There is a class of dielectric materials known as ferroelectrics, such as lead scandium tantalate (Pb2ScTaO6), which exhibit a characteristic internal electrical polarisation within a certain temperature range. Ferroelectric materials demonstrate pyroelectricity which makes them suitable for use in the detector arrays of thermal imaging systems of the type used by emergency services, for instance, to locate survivors in collapsed buildings. At least in the UK, the development of ferroelectric detector arrays has largely been based on a hybrid approach, where bulk ceramic ferroelectric material is bump-bonded to an electronic substrate.

The ultimate performance of hybrid ferroelectric detector arrays may be limited by unwanted thermal conduction mechanisms. In order to improve the imaging performance of the array, a low thermal conductance pixel structure is required. It is advantageous to couple this to a low pixel thermal capacitance so as to match the thermal time constant to the array readout. Both of these improvements can be achieved through the use of thin film materials, where the pixel is made, by micro- fabrication techniques, and supported above a substrate by thin, narrow legs. Such a structure is called a ferroelectric microbridge.

It has been known to grow thin films on substrates by vacuum deposition techniques such as sputtering. Basically, sputtering involves bombarding energetic ions in a plasma (commonly of argon) into a target of the desired film component, which has a bias power applied to it. Generally, the bias power is adjusted by varying the applied bias voltage. Electrically insulating oxide targets necessitate the use of an RF bias

voltage whereas metal targets may have DC bias voltages. Atoms from the surface of the target are dislodged and deposited on surrounding surfaces, including that of the substrate which is placed close by. For reactive sputtering, oxygen may be admitted into the plasma as the reactive gas so as to react with the dislodged atoms to form oxides which are deposited. The deposition process may be assisted by a magnetron which directs electrons dislodged by the target to produce further energetic ions.

The applicants have attempted to produce lead scandium tantalate (hereinafter referred to as"PST") thin films using two alternative configurations of apparatus having two sputter targets. Firstly, the applicants tried using apparatus having a target of lead and a composite target of scandium and tantalum arranged close together so that there was a region in which dislodged atoms from both targets were present for deposition. However, this created uniformity problems. Alternatively, the applicants tried using apparatus having one target made from lead and another target divided by area into sectors of different size, one sector of scandium and the other of tantalum.

A substrate was seated on a rotatable plate, held above a magnetron on a support which was pivotally mounted so as to be movable at constant speed relative to each of the targets. The movement allowed the substrate to be scanned between the two targets. The scan angle, that is the angle through which the support was pivoted between its two outermost positions, and the speed with which the support was moved between those positions, were adjusted. The relative areas and positions of the scandium and tantalum, combined with a careful setting of the scan angle, allowed the ratio of the scandium and tantalum to be controlled. Each target had a separate RF bias generator, so altering the bias to the lead target varied the lead ratio in the film, for a particular scan routine. Scanning between the two targets thus built up a film of PST. The combination of scanning and substrate rotation resulted in good film uniformity.

Two target sputtering apparatus of the second type described above suffers from the disadvantage of inflexibility, not least because the scandium/tantalum target is adjusted by process trials to be specific to the production of a certain film

composition. In addition, reactive sputtering from metal targets in a gas such as oxygen or a mixture containing oxygen, can result in the formation at the target surface of an oxide coating, which has an effect upon the sputtering rate. The gas pressure and its flow and the target bias power will, in a complex manner, determine the extent of oxidation. If a relatively high target bias power is used, oxidation may be negated, but the sputter rate may be difficult to control and may"run away".

Alternatively, a low target bias power may result in the target becoming"poisoned" with oxide thereby restricting the sputtering rate. Generally, there is an optimum bias power for each target material for a given reactive gas pressure and flow, which provides a stable sputtering rate, that is to say, a rate of sputtering which is substantially constant, and which will remain substantially constant for a period of time, such that a known amount of material will be deposited in a particular time.

