IE83994B1 - An inspection system support mechanism - Google Patents

Abstract

ABSTRACT A support mechanism (1) has a support (11) for a radiation source and a support (13) for a detector. A wafer chuck (40) is mounted on a motion stage (4) for X, Y, and Z movement. The source and detector supports (11, 13) move in a plane perpendicular to this plane of the chuck (40). They are mounted on arms (17, 18) of a moving parallelogram. The arms (17, 18) rotate about a fixed joint (25), and the top surface of a sample is aligned with the axis of the fixed joint (25) to ensure highly accurate coincidence of the axis of the source and detector on the sample surface. The mechanism (1) allows highly accurate and simultaneous control of excitation and detection angles for applications such as photo—reflectance spectroscopy.

Description

“An inspection system support mechanism” INTRODUCTION Field of the Invention The invention relates to support mechanisms for inspection systems of the type which direct radiation at a device under inspection and detect radiation reflected from the device.

Prior Art Discussion An example of such an inspection system is a photo reflectance spectroscopy system for analysis of materials such as wafers to provide characterisation data.

Some such systems have a requirement for highly accurate direction of radiation onto the device and detection of the reflected radiation. However a limitation on accuracy has been imposed by the support mechanism. A known approach is to use a rack and pinion arrangement in a curved configuration and a motor for each of the radiation source and detector. In this arrangement it is difficult to ensure the same angular relationships between the radiation source and the device and between the detector and the device. Other disadvantages are their lack of long term reliability and slow speeds.

EPO96ll5l describes a mechanism in which a mobile support is connected to a base by a pair of flexible blades and a movable mirror. US5140989 describes an arrangement in which optical heads are mounted on carriers which allow a limited range of manual movement. US 6750968 describes an optical metrology instrument in which little detail is given of how mechanical control is achieved.

There is therefore a need for a support mechanism for more accurate source and detector support and/or for simpler positional adjustment for such inspection systems.

SUMMARY OF THE INVENTION According to the invention, there is provided a support mechanism for an inspection system, the mechanism comprising a support for a radiation source and a support for a detector, wherein the supports are interlinked for simultaneous movement, the supports being mounted on arms of a parallelogram, and the arms being interconnected to form the parallelogram by four joints including a fixed joint and one of the other joints is driven by a linear drive.

In one embodiment, the supports are mounted over a base for simultaneous movement with changing angles relative to the base.

In another embodiment, the mechanism allows the supports to be mounted at the same angle with respect to the base and for this angle to remain the same for both supports as they move in a plane orthogonal to the base.

In another embodiment, the driven joint is opposed to the fixed joint.

In a further embodiment, the driven joint is mounted on a carriage running on a rail.

In one embodiment, the mechanism includes a sample support and the driven joint is driven in a plane orthogonal to a plane of the sample support.

In another embodiment, the axial direction of movement of the driven joint is normal to the plane of the sample support.

In a further embodiment, the linear drive comprises a DC stepper motor driving a ballscrew.

In one embodiment, the source and detector supports are each mounted on an arm of the moving parallelogram at a location close to lateral joints on either side of an axis between the fixed and driven joints.

In another embodiment, the source and detector supports are mounted on arms which pivot about the fixed joint.

In a further embodiment, the axis of the fixed joint extends in or close to the plane of the sample support.

In one embodiment, comprises an adjustment mechanism for aligning the plane of a sample on the sample support with the axis of the fixed joint.

In another embodiment, the adjustment mechanism allows X, Y, and Z sample position adjustment, the source and detector supports moving in the 2 plane.

In a further embodiment, the adjustment mechanism further comprises a tip/tilt mechanism.

In another aspect, the invention provides an optical metrology inspection system comprising a support mechanism as described above, a controller for directing movement of the support mechanism, an optical source and an optical detector mounted on the support mechanism, and a processor connected to the detector.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Fig. 1 is a front top perspective view of a support mechanism of the invention; Figs. 2 and 3 are front views of the mechanism; Fig. 4 is a diagram showing movement parameters; Fig. 5 is a front view showing a different angle of incidence and reflection; Fig. 6 is a side view; and Fig. 7 is an exploded perspective view.

