DD261422A1 - ARRANGEMENT FOR INTERFEROMETIC QUALITY TESTING OF TECHNICAL SURFACES - Google Patents

Abstract

The invention is applicable to the automatic interferometric evenness testing of reflective smooth technical surfaces, for example semiconductor wafers. The arrangement according to the invention uses a Mach-Zehnder interferometer formed by a test arm and a reference arm and a laser light source (L) with a downstream expansion system (AS1). A first hologram (H1) downstream of the expansion system (AS1), which is used both in reflection and in transmission, leads the divergent beam path into a parallel beam path and divides it into the test and reference beam path. An identical second hologram (H2) arranged in the plane of the first hologram (H1) combines the reference beam path reflected by a mirror (S1) with the test beam path to the convergent observation beam path, in which an imaging system (AS2) and subsequently a matrix receiver (MA) are arranged , The matrix receiver (MA) is connected to a computer (RE) which controls a translator (TR) for shifting the mirror (S1) in the direction of its surface normals. Fig. 1

Description

For this 3 pages drawings

Field of application of the invention

The invention is applicable to the automatic interferometric evenness testing of reflective smooth technical surfaces, for example semiconductor wafers.

Characteristic of the known technical solutions

A number of methods and arrangements for the interferometric measurement of planar surfaces are already known. A grating inferometer is known, with the aid of which the effective wavelength can be considerably increased during the test. Deviations from the size of lattice constant / 2 correspond to one stripe distance (Birch, K.G. Journal of Physics E., Scientific Instruments 6 [1973] p. 1045).

The disadvantage of this arrangement is the problem of projective distortion of the object surface.

In another known grating interferometer for the evenness testing of silicon wafers, the light from two adjacent diffraction orders, both of which reach the object at different angles, is brought into interference (Järisch, W., Feinwerktechnik und Messtechnik 83 [1975] p. 199; DE-AS 2636211 / G 01 B, 9/02).

Since the path difference depends on the angle at which the light passes through the interferometer, superimposing the differences in the phases results in significant possibilities for varying the topographical sensitivity. Disturbing here act multiple overlays by the influence of other diffraction orders and similar projective distortions as in the former known arrangement.

Also known is an arrangement called "interferometer" for the evenness testing of technical surfaces, eg.

Steel surfaces where the wavelength increase is also generated by grazing incidence (Abramson, N., Optik 20

In this arrangement, a 90 ° prism is used, the hypotenuse of which is used as a reference surface in a Fizeau interferometer. The light falls at about 45 ° on the hypotenuse of the prism and emerges almost grazing from this.

After reflection on the specimen surface, the light enters the prism again and leaves it at 45 ° to the hypotenuse perpendicular to the catheter of the prism. The calculation causes the bundle cross-section to be anamorphic distorted and equalized again upon re-entry.

The projection error is therefore only V2 ~ and not z. B. 10. This can be cited as a clear advantage over the previously mentioned solutions, as this z. B. simplifies a photoelectric detection or is only possible.

The disadvantage is that the interferogram is a Mehrstahlinterferogramm, which can occur depending on the thickness of the air gap between the prism and the specimen unbalanced interference fringes. Furthermore, the effective wavelength λ is a function of the angles of incidence and refractive indices of the prism and the air. Although the effective wavelength can be selected within wide limits, the problem then arises that with automatic detection of the interference images by known methods (Bruning et al., Appl. Opt. 13 [1974] 2693; sa Gallagher, JE and Herriott, DR, DD-PS 96779 / G 01 N, 21/46), the shift of the reference mirror must be adapted within wide limits, since the reference phase must be tuned by a full period (2π).

It is already an arrangement for interferometric evenness of technical surfaces known (DD-PS 219 565, G 01 B, 9/02), which builds on the "interferometer" (Abramson), but on the one hand avoids the disturbing influence of multi-beam interference by optical filtering and On the other hand, the solution for electronic evaluation can be made accessible by means of a special Moire interferometer connected downstream: The phase shift required for the automatic evaluation can be achieved by translating a low-frequency grating in the downstream Moire interferometer in the grating plane perpendicular to the grating lines.

