WO2012053998A1

WO2012053998A1 - A surface roughness measurement method and setup

Info

Publication number: WO2012053998A1
Application number: PCT/TR2011/000231
Authority: WO
Inventors: Zafer Dursunkaya; Murat Yildirim
Original assignee: Zafer Dursunkaya; Murat Yildirim
Priority date: 2010-10-19
Filing date: 2011-10-17
Publication date: 2012-04-26
Also published as: TR201008570A1

Abstract

This invention is related to a method and setup for measuring surface roughness and waviness, also determining straightness error of the carriage and other means of the setup, by using interference phenomenon. In this invention, a surface to be analyzed and an interferometer interacting with said surface, being displaced relative to each other are used. Using the interference fringes obtained during the displacement along a particular direction (this particular direction is named as direction in this document after this point), the surface profile of the surface along the projection of the direction of progress is determined.

Description

A SURFACE ROUGHNESS MEASUREMENT METHOD AND SETUP

Field of the Invention

This invention is related to a method and setup for measuring surface roughness and waviness, also determining straightness error of the carriage and other means of the setup, by using interference phenomenon.

Background of the Invention

Measurement of surface roughness and waviness of surfaces is an important issue especially in different industry applications which needs high precision and accuracy. Therefore, numerous methods and setups have been developed for measuring surface roughness.

In one of these methods, the measurement is performed by subtracting the deviation of the carriage from linearity, from the measurement data obtain from sensors, e.g. capacitive sensors, which are placed at specific locations. In a setup using this technique, distance between the sensors and the surface cannot be changed after they are placed in the setup.

Although atomic force microscopy (AFM) may be used to determine surface roughness values, this method is limited with measuring surface roughness values of only small areas. Since surface waviness values cannot be measured by measurements of such small areas, surface roughness values also cannot be determined with the desired accuracy.

In addition to these techniques mentioned above, there are some techniques and setups which use interference methods to measure surface roughness of different surfaces. For example, a fiber optic interferometer was developed to detect surface roughness values of a surface, especially a glass surface, in the WO2005010511A1 document. Although the fiber optic interferometer mentioned in that document was effective for detecting surface roughness of large surfaces, they are not appropriate to be applied for small benches and micro machines. Aim and Summary of the Invention

It is aimed to develop a method and a setup to be used for measuring surface roughness and waviness with this invention.

Apart from that, it is also aimed to develop a setup for surface roughness measurement which works relatively independent of the locations and orientations of the surface and the interferometer.

In addition to these, it is also aimed to develop a method for determining the location of parallelism between the surface and the direction of progress, with this invention.

In this invention, a surface to be analyzed and an interferometer interacting with said surface, being displaced relative to each other are used. Using the interference fringes obtained during the displacement along a particular direction (this particular direction is named as direction in this document after this point), the surface profile of the surface along the projection of the direction of progress is determined.

Brief Description of the Drawings

The figures and their explanations, which are used to explain the surface roughness measurement method and setup developed in this invention, are given below.

Figure 1 Schematic view of the surface roughness measurement setup

Figure 2 Schematic view of the relative position of the measurement plate and the surface during measurement along the first direction of progress

Figure 3 Schematic view of the relative position of the measurement plate and the surface during measurement along the second direction of progress

Figure 4 A graph depicting the "V" shaped curve

Figure 5 A graph depicting the interference fringes of the raw data

Figure 6 A graph depicting the frequency distribution of the raw data obtained by Fourier transform

Figure 7 A graph depicting the interference fringes of the filtered data

Figure 8 A graph depicting the frequency distribution of the filtered data obtained by Fourier transform Figure 9 A graph depicting the interference fringes obtained with the displacement of the carriage along the direction of progress

Figure 10 A graph depicting the distance with respect to the initial position versus the number of data

Figure 11 A graph depicting the frequency distribution of the distance data obtained by Fourier transform

Figure 12 A graph depicting the linear curve fitted to the distance versus number of data

Figure 13 A graph depicting the deviation of the distance from linearity

Figure 14 A graph depicting the frequency distribution of the deviation of the distance from linearity obtained by Fourier transform

Figure 15 A graph depicting the 9^th order polynomial curve fit corresponding to the straightness error of the carriage

