Apparatus and method for measuring two opposite surfaces of a body

An apparatus and a method are provided which allow two opposite plane surfaces of a body to be interferometrically measured simultaneously using light from a single light source. From a parallel light beam (P) produced by a light source (1) partial light beams (A, B) having positive and negative diffraction angles are produced using a beam splitter (8) in the form of a diffraction grating. The partial light beams strike the respective surfaces (90, 91) of the body (9) to be measured and are reflected thereat. The reflected partial light beams (A, B) are interfered with the throughgoing partial light beam (P) having an order of diffraction of zero and the thus produced interference patterns are digitized and subtracted from each other, whereby the parallelism of both surfaces (90, 91) of the body can be determined.

The invention relates to an apparatus and a method for measuring two opposite surfaces of a body according to the preamble of claim1or20, resp.

The technical progress of the semiconductor industry in the last years resulted in a sharp increase of the diameters of the semiconductor wafers as base material for chip production for economic and process technical reasons. Wafers having a diameter of 200 millimeters are already state of the art and wafers having a diameter of 300 millimeters will be processed in near future.

At present manufacturers and processors of such wafer sizes do not yet have measuring devices at their disposal which enable them to check particular quality features such as the geometry (flatness, curvature, thickness variation) of the wafer with a desired resolution and precision.

Two measuring methods for measuring the geometry of semiconductor wafers are known. The one measuring method is an optical geometry measurement using interferometry. One entire surface of the wafer is interferometrically measured, while the wafer rests on a plane plate or is sucked thereto. After measuring one surface the wafer is turned around and the other surface is measured. Since, in this method, one side only can be measured at a time, the relation between the front and rear side of the wafer indicating the parallelism and the thickness variation is not directly given. It is assumed that the sucked surface is drawn in an absolutely plane state, but this is practically not the case, because it is prevented by particles between the wafer and the support and it is generally uncertain whether the wafer—especially in case of unevenness—fits in a uniform manner. Furthermore, a horizontally placed wafer having a diameter of 200 millimeters or 300 millimeters is bent by gravity and therefore no forcefree state of the wafer prevails. This renders the measurement of the absolute evenness impossible. Moreover, the risk of damage due to the surface contact with the support and possibly also with the optical measuring system is so high that mostly sample measurements only are admitted. Owing to the sum of the many measuring uncertainties the measuring accuracy is insufficient. Measurement values produced with other methods are not directly comparable also.

A further method is the capacitive geometry measurement including scanning the surface using distance sensors. Dot scanning distance sensors scan the front side and the rear side of a wafer. The wafer is supported at its center and rotated. Since the measurement is punctual, it is necessary to scan in order to obtain two-dimensional data. The known disadvantages of a scanning method, e.g. instable measuring conditions during the entire scanning process, considerably reduce the measuring accuracy. Since the wafer is centrally supported during the measurement, the gravity exerts a strong influence on the form of the wafer by causing a flexion. This influence can be computationally taken into consideration only to an insufficient approximation. Furthermore, the number of measurement points which can be obtained within an acceptable time is too low. The size of the measurement points resulting from the method and from the sensor diameter can not be reduced to an extent necessary to meet the new quality rules. Moreover, the risk of damaging the wafer is high because of the surface contact and of the very small distance of the sensors to the wafer surface for technical reasons. Generally, also in this case the measuring accuracy is too low, owing to the sum of measuring uncertainties. Again, measuring values produced with other methods can not be directly compared.

It is the object of the invention to provide an apparatus and a method for measuring two opposite, substantially plane and parallel surfaces of a body, in particular of a semiconductor wafer, whereby the measuring accuracy can be increased, the damaging risk can be reduced and the measuring time can be decreased.

The object is achieved by an apparatus according to claim1and a method according to claim20, resp.

Further developments of the invention are defined in the subclaims.

The apparatus and the method, resp., has the following advantages:

The front side and the rear side are measured under absolutely equal conditions in a contactless, isochronal and static manner—no wafer movement occurs—and a single sensor is used. No tuning calibration is required. During the measurement the wafer is free of effects from outer forces, because it stands in an upright position. The critical surfaces of the wafer are never touched, and there is therefore a low risk of damage. All required geometry data are derived from a single measurement. Owing to the single measurement the measuring time is considerably reduced, whereby the throughput and the productivity is increased. The measuring accuracy and the resolution in lateral as well as vertical direction are as high as, or even higher than, required by international standards. Moreover, the method detects the wafer in an unaffected state and could therefore form a standard.

As shown in theFIGS. 1 and 2the apparatus comprises a light source in the form of a laser1. The light emitted from the laser1is conducted through a beam waveguide2to a defined place of the apparatus. The light produced by the laser1emerges at an end3of the beam waveguides2so that the end3acts as a punctual light source. The emerging light strikes a deviation mirror4wherefrom it is redirected onto a collimation mirror7in the form of a parabolic mirror by two further deviation mirrors5and6which are oriented at an angle of 90° relative to each other. The parallel light beam P reflected from the parabolic mirror7reaches a beam splitter8through the two deviation mirrors5and6. This beam splitter is formed as a first diffraction grating and is preferably a phase grid. The beam splitter8is arranged in the apparatus in a vertical direction and the parallel light beam P strikes the diffraction grating in a perpendicular direction. A beam collector10in the form of a second diffraction grating is disposed in a distance from the first diffraction grating and parallel thereto. Behind the beam collector10two decollimation lenses11are arranged at equal level and the light beams leaving these decollimation lenses are each deflected and focused onto two CCD cameras16through deviation mirrors12,13,14and an optical imaging system15.

