GB2159939A

GB2159939A - Detector apparatus

Info

Publication number: GB2159939A
Application number: GB08405550A
Authority: GB
Inventors: Graeme Adamson; Wilfred John Bates; Michael Antony Player
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1984-03-02
Filing date: 1984-03-02
Publication date: 1985-12-11
Also published as: GB8405550D0; FR2563908A1; JPS60228905A; NL8500528A; DE3506812A1

Abstract

An optical detector apparatus for pre-aligning a semiconductor wafer 1 during lithographic processing utilises a vee groove 3 etched into the wafer, a light beam directed normally to the wafer surface, and dual photodetectors 5 and 7 to unambiguously and precisely detect the location of the groove. Lens system 9 focusses the light beam onto the wafer surface and the dual photodetectors are located off the optical axis of the light source 11 so as to detect light reflected from the walls of the vee groove without detecting light reflected normally to the flat surface of the wafer. The product, sum or difference of the photodetector signals may be used to determine centering of the detector apparatus with the groove. <IMAGE>

Description

SPECIFICATION Detector apparatus This invention is concerned with detector apparatus.

Because of the small geometries used in modern semiconductor devices, it is critical that alignment of the semiconductor wafer and the various masks utilised during lithographic processing be kept to a very tight tolerance. Alignment is often accomplished in two steps with the final alignment step, of a micron or sub- micron tolerance, having a capture range of only a few microns or tens of microns. This requires that an initial pre-alignment step be performed to bring the alignment of the wafer and the mask within the capture range of the final alignment apparatus.

Various prior art pre-alignment apparatuses, such as the ADE Corp. PA-407 Pre-Aligner, employ optical or capacitive sensors to detect the edge of the semiconductor wafer. Such prior art edge detectors are susceptible to error caused by chipping or photoresist build-up at the wafer edge. Other prior art prealignment apparatuses, such as that disclosed by Mayer and Loeback in "Characterisation Data and Cross-Matching of a High Performance Step-and-Repeat Aligner", SPIE Conference on Optical Microlithography SPIE Vol 334 (1982), Santa Clara, California, utilise detection of normally-reflected light to distinguish alignment marks such as lines or crosses on the surface of the wafer.These prior art apparatuses may provide erroneous mark detection due to variations in the signal-to-noise ratio which is caused by variations in the reflectance or thickness of the photoresist covering the surface of the wafer.

The present invention provides apparatus for detecting a mark on a surface, the detector comprising an energy source for emitting a reflectable beam of energy along an axis; focussing means, positioned on the axis for focussing the beam onto or adjacent to a surface; and a detector, positioned at a first angle relative to the axis, for detecting a portion of the beam reflected by a mark on the surface.

The construction and arrangement of the apparatus is preferably such that the axis is substantially normal to the surface when the apparatus is in use; the apparatus is also preferably movable in a direction of translation which is parallel to the surface.

The mark is a groove in the surface, and the preferred shape thereof is a vee; the groove has a longitudinal axis which is substantially perpendicular to the aforesaid direction of translation.

Preferably, two detectors are arranged to detect reflected portions of the incident beam, and are positioned in the plane of the axis at substantially equal angles thereto. The two detectors also provide an electrical output indicative of the intensity of energy incident thereon and means is provided for either adding or multiplying the outputs together or subtracting one from the other to determine precise alignment of the mark with the beam.

The energy source is preferably a light source with both detectors being photodetectors.

The present invention also provides a method of accurately positioning a workpiece using a mark formed on a surface of the workpiece, the method comprising focussing a reflectable beam of energy onto the surface, effecting relative movement of the mark and the focussed beam to cause the beam to travel across the surface; and detecting a portion of the beam reflected by the mark along a predetermined axis.

In accordance with the illustrated preferred embodiment of the present invention, an optical detection apparatus utilises dual photodetectors to detect a vee groove in the semiconductor wafer surface. A light source and lens system image a conical light beam onto the surface of the wafer and the dual photodetectors are offset from the optical axis of the light beam. Since light is reflected substantially normally to the flat wafer surface of the wafer, no light is detected when the light beam is imaged onto the flat surface of the wafer. Light which is imaged onto the vee groove is reflected at an angle and strikes one or both of the photodetectors.

