CN105136046B - Laser interferance method change in film thickness amount on-line monitoring method and device - Google Patents

Abstract

The invention discloses an on-line monitoring method and device for film thickness variation by laser interferometry. The method is as follows: the laser is focused on the film to be measured, and two beams of reflected light generated on the upper and lower surfaces of the film interfere; the interference light is detected, Then a visible periodic "voltage-time" curve is formed; combining the number of cycles and the time spent can obtain the change in film thickness. The device includes: a continuous laser diode, a light emitting diode, a dichroic mirror, a 50:50 beam splitter, an achromatic lens, an infrared objective lens, a third beam splitter, an imaging element, a visible light cut filter, and a photodiode. The infrared objective lens combined with the light-emitting diode to provide illumination can simultaneously image the visible light topography of the sample surface to be tested and the infrared laser spot used for measurement on the imaging element, and achieve real-time measurement of specific points on the sample surface. The invention has the advantages of high precision, low cost, real-time lossless, stable optical path, convenient system maintenance and the like.

Description

On-line monitoring method and device for film thickness variation by laser interferometry

technical field

The invention belongs to the technical field of photoelectric precision measurement, and in particular relates to a method and device for on-line monitoring of film thickness variation by laser interferometry.

Background technique

With the advancement of science and technology, films are widely used in various fields such as packaging, agriculture and hospitals, especially polymer films are more widely used in the electronics industry. In the application, the thickness of the film is an important reference for judging some key performance indicators of the film, and its influence on the device is very important. Therefore, online, real-time, dynamic, accurate, stable, and non-contact measurement of thin films is very important for thin film production.

At present, the methods of measuring film thickness are mainly divided into two categories: off-line measurement and on-line measurement, specifically: (1) Off-line measurement includes probe method, optical thickness measurement method, and other off-line methods; (2) On-line includes laser thickness measurement method, capacitance thickness measurement method, ray thickness measurement method, infrared thickness measurement method. Although off-line measurement can accurately measure the thickness of the existing fixed film, it has its limitations and shortcomings. For example, the probe method equipment is easy to damage the film, it is not easy to move, and the measurement process requires strict and no vibration. The optical measurement method is commonly used ellipsometry method, its measurement speed is slow, and fast measurement cannot be performed. At the same time, off-line measurement cannot monitor the thickness change of the film during processing, so it cannot meet the needs of on-line measurement.

The most widely used online optical measurement method is the laser thickness measurement method. The principle is that a beam of light is reflected on the upper and lower surfaces of the film, and the two reflected lights interfere. The number of interference fringes is measured by a reversible counter to obtain the film thickness to be measured. The measurement error is 0.1% to 0.3%, which still cannot meet the measurement requirements of film thickness at the nanometer level. Moreover, the traditional measurement method has relatively strict requirements on the measurement environment, and it cannot be measured during the processing of the film, which will inevitably cause a lot of waste of resources.

Therefore, high-precision, non-destructive, online, and low-cost monitoring of film thickness has become an urgent technical problem to be solved.

Contents of the invention

The main purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art, and provide an online monitoring method for film thickness variation by laser interferometry, which has the advantages of high precision, non-destructive, online monitoring, and low cost.

Another object of the present invention is to overcome the shortcomings and deficiencies of the prior art, and provide a monitoring device for realizing the above-mentioned online monitoring method of film thickness variation. The device has the advantages of low cost and easy construction.

The purpose of the present invention is achieved through the following technical solutions: laser interferometry online monitoring method of film thickness variation, focusing the laser on the measured film, generating two beams of reflected light on the upper and lower surfaces of the measured film, and the two beams of reflected light generate The interference forms interference light, and the intensity of the interference light changes periodically with the change of the thickness of the measured film; the photodiode converts the detected interference light power into a current, and the current signal is converted into a digital voltage signal by the data acquisition circuit to construct "Voltage-time"curve; since the time required for a single cycle is known, and each cycle represents a change in the thickness of the measured film Among them, λ is the laser wavelength, and n is the refractive index of the film to be measured, so as to obtain the change in film thickness.

