SAW device and method of manufacture

Abstract

A method of reducing non-uniformity in the resonance frequencies of a surface acoustic wave (SAW) device, the SAW device comprising a silicon oxide layer comprising an oxide of silicon deposited over interdigital transducers on a piezoelectric substrate by reactive sputtering. The method comprises positioning a piezoelectric substrate having interdigital transducers on a substrate support, then depositing a silicon oxide layer comprising an oxide of silicon over the piezoelectric substrate and the interdigital transducers to form a SAW device. The substrate support is positioned relative to a sputtering target so that the silicon oxide layer of the SAW device has an arithmetic mean surface roughness (R.sub.a) of 11 angstroms or less.

Claims

1. A method of reducing non-uniformity in the resonance frequencies of a surface acoustic wave (SAW) device, the SAW device comprising a silicon oxide layer comprising an oxide of silicon deposited over interdigital transducers on a piezoelectric substrate by reactive sputtering, the method comprising: (i) positioning the piezoelectric substrate having the interdigital transducers on a substrate support and depositing the silicon oxide layer comprising the oxide of silicon over the piezoelectric substrate and the interdigital transducers to form a first SAW device of the SAW device, the substrate support being positioned relative to a sputtering target so that the silicon oxide layer of the first SAW device has an arithmetic mean surface roughness (R.sub.a) of 11 angstroms or less; (ii) adjusting the position of the substrate support; and (iii) positioning a subsequent piezoelectric substrate having further interdigital transducers on the substrate support and depositing a further silicon oxide layer comprising an oxide of silicon over the subsequent piezoelectric substrate and the further interdigital transducers using the sputtering target to form a second SAW device of the SAW device; wherein the position of the substrate support in step (ii) is chosen so that the further silicon oxide layer of the second SAW device has an arithmetic mean surface roughness (R.sub.a) of 11 angstroms or less.

2. The method of claim 1, wherein the SAW device is a temperature-compensated SAW device.

3. The method of claim 1, wherein steps (ii) and (iii) are repeated at least once during a lifetime of the sputtering target.

4. The method of claim 1, wherein the position of the substrate support is adjusted by changing a separation distance between the substrate support and the sputtering target.

5. The method of claim 1, wherein the adjustment of step (ii) is based on a surface roughness measurement of the silicon oxide layer of the first SAW device.

6. The method of claim 1, wherein the position of the substrate support is adjusted based on a look-up table.

7. The method of claim 6, wherein the look-up table provides a position adjustment value corresponding to a value of an elapsed lifetime of the sputtering target.

8. The method of claim 1, wherein the substrate support is vertically movable.

9. The method of claim 1, wherein each of the silicon oxide layers is deposited using a magnetron.

10. The method of claim 1, wherein each of the silicon oxide layers is deposited by a DC sputtering deposition.

11. The method of claim 10, wherein the DC sputtering deposition is pulsed DC sputtering deposition.

12. The method of claim 11, wherein the pulsed DC sputtering deposition is pulsed DC magnetron sputtering deposition.

13. The method of claim 1, wherein the SAW device is a SAW filter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of an apparatus that may be used to carry out the present invention.

(3) FIG. 2 is a schematic diagram of a SAW device according to the present invention.

(4) FIG. 3 is a graph showing the relationship between frequency non-uniformity of a SAW device and the surface roughness of its silicon oxide layer.

(5) FIG. 4 is a graph showing how RMS surface roughness at the centre and at the edge of a wafer varies with the target-to-platen distance used.

(6) FIG. 5 shows topography images obtained by atomic force microscopy for centre and edge regions of wafers in which different target-to-platen (TTP) distances were used when depositing the silicon oxide layer.

(7) FIG. 6 shows surface roughness distributions obtained by X-ray reflectometry (XRR) for a silicon oxide film deposited using a target-to-platen distance away from an optimum distance (left side of the Figure, labelled A) and for a silicon oxide film deposited using a target-to-platen distance optimised for minimum silicon oxide roughness (right side of the Figure, labelled B).