Figure 16 A graph depicting the sinusoidal curve corresponding to the effect of the stepper motor

Figure 17 A graph depicting the sum of the effects due to the measurement setup Figure 18 A graph depicting the surface profile curve

Figure 19 A graph depicting the surface roughness values of the steel surface obtained with the first probe

Figure 20 A graph depicting the surface roughness values of the steel surface obtained with the second probe

Figure 21 A graph depicting the surface roughness values of the gold coated surface obtained with the first probe

Figure 22 A graph depicting the surface roughness values of the gold coated surface obtained with the second probe

Figure 23 A graph depicting the surface roughness values of the gold coated surface obtained by AFM

Figure 24 A graph depicting the surface roughness values obtained with the first and the second probes with different surface areas of the gold coated and steel gauge surfaces Figure 25 A graph depicting the straightness error values obtained with the first and second probes in the first direction of progress

Figure 26 A graph depicting the straightness error values obtained with the first and second probes in the second direction of progress

Definitions of the Parts Used in the Invention

The parts and the components which are shown in the figures are numerated in order to explain the surface roughness measurement method and setup, developed with this invention. Each part and component definition is given below with respect to its corresponding number.

1. Surface roughness measurement setup

2. Sample

3. Base

4. Sample plate

5. Measurement plate

6. Shaft

7. Carriage

8. Jack

9. Holder

10. Coherent light source

11. Circulator

12. Optical fiber

13. Detector

14. Processor

15. Laser

16. Beam splitter

17. Isolator

18. Stepper motor 19. Data acquisition card

20. Computer

Detailed Description of the Invention

The surface roughness measurement method subject to this invention, is basically consisting of the steps:

1. placement of the surface to be analyzed close to parallel but nonparallel with the direction of progress;

2. delivery of waves from a coherent source whose frequency is known, onto the surface to be analyzed, where some fraction is reflected back after moving a specific distance and the remains are reflected from the surface in such a way to overlap with the firstly reflected fraction;

3. measurement of the optical intensity resulting from the interference of the overlapping waves;

4. movement of the unit delivering the wave to the surface, and the surface with respect to each other by a specific amount;

5. repeating the steps 2,3 and 4 for the whole length of the surface aimed to be analyzed;

6. process of the data obtained, to produce a curve of the distance with respect to the initial position (this change is named as distance after this point) versus number of the data curve;

7. application of a linear curve fit to the distance versus number of data curve;

8. obtaining the deviation from linearity of the distance by subtracting the obtained linear curve fit from the distance curve;

9. producing curves corresponding to the effects due to the measurement setup;

10. obtaining the surface profile curve by subtracting curve fittings corresponding to the effects due to the measurement setup from the curve of deviation from linearity of the distance curve.

After obtaining the surface profile curve, surface roughness value is calculated by taking the average of the absolute values of the data. In respect of the technical character of this invention, there is no difference between sets of data stored in matrixes or tables and curves corresponding to these sets, in performing the method whose basic steps are listed above. Curves are used in the explanation for ease of explaining the invention.

By placing the surface to a position close to parallel but nonparallel to the measurement plate, the occurrence of a detectable trend in the change of the interference fringes is provided; allowing the direction of the change in distance which correspond to small disturbances of said interference fringe to be determined.

The length of the movement of the measurement plate, which is explained in step 4 is inversely proportional to the measurement resolution, can be adjusted in accordance with the accuracy requirements of the measurement. Apart from that results of the measurements can be improved by filtering the measurements obtained in step 5.

The operation mentioned in the step 6 relies on the known correspondence of the maximum and minimum intensities observed in the interference fringes of waves of a known frequency to the change in distance.

Possible effects that can be among those mentioned in step 8, include straightness error of the carriage and vibrations coming from the stepper motor.