There now follows a detailed description which is to be read with reference to the accompanying drawings of an apparatus according to the present invention; it is to be clearly understood that this apparatus has been selected for description to illustrate the invention by way of example and not by way of limitation.

In the accompanying drawings: Figure 1 is a diagrammatic representation of an apparatus, including a light source, which is constructed in accordance with the preferred embodiment of the present invention; Figures 2A-2C illustrate diagrammatically various light ray paths as the surface of a semiconductor wafer is translated under the apparatus shown in Figure 1; Figure 3A is a diagrammatic representation of an output signal from a prior art apparatus; and Figures 3B-3D are diagrammatic representations of output signals from the apparatus shown in Figures 1 and 2.

Figure 1 shows an apparatus which is constructed in accordance with the preferred embodiment of the present invention. As an initial step in the pre-alignment of a semiconductor wafer 1 in a lithography exposure system, it is important to determine precisely the location of a reference mark on the wafer 1. The reference mark utilised is a vee groove 3 which is etched into the surface of the wafer 1. Using well known orientation dependent etchants it is possible to etch the vee groove 3 with tightly controlled dimensions, although tight control of the dimensions is not required.For a vee groove 3 which is 3 millimeters long and 110 microns wide by 80 microns deep in a silicon wafer 1 of [100] orientation, a cone of light which is projected into the groove 3 with the cone axis nearly normal to the surface of the wafer 1 is predominantly reflected by walls of the groove 3 so that two cones of light emerge at approximately 40 de gress from normal.

A light source 11 emits light which is imaged by lenses 9 onto the wafer 1. Although a light source is used in this embodiment, it will be readily understood that any suitable energy source may be used. A light source is however most convenient and suitable light sources may be lasers, LED's or polychromatic sources. The lenses 9 may comprise, for example, two lens doublets having diameters of 10 millimeters and focal lengths of 20 millimeters. Between the two lens doublets there is provided a stop 8 of 4 millimeters diameter for controlling the angle of the cone of rays. The lenses 9, the stop 8, and the wafer 1 are selected and positioned so that the diameter of the image on the wafer 1 is less than or equal to the width of the groove 3 and the cone of rays can be adjusted for maximum sensitivity.

When the light source 11 is imaged onto the flat surface of the wafer 1, as shown in Figure 1, light is reflected nearly normally to the surface. Only a small amount of scattered or diffused light is received by two photodetectors 5 and 7 which are located off the optical axis of the optical system defined by the source 11 and the lenses 9.

Figures 2A-2C show the apparatus and the wafer 1 as the wafer 1 is translated under the apparatus.

In Figure 2A, light is imaged onto the left wall of the groove 3 and is reflected at approximately 40 degrees from normal to the photodetector 5. The photodetectors 5 and 7 are located approximately 40 degrees off the optical axis of the source 11 in order to receive light reflected from the walls of the groove 3. The photodetectors 5 and 7 may comprise any one of a number of well known photodetectors which operate in the frequency range of the source 11; output signals from the two photodetectors are transmitted to a multiplier or subtractor 10 which produces a product or difference signal.

In Figure 2B, light is imaged onto the center of the groove 3 and is reflected onto both of the photodetectors 5 and 7. As further translation occurs, light is reflected to the detector 7 alone and finally reflection normal to the surface of the wafer 1 again occurs as shown in Figure 2C. the presence of any photoresist in the groove 3 does not seriously affect the performance of the apparatus.

Figure 3A shows the output of a typical prior art detector apparatus which detects reflected light normal to the surface of the wafer 1 with maximum detection on the flat surface of the wafer 1 and minimum detection at the groove 3. But, because of variations in the thickness and reflectance of a photoresist layer on the surface of the wafer 1 (and in the groove 3), radical variations in the amplitude of the output signal occur. The result is that the detection of the groove 3 can be obscured.