The disadvantage of this solution is an anamorphic distortion of the specimen.

In another known arrangement (DD-PS 233644, G 01 B, 9/02) is formed by a Prüflingsarm and a reference arm Mach-Zehnder interferometer used in the Prüflingsarm between beam splitter and DUT and between the test specimen and Strahlenvereiniger ever Diffraction grating arranged to eliminate the anamorphic distortion. A disadvantage of this solution are the shock sensitivity, due to the wide-fanned beam path, and the relatively large expenditure on equipment. This is due to the relatively large number of required individual components such as collimator lens and large-diameter imaging lens and beam splitter in the embodiment for coherent illumination and additional components for retardation and wave front folding in the embodiment for partial coherent illumination.

Object of the invention

The aim of the invention is to have available an arrangement for the interferometric evenness testing of reflecting technical surfaces, with which an automatic selection of workpieces is possible in the current process, which do not correspond to the given demands on the flatness.

Explanation of the essence of the invention

The invention has for its object to form a connectable with a computer test interferometer for direct measurement of flatness deviations on technical plan surfaces such that the information about the Prüflingsfläche containing pure Zweistrahlinterferogramm is provided for evaluation, without the use of complex equalization process is necessary and wherein the Vibration sensitivity and the equipment cost are significantly reduced.

The object is achieved by an arrangement using a Mach-Zehnder interferometer formed by a Prüflingsarm and a reference arm, with a laser as a light source, which is followed by an expansion system with pinhole for generating a divergent beam path, and formed according to the invention in the manner described below is. Behind the expansion system is located in the divergent beam path, a first holographic optical element (hereinafter referred to as hologram), which is such that the divergent before the first hologram beam path to the parallel beam path. At the same time the beam path is divided into a reference and a Prüflingsstrahlengang, wherein the first hologram is used for the one beam path in reflection and for the other beam path in transmission. So this first hologram is at the same time beam splitting and

collimating optical holographic device. The reference beam path includes a reference mirror and terminates with a second hologram that is an identical copy of the first hologram. The specimen beam path contains the specimen and terminates with the same second hologram, which thus combines the parallel Prüflings- and the parallel reference beam path to the convergent observation beam path.

The observation beam path includes an imaging system and a matrix receiver. As matrix receivers, CCD matrices, diode matrices or digitized vidicons can be used in a manner known per se. The candidate should be as sharp as possible in the plane of the matrix receiver. The output of the matrix receiver is connected to a computer for processing the obtained electrical signals. The arrangement further includes a phase shifter. This is, for example, a translator, which is controlled by the computer and coupled to the reference mirror, so that it can be moved in the direction of its normal.

To vary the detection sensitivity, a tunable laser can be switched in front of the interferometer and the test object and reference mirror can be displaced in the direction of their normals.

The operation of the arrangement according to the invention will be described below.

The light from the laser passes through the expansion system and leaves it as a spatially filtered, divergent bundle.

The bundle then strikes the first hologram, making it a parallel beam and simultaneously splitting it into several transmission and multiple reflection orders of diffraction.

Such a hologram can be determined by the holographic methods known per se (see, for example, Collier, R. J .; Burchhardt, C. B .;

L. H .: Optical Holography, New York 1971) as a synthetic or optically generated hologram. By means of suitable production, it is also possible to achieve that only a transmission and a first-order reflection bundle leave the hologram under absolutely the same, very large diffraction angles, whereby they are anamorphotically distorted. The peculiarities to be considered in the production of the hologram are described below. The anamorphic distorted bundles in the Prüflings- or in the reference beam path then fall obliquely on the test piece or on the reference mirror, so that a cross section of the shape of the original bundle cross-section is made, although the bundles are noticed obliquely.

The reflected, the specimen or the reference mirror obliquely leaving bundle arrive obliquely on the second hologram. In this case, the original, undistorted bundle cross section is restored and the two bundles are made in reflection or in transmission to a convergent bundle and combined at the same time to the observation bundle.

The observation beam is detected with the matrix receiver.