Preferably, the on-line monitoring method also includes imaging the surface of the sample, the steps are: projecting visible illumination light onto the film under test, illuminating the surface of the film under test to obtain a reflected light, and the imaging element detects the reflected light, and then Image the surface of the film under test.

A monitoring device for realizing the above online monitoring method of film thickness variation, comprising a continuous laser diode, a dichroic mirror, a 50:50 beam splitter, an achromatic lens, an infrared objective lens, a third beam splitter, a visible light cut filter, Photodiode, imaging element, and dichroic mirror reflect the laser beam through visible illumination light; the laser beam generated by the continuous laser diode passes through the dichroic mirror, 50:50 beam splitter, achromatic lens, and infrared objective lens in sequence , focus into a spot on the surface of the film to be tested, and the two beams of reflected light produced by the laser beam on the upper and lower surfaces of the film to be tested interfere; the interference light passes through the infrared objective lens, achromatic lens, 50:50 beam splitter, and then enters the third beam splitter , and finally the transmitted light through the third beam splitter enters the imaging element, and the spot on the film under test is imaged on the imaging element; the reflected light through the third beam splitter is detected by the photodiode through the visible light cut-off filter and converted Current signal for film thickness information.

Preferably, the monitoring device also includes a light-emitting diode for providing illumination to the light path in the monitoring device, and the visible illumination light generated by the light-emitting diode passes through a dichroic mirror, a 50:50 beam splitter, an achromatic lens, and an infrared objective lens. After that, it is projected onto the surface of the film to be tested to generate reflection. The reflected light passes through the infrared objective lens, achromatic lens, 50:50 beam splitter and then enters the third beam splitter. The transmitted light through the third beam splitter enters the imaging element, and the measured film Image on imaging element.

Preferably, the continuous type laser diode is a continuous type single-mode laser diode, and the wavelength of the diode is selected from the wavelength corresponding to the smaller absorption in the absorption spectrum of the film to be measured. This diode has better coherence and monochromaticity.

Preferably, an aspheric lens is also provided between the continuous laser diode and the dichroic mirror. The aspherical lens can improve the directivity of the continuous laser diode, and improve stray light and waste of laser light energy.

Preferably, a plano-convex lens is further provided between the light emitting diode and the dichroic mirror. The plano-convex lens can improve the light-gathering characteristics of the light-emitting diode.

Preferably, the distance between the main base plane of the image side of the achromatic lens and the main base plane of the object side of the infrared objective lens is the sum of the focal length of the image side of the achromatic lens and the focal length of the object side of the infrared objective lens. The lens group composed of the two can simultaneously image the visible light topography of the film surface to be tested and the infrared laser spot used for measurement on the imaging element.

Preferably, the resolution of the infrared objective lens is higher than that of the imaging element. Thus, image quality can be guaranteed.

Preferably, the monitoring device is provided with an outer frame. In this way, the influence of ambient stray light can be minimized.

Furthermore, several cooling fins are arranged behind the light emitting diodes, and a heat exhaust fan is installed, and several ventilation holes are arranged on the side wall of the outer frame. Ensure that the heat generated by the light-emitting diode is removed in time to ensure its luminous effect and service life.

Preferably, the monitoring device is installed on an XYZ displacement platform. Therefore, the relative position of the measuring device and the film to be measured can be adjusted in actual use to ensure the accuracy and stability of the measurement.

Compared with the prior art, the present invention has the following advantages and beneficial effects:

1. The present invention is based on laser interferometry for online monitoring of film thickness variation, and can convert the interference light intensity into a visible "voltage-time" curve. This method can achieve high-precision measurement in the process of film processing, and The system has strong stability, lossless and low cost.

