PLD is an increasingly popular technique for depositing high- quality thin films of complex crystalline materials such as high-temperature superconductors or colossal magnetoresistive oxides. The PLD method of thin film growth involves evaporation of a solid target in an high vacuum chamber by means of short and high-energy laser pulses. In a typical PLD process, a researcher places a target in a vacuum chamber. A pulsed laser beam vaporizes the surface of the target, and the vapor condenses on a substrate. The main components are a laser, optics, and a vacuum system.

PLD's main advantage over other techniques is its ability to transfer material from a bulk"target" (typically a disc of one or more centimeters in dimension) onto a surface while maintaining the chemical and crystallographic properties.

This is achieved by focusing a pulsed high power laser onto the target, causing the irradiated surface region of the target to rapidly vaporize and produce an ionized cloud, which then propagates away from the target surface. The material can recondense on a substrate (e. g. a crystalline wafer) placed in the path of the expanding cloud. Each laser pulse typically produces approximately 0.01 monolayer of deposited material. Hence after, say, 50 000 shots (equivalent to about 80 minutes at a laser repetition rate of 10 Hz), several 100 nm of material can be deposited.

A technical problem associated with PLD is that some of the material liberated by the laser pulse travels back towards the laser entrance window. Depending on the type of material being deposited, this can lead to a gradual reduction of the optical transmission of the window and hence to a change in the processing conditions. For the deposition of metals, this can become serious after only 50 000 laser shots, hence the window must be replaced or cleaned after each experiment.

This is time-consuming and often impractical.

A solution to the technical problem is a device known as the "intelligent window° sold by PVD Products Inc, US (www. pvdproducts. com). This is simply a large quartz disc placed in the vacuum chamber just behind the laser entrance window. Most of the disc is protected by a housing from the expanding cloud, and is only exposed in the path of the laser beam. When material deposited on this part of the quartz disc causes it to absorb/reflect the laser light too strongly, the disc can be rotated by a small angle, thereby exposing a fresh region with high optical transmission. This solution therefore extends the amount of deposition one can perform by approximately a factor of 10 before the system has to be opened and a fresh window introduced.

A problem with the above known solution is that the laser entrance window port must be periodically replaced and/or cleaned. Accordingly, this solution delays the inevitable time and cost consuming task of cleaning a port window, albeit a second internal port window.

An object of the present invention is to provide a solution to the aforementioned problem wherein time and cost consuming cleaning of the port window is completely avoided.

This and other objects are achieved by a system for vapor deposition comprising: a laser, a lens, a laser window, and a

vacuum chamber housing a target and a substrate film arranged therein such laser light entering the chamber impinges upon said target so as to vaporize target material thereby causing an ionized cloud of target material which adheres to said substrate film, characterized in that a pulsed gas valve and tube are arranged within said chamber housing such that said entering laser light passes through said tube and gas from said valve is forced into said tube by said pulsed gas valve when said entering laser light passes through said tube so that liberated target material collides with said gas and is deflected towards said tube where said material adheres to said tube.

The present solution also includes a method for preventing material build up on a vacuum chamber window, comprising the steps of: generating an ionized cloud in a vacuum chamber, said cloud comprising liberated materials, generating a gaseous cloud between said ionized cloud and said chamber window so that said cloud deflects said liberated materials away from said window.

Examples of the invention are described below in accordance with the drawings wherein: Figure 1 depicts a layout of the present inventive system ; and Figure 2 depicts the present inventive method in block diagram.

Figure 1 depicts an embodiment of the present invention wherein a pulsed laser 10 produces a beam 12 directed, via optics 14 into a vacuum chamber 20 window 16. The window 16 is affixed to the chamber 20 at points 18. The laser may have a repetition rate of 10 Hz. The optics may include lens, mirrors and the like commonly used to redirect laser beams. The window may comprise light transparent materials

known to one skilled in the art. The vacuum chamber may also be a standard chamber known in the art. As used in the present invention, the chamber may be operated at a vacuum of 10-4 mbar.

Beam 12 passes into the interior 38 of chamber 20 and impinges upon target 22. Target 22 may have any geometrical shape and size known to one skilled in the art. The beam strikes target 22 at location 46. The beam evaporates a surface portion of target 22, at location 46, thereby producing an ionized cloud 24 of liberated target material.

A substrate 26 is positioned proximate to target 22 in the direction of the ionized cloud 24. Accordingly, the liberated material adheres to the substrate 26. Other elements may be used in place of a substrate as is known in the art. Part of the ionized cloud is directed back in the direction of the beam and towards the chamber window 16.

This part is illustrated as liberated target material or errant material 28.

A pulsed gas valve 30 is positioned between target 22 and window 16. The valve may be mounted on a translation stage 48. The location of valve 30 depends upon what feature of the chamber 20 is intended to be protected from coating by the errant material 28. Accordingly, it is within the scope of the invention to place the valve 30, and elements functioning therewith, in other locations. In the disclosed embodiment, the valve is placed proximate to window 16. A valve includes a nozzle which attaches to a tube 34 through which gas is received into the valve 30. The gas may comprise a heavy weight gas such as argon, neon, nitrogen and krypton. The gas is directed, via valve 30 and output nozzle 40, into a tube having an upper and lower wall, 32b and 32a respectively. The tube is cylindrical and depicted in a cross sectional view and may have a variable or fixed thickness based upon an expected amount and/or direction of errant material. The tube upper and lower walls further define inner

walls 44a and 44b respectively. The gas is fed into the tube at an approximate center tube and concentrates therein to form a gas cloud 42. The tube includes open ends from which the gas may emit. The tube is positioned such that beam 12 passes therethrough. As with the gas valve, the tube may be located in other locations within the chamber.

As the errant material 28 travels into gas cloud 42, the material is redirected by the gas cloud away from window 16.

One direction is the inner walls 44a and 44b, whereupon the errant material adheres to the inner walls. Accordingly, the inner walls may act as a trap for the errant material, this is especially true given the sticking probabilities of the vast majority of materials deposited using PLD means when the materials impinge on a solid surface.

The valve is substantially synchronized with the laser such that a cloud is present prior to or substantially simultaneously with the presence of the errant material. For example, the gas pulse would have a duration of approximately 100-200 microseconds. This duration is significantly longer than the pulse of liberated deposit material, namely about 25 microseconds.

The pulsed gas is on for less than 1% of the time, when it is needed, thereby lowering the working gas pressure accordingly. Accordingly, errant material flies directly towards the PPW in more or less collisionless conditions (i. e. it is not deflected by colliding with background gas molecules) and only gets deflected when it hits the localized pocket of gas emitted by the pulsed valve. Therefore it cannot get around the PPW and onto the laser entrance window by multiple scattering by gas molecules, which would be possible if one used a static and spatially homogeneous background of gas.

List of reference numerals 10 laser 12 laser beam 14 optics 16 chamber window 18 points 20 vacuum chamber 22 target 24 ionized cloud 26 substrate 28 errant material 30 pulsed valve 32a lower tube wall 32b upper tube wall 34 tube 36 gas 38 interior of vacuum chamber 40 output nozzle 42 gaseous cloud 44a tube upper inner wall 44b tube lower inner wall 46 target point 48 translational stage