JP4235997B2 - Optical film thickness measuring method and apparatus - Google Patents

Description

  The present invention relates to an optical film thickness measuring device using a laser beam mounted on an apparatus for forming a thin film as a light source, and a high-precision optical film by varying the output of the laser beam between layers at the time of forming a multilayer film. It is intended to provide a thickness measuring device.

  In a high-density wavelength division multiplexing transmission system (hereinafter referred to as DWDM), an optical multiplexer / demultiplexer is used for wavelength multiplexing and demultiplexing of multiplexed optical signals, but it is used inside the optical multiplexer / demultiplexer. Optical specifications such as transmission bandwidth, flatness, transmission loss, and suppression ratio with adjacent wavelengths required for a dielectric multi-layer narrowband bandpass filter (hereinafter referred to as NBPF) It is a strict value to achieve high speed and large capacity. Table 1 shows an example of optical specifications required for NBPF for 100 GHz (International Telecommunication Union: wavelength interval defined by ITU). The minimum transmission loss in the transmission band is required to be 0.8 dB or less, and the rectangular characteristic of the waveform requires −0.5 dB bandwidth: 0.4 nm or more and −25 dB bandwidth: 1.2 nm or less.

  The NBPF is one of the applications of optical thin films using light interference, and its structure is made by alternately depositing high and low refractive index dielectric materials and using multiple reflections from the layer interface to obtain the desired filtering characteristics. Is what you get. FIG. 1 shows the basic structure of NBPF. The optical film thickness of each layer is λ / 4 with respect to the transmission wavelength: λ, that is, λ / 2 for a pair of high refractive index material (25) and low refractive index material (26). As a result, the reflected light from the layer interface is added in phase to form the reflective band layer (27). There are two reflection band layers (27), and a spacer layer (28) whose optical film thickness is an integral multiple of λ / 2 is disposed between them to form a Fabry-Perot structure filter facing each other to form a cavity (29). . In order to satisfy the above optical specifications, NBPF has a multi-cavity structure in which a plurality of Fabry-Perot filters are connected via a coupling layer (30), and the number of layers is a multilayer structure of 100 layers or more. Furthermore, the optical specifications described above cannot be satisfied unless the optical film thickness of each layer is controlled with an accuracy of 0.01% or less.

Next, an optical film thickness control method for each layer will be described. Refractive index: n _g refractive index on a transparent substrate: n thin film thickness of _m: d only deposited, the wavelength in air or in a vacuum: when the incident light of lambda, the energy reflectance: R is the following It is expressed by a formula.

FIG. 2 shows the relationship between the optical film thickness: _nm · d and the reflectance: R when the refractive index of the transparent substrate is 1.52. The energy reflectivity: R increases with light interference when n _g < _nm , and decreases when _ng > _nm , but both have extreme values when _nm · d = λ / 4. Thus, every time the optical film thickness: _nm · d becomes an integral multiple of λ / 4, the energy reflectance (transmittance): R becomes an extreme value. The film thickness control of each layer in NBPF manufacturing is performed at this extreme value.

FIG. 3 shows a configuration diagram of a vacuum film forming apparatus for NBPF provided with a conventional optical film thickness measuring apparatus. The vacuum vessel (1) is evacuated to a level of 1 × 10 ⁻⁵ Pa by a vacuum pump (not shown) such as an oil diffusion pump or a cryopump.
The film formation substrate (6) attached to the center of the substrate dome (5) is rotated at 1000 rpm together with the substrate dome (5) by a high speed rotation mechanism (not shown) in order to make the film thickness distribution in the substrate uniform. Heated by a heating sheath heater (7) and a halogen heater (21). Further, the temperature of the film formation substrate (6) is measured using a radiation type thermometer (17), the actual measurement data is input to the temperature controller (18), and the temperature controller (18) is set at a preset temperature. Compare and calculate the measured temperature and the measured temperature, and based on the results, adjust the power for the halogen heater so that the substrate temperature is always constant even if the deposition substrate (6) receives radiant heat from the electron beam or heat generated during plasma generation. Control (19) (Japanese Patent Application No. 2002-229025).

An electron beam evaporation source (2) is used to form a dielectric film which is an optical thin film. At that time, when high frequency power (frequency: 13.56 MHz) output from the high frequency power source (22) is directly applied to the substrate dome (5), glow discharge is generated in the space between the substrate dome (5) and the evaporation source (2). Then, a plasma state occurs, and a self-induced negative DC electric field is generated on the surface of the film formation substrate (6) attached to the substrate dome (5), and a thin film having high energy and high packing density is formed.
The matching box (23) matches the output impedance of the high-frequency power source (22) and the impedance of the discharge mechanism including the substrate dome (5) as a load.
The quartz film thickness sensor (4) detects the evaporation rate and feeds back a detection signal to the electron beam evaporation source controller (not shown) to control the film formation rate to be constant.