2. The present invention utilizes an infrared objective lens and an achromatic lens in conjunction with a light-emitting diode to provide illumination to image the laser spot and the surface of the film under test to the imaging element, thereby realizing real-time measurement of a specific point on the surface of the film under test.

3. The present invention can not only measure the change amount of the film thickness with high precision, but also measure the change rate of the film thickness, and identify whether the film processing is terminated.

4. The method of the present invention has no rigid requirements on the film to be tested, and the measurement effect is better for films with smaller absorption of the selected laser wavelength; it is also possible to replace a continuous laser diode with a suitable wavelength according to the film to be measured. At the same time, there is no rigid requirement on the shape of the film, which can be solid, liquid or polymer.

Description of drawings

Fig. 1 is a schematic diagram of the laser path of the present invention.

Fig. 2 is a schematic diagram of the illumination imaging optical path of the present invention.

Fig. 3 is a schematic diagram of double interference involved in the present invention.

Fig. 4 is a schematic diagram of an embodiment of the present invention.

Detailed ways

The following drawings clearly and completely describe the conception, specific structure and technical effects of the present invention, so as to fully understand the purpose, features and effects of the present invention. Apparently, the described embodiments are only some of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative efforts belong to The protection scope of the present invention. The various technical features in the invention can be combined interactively on the premise of not conflicting with each other.

Example 1

The principle of the laser thickness measurement method in the prior art is that one beam of light is reflected on the upper and lower surfaces of the film, the two reflected lights interfere, and the number of interference fringes is measured by a reversible counter to obtain the film thickness to be measured. The laser interferometry online monitoring method for film thickness variation described in this embodiment is based on two beams of reflected light generated on the upper and lower surfaces of the film for sub-amplitude interference. When the optical path difference of the two beams is an even multiple of the half wavelength, Interference enhancement occurs, and when the optical path difference between the two is an odd multiple of the half-wavelength, interference weakening occurs. Therefore, the intensity of interference light changes periodically with the change of film thickness. Use the photodiode to detect the interference light power and convert it into a current, and convert the current signal into a digital voltage signal through the data acquisition circuit to construct a "voltage-time"curve; each cycle represents the change of the film thickness (λ is the laser wavelength, n is the refractive index of the measured film), and the time required for a single cycle can be intuitively obtained from the collected curve. The change in film thickness can be obtained from the combination curve.

In addition, in this embodiment, an infrared objective lens and an achromatic lens are used together with a light-emitting diode to provide illumination to image the laser spot and the surface of the film under test to the imaging element, thereby realizing real-time measurement of a specific point on the surface of the film under test.

As shown in Figure 1 and Figure 2, it is a schematic diagram of the laser path involved in the above method and a schematic diagram of the illumination imaging optical path. Since the description is under the same method, the same number in the two figures refers to the same object, but the light source is different. different.

As shown in Figure 1, the laser path includes: continuous laser diode 1, aspheric lens 2, light emitting diode 3, plano-convex lens 4, dichroic mirror 5, 50:50 beam splitter 6, achromatic lens 7, near An infrared infrared objective lens 8 , a third beam splitter 9 , a visible light cut filter 10 , a photodiode 11 , an imaging element 12 , and a sample 13 to be tested. As shown in Figure 1, the laser diode 1 produces a laser beam 18 with a certain divergence angle; the aspheric lens 2 can greatly improve the directivity of the laser diode 1; the dichroic mirror 5 almost completely reflects the laser beam 18; the laser diode The laser beam 18 generated by 1 is focused into a spot on the sample 13 through an aspheric lens 2, a dichroic mirror 5, a 50:50 beam splitter 6, an achromatic lens 7, and an infrared objective lens 8, and the spot size is as small as possible. The two beams of reflected light generated by the laser beam 18 on the upper and lower surfaces of the film to be tested interfere; the interfering light returns in the same way, passes through the infrared objective lens 8, the achromatic lens 7, and the 50:50 beam splitter 6 and then reaches the third beam splitter 9. The transmitted light through the third beam splitter 9 enters the imaging element 12, and the laser spot on the sample 13 to be measured is imaged on the imaging element 12; The detection is transformed into a current signal with thickness information of the thin film 13, wherein the visible light cut-off filter 10 only passes the infrared laser wavelength used for measurement, but the visible light used for imaging cannot pass through.

