DE2200222A1 - DEVICE FOR DETERMINING THE SURFACE QUALITY - Google Patents

Description

Böblingen, December 30th, 1971 pr-bue / fr,

Official file number: New registration file number of the applicant Docket GE 971 Oll

Device for determining the surface quality

The invention relates to a device for determining the surface quality by means of light point scanning from for example Machining of periodically structured surfaces.

In the production of flat surfaces , for example in the production of flat surfaces on lathes, a certain step formation inevitably occurs. Machines that work very precisely, such as those used, for example, in the production of blanks for magnetic storage disks, make this possible. To process workpieces in which the surface roughness caused by this step formation is below 0.05 pm. The quality of such surfaces can be checked either by mechanical scanning methods, at · =

for example Taly-Surf method, or by interferometric Ver = drive carried out «The first-mentioned methods differ because of the very large amount of time required to capture larger areas and because of the difficulty of measuring non-destructively. The interferometric Methods, in particular interference microscopy Seems appropriate, require size- during the examination. rer surfaces such a large metrological and lateral elevation

309828/0189

found that they are ruled out for most practical applications.

The invention is based on the task of specifying a device for examining periodically structured surfaces, with which it is possible to make a number of possible errors in a simpler way Way and with little expenditure of time. The most important faults to be determined are: horizontal dislocations of step edges, vertical shifts of individual steps, changes the step shape, e.g. B. by chips wedged under the turning tool, (defects in the lattice structure), soiling, for example due to oil or dust, irregularities in the material, for example voids and scratch marks (other defects).

According to the invention, this object is achieved by a device for determining the surface quality by means of light point scanning for example, solved by machining periodically structured surfaces, which is characterized by a the Scanning beam controllably displacing or pivoting about a pivot point element, one or more orders of the surfaces to be examined diffracted scanning beam separately recorded light-sensitive arrangement and by an optical system, that is, the shifted scanning beam or the pivot point of the pivoted scanning beam after its diffraction at an error-free area of the area to be examined, regardless of the respective position of the scanning light point, always on the same area of the light-sensitive area Arrangement maps.

Especially advantageous further developments of the inventive concept are characterized in that the Äbtaststrahl shifting or pivoting movable mirror elements or electro-optical devices are operable periodically to deflect the scanning beam in either one or in two directions.

Another particularly advantageous development of the inventive concept is characterized in that the scanning by the

Docket ge 971 on 3 0 9 8 2 8/0169

Combination of a periodic movement of the scanning light point {and one that takes place, for example, through constant rotation The surfaces to be examined are shifted. Further Features ^ of the invention emerge from the subclaims in j. Connection with the figures. . j

The invention will then be explained in more detail with reference to the figures. Show it: -

: Fig. 1 a particularly advantageous embodiment.

the invention?

Figs. 2 and 3. schematic representations of the surface structures from two panels of different surface quality?

Fig. 4 shows the intensity curves of the 1/2 diffraction order

and the scattered light when scanning the in FIGS. 3 and 4 shown 'surfaces?

, Fig. 5; the course of the I / 2 curve in the absence of a Git

level.

The arrangement shown in Fig. 1 consists of a laser 1 » ^: the.eine beam 2 erected via a fixed mirror 3 on a mirror 5 pivotable about an axis 4, which is moved by means not shown in such a way that the beam reflected on it in a plane defined by the lines 2a, 2b and 2c between the area delimited by the lines 2a and 2c is periodically pivoted, in the course of the beam paths lying in said plane a cylindrical lens 7 is arranged _t which the rays emanating from the mirror 5 in every position drops almost vertically onto a 2u testing plate 8. The plate 8 is, for example, the blank of a magnetic storage disk, for example, the surface structure aiii "shown in FIG * 2 * DIU Ah 'order is such that the desired at each ι on a.