  The optical film thickness measuring device mainly includes a laser light source (11), an optical fiber (13), an emission tube (14), a light receiver (15), and a controller (10). The laser beam (monochromatic light) emitted from the laser light source (11) passes through the depolarizer (12), the optical fiber (13), the emission tube (14), and the lower viewing window (8) to form the film formation substrate (6). Then, the light passes through the upper viewing window (9) and enters the monochromatic photometric light receiver (15). The amount of transmitted light that changes depending on the thickness of the optical thin film deposited on the film formation substrate (6) is photoelectrically converted into an electrical signal by the light receiver (15). The controller (10) performs arithmetic processing on the photoelectrically converted electrical signal, and when the transmittance reaches the extreme value of λ / 4, the shutter (3) is closed, the film formation is terminated, and the dielectric materials are sequentially stacked. I will do it.

JP 2001-73136 A

FIG. 4 shows the result of simulating the change in transmittance of each layer in a single cavity NBPF having a 27-layer structure. In the figure, the horizontal axis represents the layer number, and the vertical axis represents the transmittance. The layer number indicates the number of layers counted from the substrate, and the single cavity shown in the figure is composed of a reflective band layer (layer numbers: 1 to 13, 15 to 27), a spacer layer (layer number: 14) Consists of.
Although the optical film thickness measuring device is used to control the film thickness of each layer at the time of manufacturing NBPF, the amount of change in transmittance in the reflective band layer in the vicinity of the spacer layer and the spacer layer is compared with other reflective band layers. Very small.

Comparing the amount of change in transmittance with reference to FIG. 4, for example, the amount of change in transmittance of the first layer is about 20%, whereas in the 14th spacer layer, it is 1.65%, which is 1/10. It turns out that it is the following. Since extreme value detection in film thickness control monitors changes in transmittance, if the amount of change in transmittance is small, errors in detection of extreme values are likely to occur, resulting in deterioration in the optical characteristics of NBPF. Will have a big problem.
Therefore, there is a method of expanding the amount of change in transmittance electrically or by calculation in the signal processing circuit after the light receiver (15) for detecting the extreme value of the layer having a small amount of change in transmittance such as the spacer layer. Since the SN ratio (Signal to Noise ratio) does not change, it is not possible to improve the extreme value detection accuracy.

The present invention is intended to solve the above problems, and in a layer having a small change in transmittance, the SN ratio is improved and the change in transmittance is increased to achieve high-precision film thickness control. An object of the present invention is to provide a new optical film thickness measuring apparatus.
In order to achieve the object, the output intensity of the laser light applied to the deposition substrate can be set to an arbitrary value during the deposition process.
Specifically, a laser light source for irradiating the film formation substrate with laser light and a control device for controlling output of the laser light source are provided, and the irradiation intensity of the laser light is varied during the film formation process. Laser light output control calculates the change in transmittance or reflectance in each layer in advance, increases the output of the laser light source when forming a layer with a small amount of change in transmittance or reflectance, and transmits the amount of transmitted light or reflection. The amount of change is increased by increasing the amount of light, and extreme value detection is performed.

  By adopting a configuration in which the output of the laser light source can be set to an arbitrary value according to the present invention, it is possible to detect extreme values with high accuracy and to significantly improve the accuracy of film thickness control. .

The embodiment of the present invention will be described with reference to FIG. The present invention is characterized in that a control device (33) for controlling the output of the wavelength tunable laser (11) as a laser light source is provided. Since other configurations are the same as those of the conventional apparatus shown in FIG. 3, the same reference numerals are given and description thereof is omitted. Hereinafter, an example in which the laser light source output control device is installed separately from the laser light source will be described. However, the control device may be built in the laser light source.
The control device (33) is configured such that the output of the laser light source can be set to an arbitrary value during the film forming process. The output of the laser light is set so that the amount of change in transmittance in each layer is simulated in advance, and the output is increased in a layer with a small amount of change.
Specifically, the output of the wavelength tunable laser is increased before starting the formation of the spacer layer and the reflection band layer near the spacer layer.

This operation will be described with reference to FIG. As shown in FIG. 4, the spacer layer (14th layer) according to the conventional method starts film formation from a transmittance of 1.5% and reaches an extreme value at 3.15%. The amount of change in transmittance at this time is 1.65%. On the other hand, when the output of the wavelength tunable laser is increased 10 times before the start of the spacer layer, for example, the film formation starts from 15% transmittance and reaches an extreme value at 31.5%. Is magnified 10 times.
By increasing the amount of laser light projected onto the substrate using the control device (33), the amount of transmitted light increases, and the amount of change in transmittance can be increased without increasing noise. It is possible to improve the SN ratio and perform highly accurate film thickness measurement.
Although the transmittance is measured in the above embodiment, the reflectance may be measured.

An NBPF having a 5-cavity configuration for 100 GHz was prepared based on the above-described embodiment. The produced NBPF uses optical thin film materials of Ta ₂ O ₅ and SiO ₂ , and their optical film thickness is deposited and controlled at λ / 4 (λ: 1550 nm). Assuming that the optical film thicknesses of Ta ₂ O ₅ and SiO ₂ are λ / 4, and H and L respectively, the film configuration is substrate / {[HL] ⁶ H8LH [LH] ⁶ } L {[HL] ⁷ H8LH [LH ⁷ } L {[HL] ⁷ H8LH [LH] ⁷ } L {[HL] ⁷ H8LH [LH] ⁷ } L {[HL] ⁶ H8LH [LH] ⁶ } / atmosphere.