The surface roughness measurement setup (1) which is developed, in scope of the present invention, to be used for the application of surface roughness measurement method whose essential steps are explained above, basically includes, a sample holder that consists of

a base (3),

a sample plate (4) which carries the sample (2) to be analyzed along a certain direction and which extends through said direction,

- a measurement plate (5) which directs the means for roughness measurement to the sample (2),

a shaft (6) which carries the measurement plate (5),

a carriage (7) which carries the shaft (6) and therefore the measurement plate (5) through the direction of progress,

- two jacks (8) which are connected to the sample plate (4) and to the base (3) in order to provide adjustment of the relative position of the sample plate (4) with respect to the measurement plate (5), a holder (9) used to hold a probe which enables detection of the position of the measurement plate (5) along the direction of progress by interacting with the measurement plate (5),

for holding the sample (2) to be analyzed, in the desired way and an interferometer that consists of

at least one coherent light source (10),

one circulator (11) for each measurement to be performed, which directs light beams coming from the coherent light source (10) and entering its first port to its second port and directs light beams coming from the second port to its third port,

- optical fibers (12), whose one end is connected to the second port of the circulator (11) and the other end is fixed to the measurement plate (5) and which carries light beams some of which are internally back reflected from its said last end, some part of which reflects back from the sample (2) and comes back to the optical fiber (12) to produce interference,

- a detector (13) which measures the intensity of the light beams entering from the second port and leaving from the third port of the circulator (11),

a processor (14) which processes the data obtained with the detector (13), for performing the measurements. The probe which is used to measure the displacement of the measurement plate (5) may also be an optical fiber (12). In this situation, another optical fiber (12) is connected to the holder (9) like other optical fibers (12) connected to the measurement plate (5). Thus, the position of the measurement plate (5) can be measured using interference.

It can be possible to use different kinds of photo detectors (13) in the surface roughness measurement setup (1). Optical intensity of the incident beams can be measured by voltage, current etc. values by using related kinds of photo detectors (13).

The light source (10) consists of, one laser (15) and a beam splitter (16) which splits the beams from said laser (15) into number of beams equal to the desired number of measurements; a number of lasers (15) equal to the desired number of measurements or at least two lasers (15) of which at least one can be used directly and at least one being split in at least two by a beam splitter (16), to produce the number of beams equal to the desired number of measurements. Different wavelength coherent beams can be obtained by using more than one laser (15) and thus different resolutions can be obtained or samples (2) which can have different reflectivity values can be analyzed. If protection of the lasers (15) from back reflections is desired an isolator (17) can be used after the laser (15).

The motion of the measurement plate (5) should be controlled to have a desired resolution during the measurements. In order to achieve this, a stepper motor (18) is used to drive the lateral carriage (7) in one embodiment this invention.

The jacks (8) are screw micrometers in order to accurately adjust the sample plate (4) position with respect to the measurement plate (5).

Processor (14) consists of a data acquisition card (19) which is connected to the photo detectors (13) and a computer (20) which is connected to the data acquisition card (19).

In a preferred embodiment of the invention, in order to perform the first step concerning placement of the sample (2) close to parallel but nonparallel to the direction of progress of the measurement plate (5) safely, the position of the jacks (8) at which parallelism is obtained should be known. For this purpose, a method of determining the location of the parallelism comprising the steps:

1. setting the jacks (8) to a random setting at which the sample (2) is nonparallel to the direction of progress and recording the difference between the micrometer values;

2. delivery of waves from a coherent light source onto the sample (2) where some fraction is reflected back from the end of the optical fiber (12) connected to the measurement plate (5) and the remains are reflected from the sample (2) in such a way to overlap with the firstly reflected fraction;

3. measurement of the optical intensity resulting from the interference of the overlapping waves via a photo detector (13);

4. moving the measurement plate (5) by a specific amount;

5. repeating the steps 2, 3 and 4 for the whole length of the sample (2) aimed to be analyzed;

6. process of the data obtained, to determine the total change in distance;

7. setting the jacks (8) to a setting at which the sample (2) is closer to parallel to the direction of progress and recording the new difference between the micrometer values; 8. repeating the steps 2, 3, 4, 5, 6 and 7 until the sample (2) reaches a nonparallel random position in the other direction after becoming parallel to the direction of progress;

9. obtaining a "V" shaped curve depicting the difference between the micrometer values versus the change in distance and determining its minimum point,

is used. The measurement plate (5) and the sample (2) will be parallel to each other for all micrometer values satisfying the difference between the micrometer values corresponding to the determined minimum point. After finding the parallel position, the sample plate (4) is adjusted such that the sample (2) is close to parallel but nonparallel to the direction of progress. In order to prevent the optical fibers (12) from scraping or touching the sample (2) during progression, the jacks (8) are adjusted in such a way that the one further along the direction of progress is lower than the other. These measurements can be performed in both directions as far as it is compatible with the adjustments explained above. The schematic views of the sample plate (4) positions corresponding to the movement in both directions are shown in Figure 2 and Figure 3.