The phase shifter in the reference beam path can be used to set different reference phase values, for example four equidistant and one period of the interframe image. This is necessary for the calculation of the flatness deviations from the intensity values after their A / D conversion and transfer into a computer which also controls the setting of the reference phase values.

In order to adapt the detection sensitivity to the test object deviations to be measured, the test object must be illuminated at an oblique incidence. For this purpose, the holograms are selected according to the required sensitivity. Mostly, the candidate will be illuminated at near grazing incidence so that a grating pitch g, slightly larger than the wavelength of light λ used, is required.

As is known, when the incidence is perpendicular to the diffraction angle α

gsina = A (1)

For the effective strip shift, the path difference variation AG is decisive due to the surface deviation Δζ (χ, y).

It applies

AG = 2Az (x, y) cosa (2)

With Ag = m λ follows:

Az (x, y) = mλ / 2 cos α (3)

In comparison with vertical incidence (α = 0) an effective wavelength is obtained

λ * = λ / cosa. (4)

Therefore, the shift by one stripe corresponds to a significantly larger area deviation Az than at vertical incidence. But this is just needed for the examination of technical plan surfaces with deviations Az of a few μηη. Advantageously, given a lattice constant g, the interferometer can be coupled to a tunable laser. This results gem. Eq. (1) a possibility of variation for α:

sin α = A _variabe | / g (5)

Depending on the sign of the wavelength change Δλ results in an increase (Δλ> 0) or reduction (Δλ <0) of the effective wavelength λ *.

In order to cover the sensitivity range 1 μm-10 μσι / Δλ / <50 nm is sufficient.

As is known, a beam after a hologram has aberrations if the wavelength differs from that used in the production of the hologram (see, for example, Collier, R. J .; C.B. Burckhardt, L.H. Lin, loc. Cit., P. In the present case, these are primarily spherical aberration and defocusing. Both can be practically neglected at low wavelength variations.

The use of partially coherent lasers (for example semiconductor lasers) is also possible without additional effort since the path difference is compensated in accordance with the arrangement and wavefront convolution is avoided.

Due to the automated evaluation, the errors of the interference arrangement in a first step (ideal level mirror

as examinee) can be won. After storing these calibration values, any surfaces relative to a plane mirror can be checked. Another possibility for calibration is provided by the optical production of the second hologram if, in a manner known per se (see, for example, Collier, RJ: et al., Loc. Cit.), The almost plane wave associated with the errors in the arrangement is present error-free spherical wave are used.

In the synthetic or optical production of holograms, some peculiarities must be considered. Thus, it is advantageous if the holograms are blazed for only one (positive or negative) diffraction order. Higher than first diffraction orders do not occur anyway because of the very large diffraction angle. In the optical production, care must be taken that the direction of polarization of the laser light used forms the angle of greatest correctness with the plane in which the optical axis of the arrangement lies (see, for example, Collier, RJ et al., Loc. Cit p.158).

The correct polarization angle must also be observed in the same way when operating the device as a measuring instrument. It should also be noted that practically the thickness of the hologram carrier can not be neglected. Throughgoing and reflected beams are intended to leave the hologram symmetrical to the hologram plane to ensure a balanced beam path. Therefore, the hologram carrier must be designed symmetrically to the hologram plane. So you can z. B. cover the on a glass body, such as a glass plate, as a carrier hologram with a second, adapted in thickness glass plate. Vitreous bodies of other shapes are also possible. For example, the areas of the glass bodies of the hologram H ₁ through which the beams emerge and those of the hologram H ₂ through which they enter may be flat surfaces whose normal lines are the direction of the optical axis.

Further, it is expedient to make the surfaces of the glass body in front of or behind hologram H ₁ or H ₂ , through which the divergent bundle on or exits the convergent bundle, to avoid spherical aberration spherical, wherein the center of curvature in the focus of the bundle must lie.

embodiments

The invention will be explained in more detail below with reference to four exemplary embodiments. In the accompanying drawings show

1: the schematic representation of the arrangement according to the invention with a gas laser as a light source, Fig. 2: the schematic representation of another arrangement according to the invention, each with a lens before the first or

after the second hologram, Fig. 3: the schematic representation of another arrangement according to the invention, in the reference mirror and

4 are the schematic representation of the symmetrized and aberration-free hologram carrier.