Figure 3B shows the output signals from the photodetectors 5 and 7 as the wafer 1 is translated under the apparatus shown in Figrue 2. The characteristic double peak signature of detection of the groove 3 is not obscured by the presence of photoresist on the wafer 1. Detection of the center of the groove 3 occurs in the waveform shown in Figure 3B at the crosspoint of the two traces. Figure 3C shows a resultant signal which is the product of the two signals shown in Figure 3B. The multiplication of the two signals may be performed in the multiplier 10 to facilitate peak detection. Figure 3D shows a resultant waveform which is the difference of the two signals shown in Figure 3B. The subtraction of the two signals may also be performed in a subtractor 10 to facilitate zero-crossing detection.

Claims

1. Apparatus for detecting a mark on a surface, the detector comprising: an energy source for emitting a reflectable beam of energy along an axis,.

focussing means, positioned on the axis for focussing the beam onto or adjacent to a surface; and a detector, positioned at a first angle relative to the axis, for detecting a portion of the beam reflected by a mark on the surface.

2. Apparatus according to claim 1, wherein the construction and arrangement is such that the axis is substantially normal to the surface when the apparatus is in use.

3. Apparatus according to either one of claims 1 and 2 wherein the apparatus, when in use, is movable in a direction of translation which is parallel to the surface.

4. Apparatus according to any one of the preceding claims, wherein the mark is a groove in the surface.

5. Apparatus according to any one of the preceding claims, wherein the mark is a vee groove in the surface.

6. Apparatus according to claim 5, wherein a longitudinal axis of the vee groove is substantially perpendicular to the direction of translation.

7. Apparatus according to either one of claims 5 and 6, wherein the beam as focussed onto the surface is of the same width as or of narrower width than the vee groove.

8. Apparatus according to claim 3, wherein the detector is located on a detection axis which lies in a plane parallel to the direction of translation.

9. Apparatus according to claim 8 and further comprising a second detector which is located on a second detection axis lying in said plane at an angle to the optical axis opposite said first mentioned angle, the second detector being operative for detecting a second reflected portion of the beam.

10. Apparatus according to claim 8 wherein the two angles are substantially equal.

11. Apparatus according to either one of claims 9 and 10 wherein each detector is arranged to provide an electrical output indicative of the intensity of energy incident thereon and means is provided for either adding or multiplying the outputs together or subtracting one from the other to determine precise alignment of the mark with the beam.

12. Apparatus according to any one of the preceding claims wherein the energy source is a light source and the or each detector is a photodetector.

13. Apparatus according to any one of the preceding claims wherein the focussing means comprises a lens arrangement, and aperture stop means for defining the width of the beam.

14. A detector according to any one of the preceding claims, wherein the surface is a surface of a semiconductor wafer.

15. Apparatus for detecting a mark on a surface, substantially as hereinbefore described with reference to Figures 1, 2A-2C and 3B-3D of the accompanying drawings.

16. A method of accurately positioning a workpiece using a mark formed on a surface of the workpiece, the method comprising: focussing a reflectable beam of energy onto the surface; effecting relative movement of the mark and the focussed beam to cause the beam to travel across the surface; and detecting a portion of the beam reflected by the mark along a predetermined axis.

17. A method according to claim 16 wherein the beam is a beam of light.

18. A method according to either one of claims 16 and 17 wherein the beam is moved in a direction which is perpendicular to a longitudinal axis of the work.

19. A method according to any one of the preceding claims wherein the beam is directed normally to the surface of the workpiece.

20. A method according to any one of claims 16 to 19 wherein the mark is formed as a groove in the surface of the workpiece.

21. A method according to any one of the preceding claims wherein the beam is directed normally to the surface of the workpiece.

22. A method according to claim 19 or either one of claims 20 and 21 as appended to claim 19, wherein two detectors are mounted, one on each side of the axis of the normal beam for detecting portions of the beam reflected by the mark at substantially equal angles to the axes of the normal beam.

23. A method of accurately positioning a workpiece using a mark formed on a surface of the workpiece, substantially as hereinbefore described with reference to Figures 1, 2A-2C and 3B-3D of the accompanying drawings.