However, the bias power needed for the optimum sputtering rate of each target at any particular gas pressure and flow may be inconsistent with the bias power required to form the desired composition of film.

US-A-5441804 discloses apparatus for sputtering multi-layer films having two targets, each composed of one of the intended layer materials, wherein a substrate is rotated over the targets in turn and may be stopped over each of them in order to build up one layer of the film. However, the films are not reactively sputtered and control of the target biases is not a concern.

Some thermal imaging devices are fabricated from ferroelectric thin films deposited on silicon substrates. The deposition of the thin film has to be done at temperatures which are compatible with the silicon. It has been found, at least in experimenting with different compositions of PST film, that excess lead promotes the formation of a ferroelectric crystal phase at a lower temperature, thereby easing compatibility problems. On the other hand, having excess lead throughout the film may undesirably affect its electrical and/or structural properties. Therefore, what may be ideally required is a film with a relatively high lead content nearest the substrate and a lower lead content nearest the exposed surface. However, it is not possible with the known

dual target sputtering apparatus to vary the deposition process in such a way as to alter the lead content throughout the film.

Also, there may bc benefits in depositing an initial layer with a different composition to the remainder of the film. The initial, or seed, layer may interact with the substrate on which the film is grown so as to enhance the growth and/or orientation of the ferroelectric film, which in turn affects the film properties.

It has been known to produce compositionally graded thin films. These have been made by building up layers of slightly different composition, each deposited by"spin and fire"metalorganic deposition. However, in order to achieve a continuous and smooth graduation of the composition, the film has to be annealed at about 1000°C, which precludes the films from being deposited on to silicon.

The invention provides reactive sputtering apparatus for depositing a multi- component thin film on to a substrate, having a first target comprising at least one component element of the film, a second target comprising at least one other component element of the film, bias power means for applying a bias power to each target, means for bombarding the first and second targets with energetic ions, movable means for supporting the substrate, and which movable means is movable between at least two positions at which atoms dislodged by the energetic ions from one or both of the targets will be deposited on to the substrate, and means for controlling the movement of the supporting means, characterised in that the bias power means applies to each target a bias power whereby atoms are dislodged therefrom at a substantially stable rate and the controlling means controls the positions to which the movable means is moved and the extent to which the movable means dwells at one or more of the positions thereby to determine the composition of the film.

By controlling the movement of the substrate the composition of the mixture may be adjusted independently of the bias power. The bias power can be set at a level which

ensures a stable sputtering rate, thereby minimising the effects of oxidation and maximising control over the amount of component deposited per unit time. The inconsistency of the set bias power with the optimum required for the composition can be disregarded, and the composition can be determined by moving the substrate appropriately. Moreover, control of the movable means enables the composition to be altered throughout the deposition simply by altering the movement of the support means rather than changing target areas and/or bias voltages, which are non-linear effects that require extensive trials to optimise; dwell times in each position can be varied throughout the deposition process to produce graded film compositions; and, dwelling initially for a relatively long time in one position can produce a film whose lowest layers are rich in or consist exclusively of the constituent whose dislodged atoms are prevalent in that position.

The bias power may be determined by the bias voltage applied by the bias power means. The bias voltage may be an RF or DC voltage or a pulsed DC voltage.

The speed with which the movable means moves between the positions may be controlled so that the movable means, and hence the substrate, may cycled through the sputtering positions relatively quickly in comparison to the deposition rate in order to achieve intimate mixing of the component elements. The movements of the movable means may be continually varied as the film is deposited, thereby to achieve the graduated or element rich films as discussed above. The movable means may comprise a pivotable substrate support upon which is mounted a rotatable substrate holder.

Preferably, the first target comprises two component elements of the thin film.

Further preferably, each component may occupy a substantially equal area of the target. The first target may be divided diametrically into two halves of equal surface area. Alternatively, discontinuous sectors of one component may overlay the other component. To produce PST, the first target comprises scandium and tantalum, and the second target comprises lead. A third or other targets may be included.