Docket GE 971 Oll 3 0 9 8 2 8 / Ö .1

Line 15, 16 on the surface of the plate 8 that acts as a grid ■ lying point reflected radiation on light-sensitive , Elements 10, 11 and 12 of a light-sensitive arrangement 9 as ■ diffraction O. +1. and -1. Order is mapped provided the scanned Surface 8 has the desired properties. If deviations from the prescribed grid structure occur additional diffraction orders of the "1/2." Order, which are detected by the photosensitive element 13.

When other disturbances occur, scattered light reaches the photosensitive element 14 of the arrangement mentioned.

The defects on the surface 8 to be examined are thus determined by evaluating the diffraction image of the periodic structure of the surface to be examined and / or by the scattered light occurring on this surface, which is obtained when the surface is scanned with the laser beam 2. The surface 8, which is periodically structured, for example, by turning, acts like a diffraction grating due to its step structure, more precisely cfesää like an echelette grating, if one assumes a flat cut surface. As is known from elementary diffraction theory, the light intensity of the diffraction orders depends on the shape of the individual steps t, in particular on their angle of inclination β

where χ is parallel to line 15-16 in the evaluation level running coordinate is.

Depending on the size of the angle of inclination, the diffraction orders are Lattice replaced with the diffraction pattern of the single stage

Docket GE 911 011

309828/0189

Intensity. In particular, the ratio is then +1. to -1. Diffraction order of the overall grating given by

(1) J ₊₁ / J _-1 = (2β + π) ² / (2β -ir) ²

and thus a measure of the angle β = together with that through the The lattice constant that can be determined from the distance between the individual orders can thus determine the roughness of an ideal lattice.

Disturbances in the periodicity of this lattice now make themselves felt Deviations from the ideal diffraction pattern noticeable. The place in the middle between the O. grid order is of particular interest here and 1st grid order ("1/2 order"). Here the contributions of neighboring levels are even in the case of the undisturbed grid same size, but 180 out of phase and therefore cancel each other out. However, this is no longer the case ^ as soon as Malfunctions occur. ■

The above error based on lattice faults depend on the intensity of the 1/2 "grating order, the inclination of the individual stages" their reflectivity, their width, the intensity distribution of Äbtaststrahles and the deviation from the mean step height from. The following applies in detail

2ßn

cos (2 π A / b

/ _O » _T _i r. / _t «n - η

U) I ₁ /, -I λ l-iJ A _{Q π} 2ß _n

, ng .2 2iri Ahn / R ~ ο b _n . e ~ " ^l a / 2 ~ '. e λ

Docket GE 971, 011

3098-28 / 0189

The following are used as abbreviations:

λ wavelength

^I l / 2 intensity of the 1/2. order

A amplitude of the illuminating beam lattice constant
Diameter of
Deviation of the η-th step from the mean step height

2 diameter of the illumination beam (l / e * e)

β angle of inclination of the η-th stage

b _n width

R reflectivity of the η-th level

The other malfunctions listed above can essentially be found here are perceived as changes in reflectivity and partially cancel each other out. However, you can can be reliably detected by measuring the scattered light with the aid of the light-sensitive element 14 outside the grating spectrum.

If you feel the area to be measured by moving the deflection mirror 5 perpendicular to the lattice direction, one will make a contribution to the intensity in the form of a for each lattice disturbance "Gaussian distribution" is obtained, which is given by the illuminating beam. These contributions are, as is easy to see, at a disturbance of the reflection capacity proportional to the disturbance, the same is approximately the case with a small deviation in the step width. The intensity is also dependent from the sine occurring step height changes in relation to the undisturbed grid, all these Gaussian curves are superimposed however, in the case of several faults with different positions and phases. In the case of statistical disturbances, the resulting Intensity when scanning carry out small fluctuations around the value corresponding to the mean probability »Isolated Larger disturbances will result in a deviation from this mean value according to their cause, their magnitude and their phase position to have.