In the arrangement according to FIG. 1, the gas laser L serving as the light source follows in the beam path an expansion system AS ₁ consisting of objective O ₁ and pinhole Bi. The expansion system ASi is followed by a plane first hologram H ₁ on a symmetrized carrier which forms the beam path in FIG a reference and a Prüflingsstrahlengang divides and at the same time makes the divergent beam path to parallel beam paths. In the Prüflingsstrahlengang follow after the first hologram H _{1 of} the test piece P and the second hologram H ₂ .

The optical axis of the Prüflingsstrahlenganges before the first hologram H _1, the direction of the surface normal, behind the hologram Hi, the direction of the plus first reflected diffraction order. The normal of the specimen surface P is the normal of the hologram H ₁ and the hologram H ₂ parallel, so that the beam path behind the specimen P has the direction of the reflection angle at the corresponding angle to H ₂ , behind H ₂ but again the direction of Surface normal of this hologram has and thus the beam path in front of H ₁ is parallel again.

In the reference beam path, following the first hologram Hi, the reference mirror S ₁ and the same hologram H ₂ follow. The optical axis of the reference beam path behind H _{1 has} the direction of the plus first diffraction order in transmission. For the normal of the reference mirror S ₁ and the reference beam path, what has been said for the test object beam applies accordingly. Behind the hologram H ₂ , an imaging system AS ₂ , consisting of aperture B ₂ and lens O ₂ and a matrix photodetector MA are arranged. The imaging system is arranged so that the specimen P is imaged as sharply as possible in the plane of the matrix photodetector MA. The arrangement further includes a translator TR (eg piezoelectric encoder) for adjusting the reference phase by shifting the reference mirror S ₁ in the direction of its normal and a computer RE, which further processes the photosignals from the matrix photodetector MA after AD conversion and the translator TR controls.

In the arrangement according to FIG. 2, the hologram H _{1 is} preceded by an objective 11, the hologram H _{2 is followed by} an objective O _{2 in} such a way that there is a parallel or nearly parallel beam path between the objective and the hologram. Although this is one of the advantages of the arrangement according to the invention compared to the prior art namely the saving of such lenses large free diameter, lost, but the production of holograms is simplified because they no longer need to focus.

In the arrangement of Figure 3 reference mirror S ₁ and hologram carrier are combined to form a glass block G. For setting the reference phase, therefore, the translator is connected to the test specimen and not, as in Fig. 1, with the reference mirror. The advantage of this arrangement is the greater compactness and stability, disadvantageous is the fixed assignment of holograms and reference mirror but insofar as the simple change of the detection sensitivity by changing the laser wavelength is no longer possible.

In Fig. 4, the carrier T _{1 of} the hologram H _{1 is} prismatic, so that the normal of the plane exit surface of the passing bundle has the direction of the optical axis. The covering glass body T ₂ has a corresponding exit surface for the reflected bundle and a spherical entrance surface for the divergent bundle of the laser whose center of curvature is in focus. Accordingly, the carrier, not shown, of the hologram H _{2 is} designed. With the arrangement according to the invention, a number of advantages can be achieved.

By using holograms as beam-splitting and collimating elements, the expenditure on equipment can be reduced in comparison with known solutions. This is only possible because the holograms according to the invention simultaneously

Reflection and used in transmission. Otherwise, the grazing incidence required to inspect technical surfaces with visible light can not be realized.

The use of holograms also makes it possible to compensate for the errors of the arrangement by appropriate production of the holograms.

As a result, such an arrangement can be used not only for automated evaluation as in the hitherto known solutions, but also for visual evaluation, even with larger errors.

The wavelength of tunable lasers can be adjusted in a simple and relatively accurate manner. Thus one can vary the detection sensitivity without changing the interferometric structure. In a somewhat different embodiment, a particularly compact and stable arrangement can be built up for a fixed detection sensitivity.