In order to exemplify the use of surface roughness measurement method and setup (1) subject of the invention, some results obtained with the aforementioned method and setup (1) are presented below.

In a study performed on a gold coated sample (2), firstly difference of the micrometer values for which the gold coated surface and the direction of progress are parallel, is determined; the corresponding "V" curve is shown in Figure 4.

The interference fringes of the rough data which is obtained by the step 5 of the surface roughness measurement method for the gold coated sample are shown in Figure 5. Fourier transform of the rough data is taken to obtain frequency spectrum of the rough data in order to interpret the existence of effects not relating to the interference. It is seen that external dynamics in addition to the optical signal frequency of interest, are included in Figure 6. The rough data is filtered to remove said effects and filtered data seen in Figure 7 is obtained. As it can be observed from the frequency spectrum, obtained by of the Fourier transform, of the filtered data seen in Figure 8, the unwanted effects have been extracted by filtering. For the present example, said unwanted effect arises from the stepper motor (18) and signals whose frequencies are much different from the frequency of the optical signal. The interference fringes due to the displacement of the carriage (5) are shown in Figure 9. As it can be seen from Figure 9, these fringes do not contain any noise.

The distance versus the measurement number curve obtained with step 6 of the surface profile measurement method is shown in Figure 10. The frequency distribution of the distance obtained by the Fourier transform is shown in Figure 11. This distribution shows that the optical signal frequency has not deteriorated during the step 6 of the surface roughness measurement method. Deviation of the distance from linearity seen in Figure 13 is obtained with the use of the linear curve shown in Figure 12 as explained in the steps 7 and 8. From the frequency distribution obtained by Fourier transform of said deviation and shown in Figure 14, it is understood that the most significant effect in the deviation is -since the frequency of this effect corresponds to one period of rotation of the stepper motor- due to the motion of the stepper motor. The other significant effect is the straightness error of the carriage.

In order to clean out these trends, according to the step 9, a 9th order polynomial curve corresponding to the straightness error of the carriage, shown in Figure 15 and a sinusoidal curve corresponding to effect of the stepper motor, shown in Figure 16, are used. The summation of these curves is shown in Figure 17. In the step 10, a graph corresponding to the surface roughness seen in Figure 18 is obtained using this sum. The surface roughness value is obtained using the values in this graph.

The surface roughness values obtained in a study for a steel gauge known to be manufactured with a surface roughness of 20nm at 9 different inclination angles and with the first and second optical fibers (12) connected to the measurement plate (5), are shown in Figure 19 and Figure 20, respectively. When the same study is conducted with a gold coated surface whose surface roughness value is not known, the surface roughness value results shown in Figure 21 and Figure 22 are obtained. The surface roughness values obtained are tabulated in Table 1 appearing below:

Table 1: Measured roughness values of the steel and the gold coated surfaces

In order to prove the measurements concerning the gold coated surface, said surface is also investigated using AFM and the results obtained are shown in Figure 23. These data are also tabulated in Table 2 appearing below:

Table 2. Measured roughness values of gold coated the surface obtained with AFM

The difference between measurements concerning the gold coated surface is based on the fact that since small areas can be scanned in measurements performed with AFM, surface waviness of the surfaces cannot be detected. This is proved by the fact that, during measurements on different surface areas of the steel gauge and the gold coated surfaces performed with the method and setup (1) subject to the invention, while the measurements concerning the steel gauge whose surface roughness is evenly distributed roughly stays the same, roughness of the gold coated surface rapidly increases beyond 500pm.

The straightness errors obtained by measurements in the forward and backward directions are shown in Figure 25 and Figure 26. As it can be seen from the figures for both two directions being symmetric to each other, shows that measurements in both directions are equivalent to each other.