(8) FIG. 7 shows a frequency distribution map for a SAW devices corresponding to the devices of FIG. 6.

DETAILED DESCRIPTION

(9) FIG. 1 shows a typical apparatus 10 for carrying out the invention in the form of a pulsed DC magnetron reactive sputtering arrangement. The apparatus includes a vacuum chamber 12 within which is located a wafer platen 13. An upper portion of the chamber 12 includes a circular target 14, which can be formed from silicon or silicon dioxide. A pulsed DC power supply 11 is provided to apply pulsed DC power to the target 14, which acts as a cathode. The apparatus 10 further comprises an anode 17 in the form of an annular ring made of a metal (typically aluminium or stainless steel) which surrounds the perimeter of the target 14. The anode 17 is supported by an insulating part 18 (e.g. a ceramic insulator) to avoid it touching the grounded chamber 12 and to keep it isolated from the target 14 which is situated above it. A magnetron 15 of known type is located behind (above) the target 14.

(10) In use, the platen 13 supports a substrate, typically a wafer, in opposition to the target 14. The platen 13 is formed from a conductive material which is biased with an RF signal provided by an RF power supply through a capacitive coupling circuit so that the platen 13 can act as an electrode. The RF bias in the presence of a plasma produces a negative DC bias to develop on the platen 13 so that sputtered ions are accelerated towards the substrate.

(11) Additionally, the platen 13 can move vertically, so that the distance between the target 14 and the platen 13 can be adjusted. The target-to-platen distance (TTP) changes the angular distribution of the target ions landing on the substrate which affects the deposited film properties.

(12) Sources of oxygen and argon are provided. Oxygen (O.sub.2) and argon (Ar) are selectively admitted into the chamber 12 through a gas inlet 16 using mass flow controllers as part of an appropriate gas manifold. Oxygen gas reacts with silicon sputtered from the target 14 to form a layer comprising an oxide of silicon on the surface of a substrate positioned on the platen 13. PVD systems that can be used in connection with the invention, or can be readily adapted for use in connection with the invention, are available commercially. For example, the applicant's own Sigma fxP PVD system can used with a desired magnetron.

(13) Example operating conditions for the silicon oxide sputtering deposition process are: a target power of 2 kW, a sputtering gas flow mixture of 10 sccm Ar and 50 sccm O.sub.2, a platen temperature of 50 C. and a platen DC bias of approximately 100V or higher.

(14) FIG. 2 shows a schematic of a SAW device 20 produced by apparatus 10. The device 20 comprises a piezoelectric substrate 21, e.g. lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3). Metal electrodes 22 forming interdigital transducers are deposited on the substrate 21. A silicon oxide layer 23 is deposited over the substrate 21 and electrodes 22.

(15) Silicon Oxide Roughness and SAW Frequency Response

(16) Experiments were performed to investigate the correlation between the average (arithmetic mean) surface roughness of the silicon oxide layer and the within-wafer (WIW) standard deviation of centre frequency (which is a measure of non-uniformity of frequency response) of a SAW device. The SAW devices used in the experiments comprised a LiNbO.sub.3 substrate with a 200 nm silicon oxide layer deposited using the above-described apparatus 10. The arithmetic mean surface roughness was measured in angstroms by X-ray reflectometry (XRR) using a commercially available system.

(17) The results, shown in the graph of FIG. 3, show a strong correlation between mean silicon oxide film roughness and WIW standard deviation of centre frequency. The graph correlates data from 11 experiments varying both hardware configuration (e.g. magnetron geometry) and process parameters. Film thickness, density and refractive index did not show a strong correspondence with frequency non-uniformity. Surface roughness was found to be the only measured blanket film parameter that could be correlated directly to WIW frequency non-uniformity across such widely varying conditions. The graph indicates that a mean surface roughness of about 11 angstroms or less provides good uniformity in the frequency response.