The fact that the path difference of the arrangement is balanced, can also be partially coherent laser, z. B. semiconductor laser, use without additional equipment. These lasers emit in the near infrared (800-900 nm). There is the maximum sensitivity of the silicon receiver and also holograms larger lattice constant, which are easy to produce use. Sufficient power (a few mW) can be generated in small volumes with semiconductor lasers. The tuning range (~ 10nm) of these lasers is sufficient for the present purposes.

The test interferometer according to the invention, coupled to a computer, allows an automatic selection of workpieces that do not meet the requirements for flatness in the current process. It can be used in particular for testing semiconductor wafers.

Claims (10)

1. Arrangement for the interferometric evenness of technical surfaces using a Mach-Zehnder interferometer formed by a Prüflingsarm and a reference arm, with a laser as a light source, which is followed by an expansion system with pinhole to produce a divergent beam path, characterized in that behind the Expansion system (AS1) is a first hologram (H ₁ ), which is arranged so that before the hologram (H ₁ ) divergent beam path after the hologram (H ₁ ) to the parallel beam path and at the same time the beam path in a reference and a Prüflingsstrahlengang wherein the hologram (H ₁ ) is used for the one beam path in transmission and for the other beam path in reflection that the Referenzstrahiengang after the first hologram (H ₁ ) contains a mirror (Si) whose surface normal to that of the first hologram (H ₁ ) is parallel and with a second hologra mm (H ₂ ), which is identical to the first hologram (H ₁ ) and arranged therewith in a plane such that the beam path after the mirror S ₁ hits the second hologram H ₂ , that the Prüflingsstrahlengang after the first hologram (H ₁ ) the test piece (P) whose surface normal to that of the first hologram (H-,) is parallel, and another hologram (H ₂ ), which closes the running after the specimen in the direction of its reflection angle Prüflingsstrahlengang and this with the Reference beam path combined to the observation beam path, wherein the previously parallel beam path to a convergent beam path, that in the observation beam path an imaging system (AS ₂ ) and a matrix receiver (MA) are arranged, the specimen (P) as sharp as possible in the plane of the matrix receiver (MA) It is shown that the output of the matrix receiver (MA) is connected to a computer (RE) and a translatable from the computer (RE) Translat or (TR) is provided for the displacement of the mirror (Si) in the direction of its surface normal. 2. Arrangement according to claim 1, characterized in that a tunable laser for varying the detection sensitivity is switched in front of the interferometer and reference mirror and DUT are displaced in the direction of their normal. 3. Arrangement according to claim 1 or 1 and 2, characterized in that a partially coherent laser is used as the light source. 4. Arrangement according to claim 1 to 3, characterized in that the holograms are blazed for a first diffraction order in reflection and in transmission. 5. Arrangement according to claim 1 to 4, characterized in that the holograms are designed such that the errors of the wavefronts that arise through the arrangement can be compensated. 6. Arrangement according to claim 1 to 5, characterized in that in the beam path in front of the first hologram (H ₁ ) a λ / 2 plate for adapting the polarization direction of the laser light is arranged on the hologram. 7. Arrangement according to claim 1 to 6, characterized in that in front of the first hologram (Hi) and after the second hologram (H ₂ ) is ever a lens which is such that after the expansion system (AS ₁ ) divergent Beam path is parallel before the first hologram (H ₁ ) and that the beam path behind the second hologram (H ₂ ) initially can still be parallel and is convergent behind the lens, so that the holograms are in the parallel beam path. 8. Arrangement according to claim 1 to 6, characterized in that the translator (TR) is provided for displacement of the specimen in the direction of its surface normal and at the same time reference mirror and hologram carrier are combined to form a glass block. 9. Arrangement according to claim 1 to 6, characterized in that the hologram carrier is designed symmetrically with respect to the passing and the reflected beam. 10. Arrangement according to claim 1 to 6 and 9, characterized in that the hologram carrier has a spherical surface for the entry of the divergent or to the exit of the convergent bundle, whose center of curvature is in the respective focus.