With the first target divided equally into two, there may bc three positions to which the movable means is moved: a first close to the first half of the first target, a second close to the second half and a third close to the second target.

The apparatus may further comprise means for altering the amount of admitted reactive gas.

The control means may comprise a stepper motor, a stepper motor controller for supplying drive signals to the stepper motor and a computer as a means for sending drive instructions to the stepper motor controller. The invention may further provide a program for the computer to generate the appropriate instructions for the stepper motor to effect the desired movement of the substrate.

The invention also provides a method of depositing by reactive sputtering a multi- component thin film on to a substrate, comprising bombarding first and second targets each of at least one component element of the film with energetic ions, applying a bias voltage to each target, and selecting at least two positions at which atoms dislodged from one or both of the targets by the energetic ions will be deposited on to the substrate, characterised by applying to each target a bias power whereby atoms are dislodged therefrom at a substantially stable rate and moving the substrate between the selected positions and dwelling at at least one of the selected positions thereby to determine the composition of the mixture.

The bias power may be determined by the bias voltage applied to each target. The bias voltage may be an RF or DC voltage or a pulsed DC voltage.

The substrate may dwell for the same period of time at each of at least two of the selected positions. Alternatively, the substrate may dwell for different periods of time. The dwell periods may be varied throughout the deposition thereby to provide

graded or varying film compositions. The speed with which the substrate is moved may be selected to achieve intimate mixing of the atoms dislodged from each target.

The first target may comprise two components of the thin film and the second target may comprise a third component of the thin film, in which case the substrate may be moved between a selected first position at which atoms dislodged from the first component by the energetic ions will be deposited on to the substrate, a selected second position at which atoms dislodged from the second component by the energetic ions will be deposited on to the substrate and a selected third position at which atoms dislodged from the third component will be deposited on to the substrate.

The deposited thin film may be annealed at a range of times and temperatures to promote mixing of the components.

The thin film may be a multi-component oxide, more particularly a ferroelectric material such as lead scandium tantalate, lead magnesium niobate, barium strontium titanate or strontium bismuth tantanate, or lead zirconate titanate.

The invention will now be described, by way of example, with reference to the following drawings, in which: Figure 1 is a side cross-sectional view of thin film deposition apparatus according to the invention; and Figure 2 is schematic plan view of a dual target sputtering apparatus according to the invention.

With reference to figures 1 and 2, two target reactive sputtering apparatus indicated generally at 1 has a housing 12 within which there is a rotatable substrate holder 2 mounted on a pivotable substrate support 4, a first target 8 and a second target 6. The support 4 is attached at the end of a rotatable shaft 14, such that the support 4 pivots

about the longitudinal axis of a shaft 14. The support 4 is arrange so that the substrate holder 2 can be moved closely, that is, in the regions to which atoms are dislodged, over either of the targets 6,8. The pivotal movement is controlled by a stepper motor 16, with its rotor connected to the shaft 14 through a gearbox (not shown) and drive belt 22. The stepper motor 16 receives drive signals from a controller 20 which is sent drive instructions by a computer 10. Rotation of the substrate holder 2 is achieved by a further motor 18 mounted on the support 4.

The first target 8 is divided diametrically into two halves of equal area, one composed of tantalum and the other scandium. The target is formed by riveting, with tantalum rivets, a semi-circular piece of scandium on to a complete circle of tantalum. The second target 6 is composed solely of lead. Both targets 6,8 are circular, 200mm in diameter.

The computer 10 is programmed so that it delivers drive instructions to the controller 20 to deliver the drive signals to the stepper motor 16 necessary to move the support 4 between three sputtering positions; a first over the scandium sector of the first target 8, in the region in which the majority of dislodged atoms are from the tantalum sector of the first target 8, a second over tantalum sector of the first target 8, in the region in which the majority of dislodged atoms are from the tantalum sector of the first target 8, and a third over the second target 6. The support 4 may dwell at any of these positions and, again, this is controlled by the stepper motor 16 under the command of the program in the computer 10. The composition of the film is determined by whether or not the support 4 does dwell at any position and the duration of any such dwell, as well as the speed with which the support moves between the positions.