(18) Roughness Optimization with Target-to-Platen Distance

(19) The inventors have also discovered that the surface roughness of silicon oxide films formed by sputtering deposition can be optimized by adjusting the distance between the target 14 and the platen 13. FIG. 4 shows a graph of RMS surface roughness (Rq) at the centre and at the edge of a 200 nm silicon oxide film using different target-to-platen distances. The surface roughness values were measured over a 1000 nm by 1000 nm area using a high resolution atomic force microscope (AFM) in non-contact mode. With a standard D-type magnetron design, an optimal distance is observed in this example at a target-to-platen distance (TTP) of approximately 40 mm. This distance resulted in a RMS surface roughness of approximately 1.0 angstrom at both the centre and edge of the wafer.

(20) The corresponding AFM topography images are shown in FIG. 5. Away from the optimal target-to-platen distance, a clear difference can be observed between the topography at the centre and the edge of the wafer, with the film generally being rougher in the centre than the edge and some fine grain structure is observable. At the optimal TTP of 40 mm, no discernible difference in roughness can be observed between the centre and edge of the wafer and there is little observable structure.

(21) The optimal target-to-platen height is found to depend on the individual apparatus configuration, e.g. on the magnetron design (field strength, geometry) but it does not directly correspond to thickness non-uniformity, which also varies with target-to-platen distance. This is shown in Tables 1 and 2.

(22) TABLE-US-00001 TABLE 1 RMS roughness Rq (measured by AFM) and thickness non-uniformity %1 versus target-to-platen distance (TTP) for a silicon oxide film deposited using a SPTS Sigma fxP PVD system with magnetron A. TTP Rq() Rq() Thickness mm centre edge %1 36 2.08 1.13 1.44 40 0.97 0.96 1.4 47 1.5 1.28 1.32 60 2.01 1.68 3.72 78 2.57 1.51 3.96

(23) TABLE-US-00002 TABLE 2 RMS roughness Rq (measured by AFM) and thickness non-uniformity versus target-to-platen distance (TTP) for a silicon oxide film deposited using a different magnetron B. TTP Rq() Rq() Thickness mm centre edge %1 47 68.63 6.1 12.92 60 7.55 2.1 6.1 78 3.52 1.49 4.64

(24) The terms magnetron A and magnetron B are merely descriptors, to indicate that different magnetrons were used. The invention is not limited with respect to the type of magnetron used.

(25) With the surface roughness of silicon oxide films characterized, SAW devices were fabricated and tested. A clear relationship was observed between the silicon oxide layer surface roughness and the frequency distribution of the SAW devices. FIG. 6 shows XRR roughness maps for a 200 nm silicon oxide layer deposited on a 150 mm wafer at a suboptimal TTP distance of 47 mm (left side of FIG. 6); and at an optimal TTP distance of 40 mm (right side of FIG. 6). FIG. 7 shows corresponding frequency distribution maps of the devices. It can be seen that away from the optimal TTP distance, both the XRR roughness map and the frequency distribution show a distinct bulls-eye pattern with higher roughness corresponding to a lower centre-frequency of the SAW devices. This frequency non-uniformity can lead to a reduction in product yield. However when the TTP distance was optimized with respect to silicon oxide surface roughness, the frequency distribution of the SAW devices was uniform across the whole wafer resulting in fewer rejected devices and increased product yield.

(26) Correction Through Target Lifetime

(27) An important aspect of high volume production of RF filter devices is maintaining device yield and performance across the whole of the target's lifetime. In the apparatus 10 described earlier, the effective target-to-platen distance increases as the target 14 gets used up. This can lead to a drift in the silicon oxide film properties over the course of the target's lifetime resulting in significant device yield loss. The optimal target-to-platen distance can be re-established at intervals through the target's lifetime by making roughness measurements and adjusting the target-to-wafer distance accordingly in order to maintain frequency performance and yield.