The beam splitter8is supported transversely to the optical axis and further comprises a piezoelectric actuating element17for shifting the phase of the parallel light beam P by displacing the diffraction grating.

A holding device50, for example in the form of a support post, is provided centrally between the first diffraction grating and the second diffraction grating. A wafer9to be measured is held on the holding device50in such a manner that both plane surfaces90,91thereof are arranged in vertical direction parallel to the light beam P. The wafer9is supported by the support post substantially at its vertical edge92only so that both surfaces90,91are not substantially contacted by the support post and are freely accessible to the interferometric measurement.

Moreover, a receiving device (50,25) is provided for the wafer9to be measured. The wafer can be inserted into the receiving device in a horizontal position. By means of a tilting device26the wafer9may be tilted from its horizontal position into the vertical measuring position, and the wafer9may be transferred, by means of a positionable traveller, into the light path between the first diffraction grating and the second diffraction grating so that the surfaces90,91to be measured are aligned substantially parallel to the undiffracted light beam P and in a substantially vertical direction.

Furthermore, a reference apparatus20is provided which comprises a reference body21having at least one plane surface24. The reference body21can be introduced into the light path between the first diffraction grating8and the second diffraction grating10in place of the semiconductor wafer9to be measured by means of a traveller23with a linear guide18. The reference body21is held so that its plane surface24is arranged in vertical direction parallel to the undiffracted light beam P. The reference body21can be turned by 180° in its mounting around an axis parallel to its surface24.

As shown in particular inFIG. 3the apparatus further comprises an electronic device30connected to the outputs of the CCD cameras for processing the interference patterns produced by the CCD cameras. The image processing device30is further connected to an evaluating processor40. The evaluating processor40is further connected to the phase shifter17through a piezo drive member170. A printer45and a video monitor46for outputting data are connected to the evaluating processor40. The evaluating processor40is further connected with a master and control unit60which is in turn connected with a host computer65, an operator terminal66and the output of an SPC (stored program control) and positioning control. Inputs of the SPC and positioning control are each connected to power electronics68for the motors69of the travellers for the semiconductor wafer or reference body, resp., to be measured or for moving other mechanical parts of the apparatus. A further input of the SPC and positioning control67is connected to the sensor members70for the travellers and tilting devices, resp.

In operation the wafer9to be measured is first inserted into the wafer receiving device25. The surfaces90,91to be measured of the wafer9are horizontally arranged. By means of the tilting device and of the traveller19the wafer to be measured is brought into the holding device50where it is arranged so that the surfaces90,91to be measured are vertical. A diffraction of the parallel light beam P striking the first diffraction grating8of the beam splitter produces partial light beams A, B, whereby the partial light beam A having a positive diffraction angle strikes the one surface90of the wafer and is reflected thereat, whereas the partial light beam B with a negative diffraction angle strikes the other surface91of the wafer and is reflected thereat. The 0-th diffraction order of the parallel light beam P passes through the first diffraction grating8and is not reflected at the surfaces90,91of the wafer9. This partial light beam P serves as reference beam for interference with the reflected wave fronts of the beams A and B. In the second diffraction grating10, the beam collector, the reflected partial light beams A and B, resp., are each combined again with the reference beam P of the 0-th diffraction order and focused, in the form of two partial light beams A+P and B+P, resp., onto the focal planes of the CCD cameras16through decollimation lenses11and deviation mirrors12,13and14as well as positive lenses15.

During the exposure of the surfaces the phase of the parallel light beam P is repeatedly shifted by 90° and 120°, resp., by displacing the diffraction grating. This produces phase shifted interference patterns. The output data of the CCD cameras16are fed to the image processing device30which produces digitized phase patterns160for each measured surface90,91on the basis of the individual interference patterns of the CCD cameras16. The digitized phase patterns160are further processed in the evaluation processor40and imaged on the video monitor46. The defined shift of the interference phase produced by the phase shifter17is evaluated to determine whether there is a protuberance or a depression in the measured surfaces90,91. For determining the parallelism of the measured surfaces90,91the two digitized phase patterns are subtracted from each other. Moreover, a mask for the phase patterns is generated in the evaluation processor and the phase patterns are calibrated, parametrized and stored in the evaluation processor. The generated graphics and tables can be outputted via the printer45.

A calibration using the reference body21can be performed before each measurement of a wafer9. The reference body21is introduced into the beam path between the first diffraction grating8and the second diffraction grating10and the known plane surface24is measured. Subsequently the reference body21is turned by 180° and the same surface24is measured as a second surface.

Modifications of the apparatus and of the method are possible. A body having two precisely plane parallel surfaces may be used for the reference body21, whereby both surfaces are measured simultaneously. However, the embodiment having a single plane surface of the reference body is more suitable.