During sputtering, the pressure within the housing 12 is reduced using a cryopump (not shown). The substrate (not shown) is clamped to the substrate holder 2 which is heated from the rear by a folded quartz halogen bulb 24, Argon, as the source of energetic ions, is admitted into the housing 12 through gas inlet pipe 36. The

chamber pressure is measured using a pressure gauge 44. A pressure controller 42 monitors the pressure and controls the flow of gas through a flow controller 34, to maintain a pre-set constant pressure. The argon flow rate is measured using a mass flow gauge 48. A flow ratio controller 46 monitors the argon flow and controls the operation of a mass flow controller 40 to admit oxygen through inlet pipe 38 to maintain a pre-set flow ratio of oxygen to argon.

Separate RF bias power supplies 30,32 operating at 13.56 MHz are connected to each target 6,8 through impedance matching units 26,28. The power supplies 30,32 can be set to operate with constant bias power or with constant bias voltage.

The apparatus 1 was optimised for the production of a thin film of PST of a desired composition by means of a series of depositions at different argon/oxygen flour ratios and chamber operating pressures and for various bias powers applied to the two targets 6,8. Optimisation involved the determination of stable sputtering rates for each target 6,8. By manually altering the pre-set on the pressure controller 42 a range of chamber operating pressures can be studied. By manually altering the pre-set on the flow ratio controller 34 a range of oxygen to argon flow ratios can be studied.

Altering either of these pre-sets will influence the amount of oxygen in the chamber and the degree and rate of target oxidation and hence sputtering rates. Long term stability was assessed by observing the state of each target surface after prolonged sputtering (for example, the lead target 6 becomes blackened by oxidation). The relative and total sputtering rates were determined for depositions using different substrate movement sequences, including different dwells at each sputtering position.

The composition of each deposited film was determined by energy dispersive x-ray analysis. Within the constraint of long term stability, a minimum deposition rate of 50A per minute was desired by adjusting the biases and flow rates. Final adjustments of the composition were achieved by altering the movements of the substrate. It was found that in the scandium sputtering position, tantalum was also deposited, so the dwell had to be adjusted accordingly.

By way of example, the apparatus illustrated was optimised to deposit PST on to circular crystalline silicon wafers substrates (not shown), 75mm in diameter, with the following settings: the total sputtering pressure was 9mTorr, the argon flow rate was 16sccm, the oxygen flow rate was 4.3sccm, the substrate was rotated at 13rpm, the substrate temperature was 300°C, the lead target bias power was 105W (66V) and the scandium/tantalum bias power was 550W (190V). The stepper motor 16 has 288000 steps per revolution of the pivotable substrate support 4. In stepper motor counts, where zero is set when the support 4 is at the home position at the rear of the housing 12, each of the three sputtering positions was identified as follows: scandium sputtering position 90000, tantalum sputtering position 130000 and lead sputtering position 194400. The support 4 was moved between these positions with a stepper motor speed of 80000 steps per second and with an acceleration of 80000 steps per second per second. Dwell times were 10 seconds in the scandium position and 5 seconds in each of the tantalum and lead positions.

A PST film containing scandium and tantalum ions in equal atomic ratios and lead with a ratio of around 10% more than the sum of the scandium and tantalum ratios was deposited on to the wafers. The film was deposited at in the region of 80 A per minute. In order to achieve the desired film thickness of 0.8 micron, the support 4 was cycled through the three sputtering positions 230 times to give a total deposition time of 100 minutes. In order to improve the crystalline quality of the deposited film, the wafers were annealed in oxygen up to a temperature of 700°C for 5 minutes.