Many embodiments of the surface roughness measurement method and setup subject to the invention may be developed and the invention cannot be considered limited to the examples explained here; it is essentially as explained in the claims.

Claims

1. A surface profile measurement method, enabling measurement of the surface roughness of a surface, characterized by the steps:

placement of the surface to be analyzed close to parallel but nonparallel with the direction of progress;

delivery of waves from a coherent source whose frequency is known, onto the surface to be analyzed, where some fraction is reflected back after moving a specific distance and the remains are reflected from the surface in such a way to overlap with the firstly reflected fraction;

measurement of the optical intensity resulting from the interference of the overlapping waves;

movement of the unit delivering the wave to the surface, and the surface with respect to each other by a specific amount;

repeating the steps 2,3 and 4 for the whole length of the surface aimed to be analyzed;

process of the data obtained, to produce a curve of the distance with respect to the initial position (this change is named as distance after this point) versus number of the data curve;

application of a linear curve fit to the distance versus number of data curve; obtaining the deviation from linearity of the distance by subtracting the obtained linear curve fit from the distance curve;

producing curves corresponding to the effects due to the measurement setup; obtaining the surface profile curve by subtracting curve fittings corresponding to the effects due to the measurement setup from the curve of deviation from linearity of the distance curve.

2. A surface roughness measurement method according to claim 1, which includes effects of the stepper motor (18) among the effects due to the measurement setup.

3. A surface roughness measurement method according to claim 2, in which effects of the stepper motor (18) can be represented by a sinusoidal curve.

A surface roughness measurement setup (1) used for the applying the surface roughness measurement method according to any of the claims 1 to 3, comprising, an interferometer for performing measurements, consisting of

at least one coherent light source (10),

optical fibers (12), whose one end is connected to the second port of the circulator (11) and the other end is fixed to the measurement plate (5) and which carries light beams some of which are internally back reflected from its said last end, some part of which reflects back from the sample (2) and comes back to the optical fiber (12) to produce interference,

a detector (13) which measures the intensity of the light beams entering from the second port and leaving from the third port of the circulator (11),

a processor (14) which processes the data obtained with the detector (13), and a sample holder for holding the sample (2) to be analyzed, in the desired way, consisting of

a base (3),

a measurement plate (5) which directs the means for roughness measurement to the sample (2),

a shaft (6) which carries the measurement plate (5),

a holder (9) used to hold a probe which enables detection of the position of the measurement plate (5) along the direction of progress by interacting with the measurement plate (5),

and characterized in that the sample holder further consists of

a sample plate (4) which carries the sample (2) to be analyzed along a certain direction and which extends through said direction, two jacks (8) which are connected to the sample plate (4) and to the base (3) in order to provide adjustment of the relative position of the sample plate (4) with respect to the measurement plate (5).

5. A surface roughness measurement setup (1) according to claim 4, in which the carriage (7) is moved by a stepper motor (18).

6. A surface roughness measurement setup (1) according to claims 4 or 5, in which the jacks (8) are screw micrometers.

7. A method for determining the location of the parallelism to be used with the surface roughness measurement setup (1) of claims 4 to 6, for determining the location of the parallelism characterized by the steps

setting the jacks (8) to a random setting at which the sample (2) is nonparallel to the direction of progress and recording the difference between the micrometer values;

delivery of waves from a coherent light source onto the sample (2) where some fraction is reflected back from the end of the optical fiber (12) connected to the measurement plate (5) and the remains are reflected from the sample (2) in such a way to overlap with the firstly reflected fraction;

measurement of the optical intensity resulting from the interference of the overlapping waves via a photo detector (13);

moving the measurement plate (5) by a specific amount;

repeating the steps 2, 3 and 4 for the whole length of the sample (2) aimed to be analyzed;

process of the data obtained, to determine the total change in distance;

setting the jacks (8) to a setting at which the sample (2) is closer to parallel to the direction of progress and recording the new difference between the micrometer values;

repeating the steps 2, 3, 4, 5, 6 and 7 until the sample (2) reaches a nonparallel random position in the other direction after becoming parallel to the direction of progress;

obtaining a "V" shaped curve depicting the difference between the micrometer values versus the change in distance and determining its minimum point.