Description of the Embodiments Referring to Figs. 1 to 6 a support mechanism 1 comprises a vertical spine 2 extending upwardly from a base 3. The base 3 supports an X-Y table 4 for a device under inspection. As viewed in Fig. 1 a support arm 10 holds a housing 11 and a support arm 12 holds a housing 13, each housing being for a detector or radiation source. The arms 10 and 12 extend from the lateral pivot joints of a moving parallelogram of arms 15, 16, 17, and 18 free to rotate about the parallelogram joints. A lower pivot joint 25 is fixed by a fulcrum holder, and the upper pivot joint is on a carriage 21 running vertically on a track 20 secured to the vertical spine 2.

The housings 11 and 13 are used for supporting a radiation source and a detector.

The support arms 10 and 12 move with the parallelogram so that they can have any desired mutual angle between 175° and 5° approximately. As shown in Figs. 3 and the arms 10 and 12 will be at the same angle (61 = 92) to horizontal as they move.

The ends of the arms have lateral extensions so that the axes of the joints are offset slightly from the centrelines of the arms. Thus, the mutual angles may be very small, as shown in Figs 2 and 5 for example.

Referring also to Fig. 7, the arms 15-18 have bushings 50 and 51 fitted into holes 52 and 53 and these receive hardened steel shafts 55 and 56. This creates a suitable bearing surface for the motion components and ensures longevity of the apparatus.

The hardened steel shafts are retained in position by retaining clips 57. Instead of a bushing, a ball bearing, roller bearing or other suitable hearing may be used.

The fulcrum joint 25 is clamped into position on a two-piece fiilcrum support (or holder) 25(a) and the arms 17 and 18 are free to rotate about the shaft. The arms 17 and 18 are held securely within the fulcrum support 25(a) without translational movement, but can still rotate freely about the joint 25. This ensures that the housings 11 and 13, which are in a cantilevered position, are not prone to lateral translation. Rotation of the arms 15, 16, 17 and 18 is caused by displacement of the carriage 21 on the rail 20.

Teflon” washers 70 are placed between all of the would—be contacting surfaces on the arms 15 to 18 at the four parallelogram joints. This ensures that there is no metal—to—metal contact between the moving surfaces, thereby reducing or eliminating wear and minimising particle build-up between the moving contacting surfaces.

This is important as it is envisaged that the mechanism may be used in a clean room environment, and there is little release of particulates/ contaminates from the moving components. Also, any mechanical wear of the lever components would affect optical alignment adversely and this is eliminated by using the friction reducing washers 70, which also act as spacers.

The linear drive for the carriage 21 comprises a DC stepper motor driving a ballscrew linked with the carriage 21. However the linear device may instead comprise an electromagnetic linear motor or a rack—and-pinion mechanism of suitable resolution. The carriage 21 moves on the rail 20 by virtue of a cross—roller bearing.

The XY stage 4 uses a similar drive for each of the X and Y directions. The mechanism further comprises a Z stage 80 and a tip/ tilt mechanism 81. The latter allows the plane of the sample to be orthogonal to the plane of the input/ output optics. This sets up a flat surface so that the plane of the incident beam is aligned with the plane of the reflected beam.

In use, the sample is set up on the wafer chuck 40 on the XY motion stage 4. The Z stage 80 ensures that the top surface of the sample is aligned with the axis of the fulcrum joint 25.

Referring to Fig. 4, for a given linear displacement Ax (actuated by the carriage 21), there is a corresponding angular displacement A8 and as long as the distance between the parallelogram axes are all equal, for an angular displacement A61, there is a corresponding angular displacement A92 and A61 = A92. Also, the reflecting surface of the sample under test is aligned so that its surface is level with the axis of the lower pivot joint. This creates a virtual fulcrum, lying in the surface of the sample about which the axes through the centre of the source and detector rotate. This is shown most clearly in Fig. 3.

