed from a silicon wafer of the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer which includes a cavity, said 5 microstructure including a composite beam spanning said cavity and having adjacent silicon and silicon nitride layers, said beam being self-supporting.

A microstructure according to claim 9 wherein said beam extends longitudinally across said cavity between two ends and includes a transverse length less than the transverse length of said cavity.

11. A microstructure according to claim 10 wherein said 17 ends of said beam are respectively electrically accessible by way of contacts.

12. A microstructure according to claim 10 or 11 wherein said beam includes doped regions within said silicon layer for forming active components selected from the following: p-n junction diode; MOS transistor; and bipolar transistor.

13. A microstructure according to claim 12 wherein a voltage is applied to said substrate to modify the electrical properties of said beam.

14. A microstructure according to any one of claims 9 to 13 wherein said beam has a span of at least 0Sm.

A microstructure according to claim 14 wherein said beam has a span of at least 100xm.

16. A method for fabricating monocrystalline silicon microstructures substantially as herein described with reference to Figures 3 to 14.

17. A silicon microstructure substantially as herein described with reference to Figures 3 to 14. 20

18. A thermocouple formed from a silicon wafer of the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer which includes a cavity, said thermocouple including a composite beam spanning said cavity for supporting a thermojunction formed between said silicon layer and an adjacent metal layer, said beam being self-supporting and including adjacent silicon and silicon nitride layers.

19. A thermocouple according to claim 18 wherein said 18 metal layer is adapted to absorb infra-red radiation.

A thermocouple according to claim 18 or claim 19 wherein said thermojunction is centrally disposed along said beam to facilitate thermal isolation from said substrate.

21. A thermopile including a plurality of thermocouples as defined in any one of claims 18 to 20, wherein said thermocouples are electrically connected in series and respective beams are substantially aligned and span a single cavity.

22. An infrared detector including a thermocouple according to claim 19.

23. A light modulator formed from a silicon wafer of *o .*the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer which .includes a cavity, said modulator including a self supporting at least partially transparent composite beam spanning the cavity which has adjacent silicon and silicon nitride layers, and actuation means to cause said 20 beam to deflect whereby constructive and destructive interference occurs between predetermined wavelengths of light reflected from said cavity and said beam.

24. A modulator according to claim 23 wherein said cavity has a depth comparable with said predetermined wavelengths.

A modulator according to claim 23 or claim 24 wherein said actuation means provides an electrical -Th4 potential between the substrate and said beam to cause 19 said beam to deflect.

26. A modulator according to any one of claims 23 to wherein said actuation means mechanically deflects said beam.

27. An optical or infrared detector formed from a silicon wafer of the type having a substrate and a monocrystalline silicon layer separated by a buried silicon dioxide layer which includes a cavity, said detector including a composite beam spanning said cavity for supporting at least one integrated semiconductor device, said beam being self-supporting and including adjacent silicon and silicon nitride layers.

28. A detector according to claim 27 wherein said device is a p-n junction diode formed in said silicon layer of said beam. DATED this llth Day of April, 1995 AWA MICROELECTRONICS PTY LIMITED Attorney: JOHN B. REDFERN Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS V ,C L ABSTRACT A simox wafer includes substrate simox silicon dioxide layer and monocrystalline simox silicon layer An additional silicon nitride layer is deposited on top of silicon layer to allow the fabrication of a monocrystalline beam by selectively etching the dioxide layer The thermal insulating property of the resultant beam offers an ideal site for construction of thermocouples light modulators (60) and active components such as p-n diode MOS transistors (47) S. and bipolar transistors. S