Docket GE 971 Oll

309828/0 169

With the device according to the invention, two faced surfaces of different quality, ie regularity of the lattice structure, were examined. The.Fign. 2 and 3 are schematic representations of interference microscopic images with a 5-fold enlargement of these surfaces / the heights and lateral displacements of the steps can be clearly seen. In Fig. 4, the intensity of 1/2. Grid order I ₁ Z ₂ ^{and the} scattered light I

shown as a function of the location in radial scanning _t where the curve shown in the left part of the figure corresponds to the better surface shown in Fig. 2 and the curve shown in the right part of the figure corresponds to the poorer area shown in Fig. 3. The difference between the curves recorded during the scanning of the two surfaces according to the invention, one of which is much more irregular than the other, is so great that a quantitative comparison does not appear sensible. However, it is not the task of the invention to carry out such comparisons, but rather small deviations from the ideal grid shape should be determined.

The surface with the strong disturbances of the periodicity {Fig. 4, right) is only intended to illustrate the sensitivity of the method. Scratch disruptions have relatively minor effects on the course of the curves. According to (2), a large part of the disturbance cancels here. It becomes much stronger through such Interference affects the scattered light I, which with the help of the light

sensitive element 14 is evaluated.

The impact of a major disorder on the 1/2. Order will shown in Fig. 5, in which a curve in the practical absence a grid step is reproduced. When scanning in the radial direction, the disturbance rises above the subsurface of the added up statistical errors.

The device according to the invention thus allows the following variables to be determined.

Docket GE 971 Oll

309828/0169

- αϊ. The surface roughness of an approximately ideal grid,

2. the mean deviation from this surface roughness,

3. the size of individual faults in the grid,

4. the presence of diffuse scattering disturbances.

DocKet GE 971 Oll _309828/0169

Claims (11)

_ 9 - PATENT CLAIMS Device for testing the surface quality by means of Light point scanning of, for example, surfaces periodically structured by machining., Characterized by means of an element which can be controlled to move a scanning beam (2) or to pivot about a fulcrum (3) ", one or more orders of the to surface to be examined (8) diffracted scanning beam (.2, 2a, 2b, 2g) separately detecting light-sensitive arrangement (9) and by an optical! System (7), the shifted scanning beam or the pivot point of the pivoted scanning beam after its diffraction at a faultless one Area of. area to be examined, regardless of the respective position of the scanning light point the same areas (10, 11, 12) of the light-sensitive arrangement (9) images, while disturbances of the lattice structure other areas (13) of the light-sensitive arrangement are acted upon. 2. Apparatus according to claim 1, characterized in that daö the element pivoting or displacing the scanning beam (2) is a movable mirror (3) a 3., Arrangement according to claim 1, characterized in that the element pivoting or shifting the scanning beam (2) consists of an electro-optical arrangement » 4. Arrangement according to claims 1 to 3, characterized in that that the element pivoting or displacing the scanning beam (2) is designed for deflection in two dimensions is. . ■ " 5. Device according to claims 1 to 3, characterized in that that the scanning beam (2) pivoting or displacing element for deflection in one dimension Docket GE 971 011 30982 8/0169 - 10 is trained. 6 »Device according to claims 1 to 5, characterized by a periodic deflection of the scanning beam causing formation of the deflection element. 7. Device according to claims 1 to 6, characterized in that that the scanning of a periodically structured surface (8) by the combination of a periodic Movement of the scanning light point and a displacement that takes place, for example, by constant rotation examined area takes place. 8 "Device according to claims 1 to 7, characterized in that that the system which images the scanning beam (2) after its diffraction at the light-sensitive arrangement | S>) a cylinder lens (7) is » 9. Device according to claims 1 to 8, characterized in that that the light-sensitive arrangement (9) areas (10, 11, 12, 13) for the separate detection of the 0., -1., +1. » and +1/2. Contains order. 10. The device according to claim 9, characterized in that the light-sensitive arrangement {9} consists of a number of light-sensitive elements (10, 11? 12, 13). 11. Device according to claims 1 to 10, characterized by at the output of the light-sensitive elements (10, 11, 12, 13) occurring signals processing electronic circuits to indicate the presence of a change of the diffraction orders and errors of the scanned surface that can be recognized by the occurrence of scattered light. Docket GE 971 Oll 309828/0 T 69