As shown in Figure 2, the illumination and imaging optical path includes: continuous laser diode 1, aspheric lens 2, light emitting diode 3, plano-convex lens 4, dichroic mirror 5, 50:50 beam splitter 6, achromatic lens 7, infrared An objective lens 8 , a third beam splitter 9 , a visible light cut filter 10 , a photodiode 11 , an imaging element 12 , and a sample 13 to be tested. The visible illumination light generated by the light-emitting diode 3 has a certain divergence angle, and the plano-convex lens 4 can improve the light-gathering characteristics of the light-emitting diode 3 . The dichroic mirror 5 transmits 100% of visible light for illumination. The visible illumination light generated by the light-emitting diode 3 is projected onto the sample 13 through the plano-convex lens 4, the dichroic mirror 5, the 50:50 beam splitter 6, the achromatic lens 7, and the infrared objective lens 8 in order to illuminate the surface of the sample 13, wherein the achromatic The main base plane of the image side of the lens 7 is separated from the main base plane of the object side of the infrared objective lens 8 by the sum of the focal length of the image side of the achromatic lens 7 and the focal length of the object side of the infrared objective lens 8 . The illuminating light beam projected on the sample 13 is reflected on the surface of the sample 13, and the reflected light returns in the same way, passes through the infrared objective lens 8, the achromatic lens 7, and the 50:50 beam splitter 6 and then reaches the third beam splitter 9. The transmitted light through the third beam splitter 9 enters the imaging element 12, and the sample 13 is imaged on the imaging element 12; the reflected light through the third beam splitter 9 cannot pass through because the visible light cut-off filter 10 is provided, and cannot be detected by the photodiode. 11 detected. Randomly select an object point 14 on the surface of the sample 13, the object point 14 is imaged on the imaging element 12 through the infrared objective lens 8 and the achromatic lens 7 to form an image point 15, and the image points formed by all object points on the imaging element 12 form a sample The image formed by the surface of 13 on the imaging element 12.

It can be known from the working process of Figs. 1 and 2 that both the laser beam and the visible illumination light undergo some transformations of light at the tested sample 13, referring to Fig. 3, which is a schematic diagram of the double-beam interference involved in the present invention, wherein the tested sample 13 is composed of the tested film 16 and the substrate 17, and the incident laser beam 18 is projected onto the surface of the tested film 16; reflected light 19 and transmitted light 20 are generated at the "air-tested film" interface; the transmitted light 20 is in the "tested film" The film-substrate interface produces reflected light 21 and transmitted light 22, and the reflected light 21 produces transmitted light 23 at the "measured film-air" interface. The reflected light 19 and transmitted light 23 meet the requirements of the same wavelength, frequency and constant phase difference. The three major interferences are sufficient and necessary conditions, so the sub-amplitude interference occurs.

The above structure is a schematic diagram of the basic structure principle. Fig. 4 has provided the example figure of a kind of concrete application, and this device comprises continuous type laser diode 24, aspheric lens 25, light-emitting diode 26, plano-convex lens 27, dichroic mirror 28, 50:50 beam splitter 29, achromatic lens 30, infrared objective lens 31, third beam splitter 32, visible light cut filter 33, photodiode 34, imaging element 35, laser diode drive circuit 36, data acquisition circuit 37, light emitting diode drive circuit 38, packaging frame 39, An external computer 40 and an XYZ displacement platform 41 are connected. The laser beam emitted by the laser diode 24 and the visible illumination light emitted by the light emitting diode 26 exist and work at the same time, and the two beam paths are independent of each other and do not affect each other. See Figures 1 and 2 for the light path diagram.