The angular resolution, A9, is a constant function of the linear carriage linear resolution. It is governed by the formula: A6 = [Sin‘1 (x + Ax) - 6] /2 d where, Ax = minimum displacement = translation stage resolution, (1 =distance between joints on the lever arm: 200 mm, x = [distance between A & C] /2.

It has been calculated as follows. ° .00O25° 367* Thus, the smaller A9 is, the better the resolution.

The XY stage 4 allows the sample to be inspected over a range of positions on its surface. These are mounted to the base plate 3 which in turn has been set up to be mutually perpendicular with the vertical base plate 2 onto which the vertical stage for the variable angle mechanism is mounted.

It will be appreciated that the invention uses only one linear motion device to effect two angular variations. This in turn means that the mechanism requires just one driver circuit for an electromechanically driven motion stage. Thus, the mechanism is simpler and less expensive to construct, having less critically controlled components, which in turn creates a more robust and reliable system less prone to failure and downtime. In fact it is envisaged that with this invention, there will be little or no mechanical breakdown over the lifetime of the product.

This invention offers a greater angular range of motion than heretofore. Prior mechanisms afford limited angles of incidence while the mechanism of the invention has a range of 5°-85° on 6. This offers a considerable advantage to applications requiring a high angular degree of incidence such as reflectance anisotropy (RA), and the large range of angles of incidence is essential for the performance of photoreflectance spectroscopy and reflectance of the active resonant cavity of multi-layer opto—electronic devices.

This mechanism also has an excellent range of angular speed.

The mechanism finds application in a wide range of applications, including metrology inspection equipment, and optical inspection equipment for the semiconductor industry.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims (1)

1. Claims A support mechanism for an inspection system, the mechanism comprising a support (11) for a radiation source and a support (13) for a detector, wherein the supports are interlinked for simultaneous movement, the supports being mounted on arms of a parallelogram (15, 16, 17, 18), and the arms being interconnected to form the parallelogram by four joints including a fixed joint and one of the other joints is driven by a linear drive. A support mechanism as claimed in claim 1, wherein the supports (11, 13) are mounted over a base (3, 4) for simultaneous movement with changing angles relative to the base. A support mechanism as claimed in claim 2, wherein the mechanism allows the supports (11, 13) to be mounted at the same angle with respect to the base and for this angle to remain the same for both supports as they move in a plane orthogonal to the base. A support mechanism as claimed in any preceding claim, wherein the driven joint is opposed to the fixed joint. A support mechanism as claimed in claim 4, wherein the driven joint is mounted on a carriage running on a rail. A support mechanism as claimed in any preceding claim, wherein the mechanism includes a sample support and the driven joint is driven in a plane orthogonal to a plane of the sample support. A support mechanism as claimed in claim 6, wherein the axial direction of movement of the driven joint is normal to the plane of the sample support. A support mechanism as claimed in any preceding claim, wherein the linear drive comprises a DC stepper motor driving a ballscrew. A support mechanism as claimed in any preceding claim, wherein the source and detector supports (11, 13) are each mounted on an arm of the moving parallelogram at a location close to lateral joints on either side of an axis between the fixed and driven joints. A support mechanism as claimed in claim 9, wherein the source and detector supports (11, 13) are mounted on arms (17, 18) which pivot about the fixed joint (25). A support mechanism as claimed in any of claims 6 to 10, wherein the axis of the fixed joint extends in or close to the plane of the sample support. A support mechanism as claimed in claim 11, further comprising an adjustment mechanism for aligning the plane of a sample on the sample support with the axis of the fixed joint. A support mechanism as claimed in claim 12, wherein the adjustment mechanism allows X, Y, and Z sample position adjustment, the source and detector supports moving in the Z plane. A support mechanism as claimed in claim 13, wherein the adjustment mechanism further comprises a tip/ tilt mechanism. A support mechanism substantially as described with reference to the drawings. An optical metrology inspection system comprising a support mechanism of any preceding claim, a controller for directing movement of the support mechanism, an optical source and an optical detector mounted on the support mechanism, and a processor connected to the detector.