The output of the wavelength tunable laser is increased three times in the first reflection band layer of each cavity and decreased three times in the second reflection band layer to return to the original value. Table 2 shows the output of the wavelength tunable laser in each layer.

The film formation conditions are as follows: the temperature of the film formation substrate (6) is 400 ° C., and the degree of vacuum in the vacuum vessel (1) is 2.5 × 10 ⁻² Pa, SiO ² when forming Ta ₂ O ₅ by introducing oxygen gas. _During film formation, the film was held at 1.5 × 10 ⁻² Pa, and the film formation rates of Ta ₂ O ₅ and SiO ₂ were set to 0.4 nm / sec and 0.8 nm / sec, respectively.

  FIG. 5 shows changes in the amount of transmitted light in each layer of the first cavity. The amount of transmitted light is indicated by relative transmittance, and the value indicating the amount of transmitted light at the laser output at the start of film formation is indicated as a reference. That is, the amount of light when the laser beam at the laser output (−10 dBm) at the start of film formation is incident on the light receiver without passing through the film formation substrate is set to 100% transmittance. In the figure, (32) shows the change in the amount of transmitted light in the 14th spacer layer. In the spacer layer shown in (32) in the figure, the film formation started at 20% transmittance and reached an extreme value at 43%, and the change in transmittance was 23%, which was about 14 times larger than before. I know that.

FIG. 6 shows the result of measuring the spectral characteristics at the center of the substrate after the film was released using the film thickness measuring apparatus of the present invention and released into the atmosphere. FIG. 7 shows the result of measuring spectral characteristics in the same manner after film formation using a conventional film thickness measuring apparatus.
From FIG. 6, by using the film thickness measuring device of the present invention to increase the amount of change in transmittance in the spacer layer and the reflection band layer in the vicinity of the spacer layer, the extreme value detection accuracy is improved, and the prior art shown in FIG. It can be seen that extremely good optical characteristics are obtained as compared with the result of film formation using this film thickness measuring apparatus.

In the above embodiment, the vacuum film forming apparatus for NBPF has been described. However, the present invention is not limited to the NBPF, and can be diverted to other vacuum film forming apparatuses.
In the above embodiment, the wavelength tunable laser is used as the light source. However, the present invention is not limited to the wavelength tunable laser, and can be diverted to other light sources with variable output.

It is explanatory drawing of the film | membrane structure of NBPF. It is a related figure of the optical film thickness of a dielectric material thin film, and a reflectance. It is explanatory drawing of the conventional apparatus structure. The simulation of the transmittance | permeability change is shown. The change in transmittance when an actual film is formed using the method of the present invention is shown. The figure shows the spectral characteristics measured in the air after actual film formation using the method of the present invention. The figure shows the spectral characteristics measured in the air after the actual film formation using the conventional method. It is explanatory drawing of the apparatus structure of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vacuum container 2 Electron beam evaporation source 3 Shutter 4 Quartz sensor 5 Substrate dome 6 Deposition substrate 7 Substrate heating sheath heater 8 Lower viewing window 9 Upper viewing window 10 Controller 11 Wavelength variable laser 12 Depolarizer 13 Optical fiber 14 Output tube 15 Monochromatic photometric detector 16 Viewing window 17 Radiation type thermometer 18 Temperature regulator 19 Halogen heater power regulator 20 Low voltage introduction electrode 21 Halogen heater 22 High frequency power supply 23 Matching box 24 High voltage introduction electrode 25 High refractive index material
26 Low refractive index material 27 Reflective band layer 28 Spacer layer 29 Cavity 30 Coupling layer 31 Substrate 32 Change in transmittance in the 14th spacer layer 33 Control device

Claims (2)

In an optical thin film measurement method for measuring the film thickness of the film formation substrate by projecting a laser beam onto the film formation substrate on which the multilayer film is deposited, measuring the transmittance or reflectance of the film formation substrate,
Calculate the change in transmittance or reflectance in each layer in advance,
When forming a layer with a relatively small amount of change in transmittance or reflectance, the intensity of the laser light irradiated to the film-forming substrate is increased, the amount of transmitted light or reflected light is increased, and extreme value detection is performed. An optical thin film measuring method characterized by performing. An optical thin film measuring apparatus for projecting a laser beam onto a film forming substrate to measure the transmittance or reflectance of the film forming substrate and measuring the film thickness of the film forming substrate,
Mounted in an optical thin film forming apparatus for forming a multilayer film on the film formation substrate,
A laser light source for irradiating the film formation substrate with the laser light;
A control device for controlling the output of the laser light source,
The control device is an optical thin film measuring device that varies the irradiation intensity of the laser light during a film forming process.
Calculate the change in transmittance or reflectance in each layer in advance,
When forming a layer with a relatively small amount of change in transmittance or reflectance,
An optical thin film measuring apparatus, wherein the output of the laser light source is increased by the control device.

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