In this embodiment, the continuous type laser diode 24 is a continuous type single-mode laser diode, and a laser with a small infrared band absorption for common semiconductor materials is used. In practice, lasers with other wavelengths can also be selected according to the optical characteristics of the measured material. Not limited by the preferred embodiments of the invention.

In this embodiment, selecting a high-precision ADC voltage acquisition chip in the data acquisition circuit 37 can improve the accuracy of film thickness measurement; Image of an infrared laser spot.

In order to minimize the impact of ambient stray light, a packaging frame 39 needs to be added. In order to ensure the imaging quality, the resolution of the infrared objective lens 31 is required to be higher than the resolution of the imaging element 35 .

In addition, since the light-emitting diode 26 that adopts can generate more heat while emitting light, in order to ensure the light-emitting effect and service life of the light-emitting diode 26, the following three measures are taken: a heat sink is arranged behind the light-emitting diode 26, and a light-emitting diode is installed. A heat exhaust fan is installed on the side wall of 26 sides, and some ventilation holes are made on the side wall of the packaging frame on the side of LED 26. In order to ensure the accuracy and stability of the measurement, the measuring device in the embodiment is installed on an XYZ displacement platform, and the relative position of the measuring device and the film to be measured can be adjusted in actual use.

The preferred embodiments of the present invention have been described in detail above, but the invention is not limited to the described embodiments, and those skilled in the art can also make various equivalent modifications or replacements without violating the spirit of the present invention. These equivalent modifications or replacements are all within the scope defined by the claims of the present application.

Claims (7)

1. The on-line monitoring device for film thickness variation by laser interferometry, characterized in that it includes a continuous laser diode, a dichroic mirror, a 50:50 beam splitter, an achromatic lens, an infrared objective lens, a third beam splitter, and a visible light cut-off filter film, photodiode, imaging element, dichroic mirror reflects the laser beam through visible illumination light; the laser beam generated by the continuous laser diode passes through the dichroic mirror, 50:50 beam splitter, achromatic lens, infrared After the objective lens, it is focused into a spot on the surface of the film to be tested, and the two beams of reflected light generated by the laser beam on the upper and lower surfaces of the film to be tested interfere; Finally, the transmitted light through the third beam splitter enters the imaging element, and the light spot on the film to be tested is imaged on the imaging element; the reflected light through the third beam splitter is detected by the photodiode through the visible light cut-off filter and converted into Current signal with film thickness information; The monitoring device also includes a light-emitting diode for providing illumination to the light path in the monitoring device, and the visible illumination light produced by the light-emitting diode is projected to the Reflection occurs on the surface of the film to be tested, and the reflected light passes through the infrared objective lens, achromatic lens, and 50:50 beam splitter in sequence, and then enters the third beam splitter, and the transmitted light through the third beam splitter enters the imaging element. on the imaging. 2. The monitoring device according to claim 1, wherein the continuous type laser diode is a continuous type single-mode laser diode, and the wavelength of the diode is selected from the wavelength corresponding to the smaller absorption in the absorption spectrum of the film to be measured. 3. The monitoring device according to claim 2, characterized in that an aspherical lens is further provided between the continuous laser diode and the dichroic mirror. 4. The monitoring device according to claim 1, characterized in that a plano-convex lens is further provided between the light emitting diode and the dichroic mirror. 5. monitoring device according to claim 1, is characterized in that, the distance of described achromatic lens image square main base surface and infrared objective lens object space main base surface is between achromatic lens image square focal length and infrared objective lens object square focal length and. 6. The monitoring device according to claim 1, characterized in that, the resolution of the infrared objective lens is higher than that of the imaging element. 7. The monitoring device according to claim 1, wherein an outer frame is provided outside the monitoring device; Set several heat sinks behind the light-emitting diodes, install heat exhaust fans, and set several ventilation holes on the side wall of the outer frame; The monitoring device is installed on an XYZ displacement platform.