JP3528955B2 - Method for designing and manufacturing optical element and optical element - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for designing an optical element having a multilayer film such as an antireflection film on its surface and a method for manufacturing the same.
It relates to.

[0002]

2. Description of the Related Art Optical elements such as lenses and reflecting mirrors are used for antireflection, for setting transmittance and reflectance for each wavelength to predetermined characteristics, and for setting phase characteristics for each wavelength to predetermined characteristics. In addition, a multilayer film is often formed on the surface. The number of layers of this multilayer film reaches several tens, and by controlling the thickness of each film constituting the multilayer film, predetermined optical characteristics are obtained.

FIG. 2 shows an example of a method for forming such a multilayer film. In FIG. 2, (a) is a view of the substrate holder as seen from below, and (b) is an end view of the apparatus at the AA ′ position. A substrate holder 2 is provided in the vacuum chamber 1 and rotates about a rotation shaft 3. Board holder 2
The optical element 4 for forming a film on the surface is concentrically attached to the lower surface of the substrate 1. The monitor substrate 5 is attached at one place where the optical element 4 is attached. A sputtering device 6 is provided below the vacuum chamber 1,
Particles of the constituent components of the film are ejected from there and hit the surfaces of the optical element 4 and the monitor substrate 5 to form the film.

A window 7 is provided on the upper and lower surfaces of a part of the vacuum chamber 1, and the light emitted from the projector 8 is transmitted through the optical element 4 or the monitor substrate 5 and is received by the light receiver 9. The film thickness can be measured.

The monitor substrate 5 is made of a transparent flat plate and is a dummy optical element used for film thickness measurement. That is, since the surface of the optical element 4 is generally a curved surface like a lens, the film thickness cannot be accurately measured. Therefore, by arranging the flat monitor substrate 5 and measuring the thickness of the film formed thereon, the film thickness on the optical element 4 formed indirectly under the same conditions as that is indirectly measured. Is.

Film formation of an optical element using the apparatus as shown in FIG. 2 is performed as follows. First, the material of the film, the number of layers, and the thickness of each layer are determined by calculation so that predetermined optical characteristics (reflectance, transmittance, phase characteristics, etc.) can be obtained. When the design is completed in this way, first, the first layer is formed. When the film thickness can be measured without stopping the rotation of the substrate holder 2, the film thickness is measured every time the monitor substrate 5 passes the positions of the light projector 8 and the light receiver 9. Of course, the approximate time required to form the predetermined film thickness can be obtained by calculation, so the film thickness may be measured after a time close to this time has elapsed.

When the film thickness cannot be measured without stopping the rotation of the substrate holder 2, the substrate holder 2 is periodically cycled after a time close to the time required to form a predetermined film thickness has elapsed. Stop the rotation of and measure. At this time, in order to prevent particles from the sputter device 6 from adhering to the specific optical element 4, the gap between the sputter device 6 and the optical element 4 immediately above it is shielded by a shutter.

In this way, the film formation is continued while the film thickness is being measured, and when the film thickness falls within the tolerance, the film formation of the first layer is finished. Then, the material used for the sputtering is changed, and the film formation of the second layer is similarly performed. Less than,
This process is repeated until the final film is formed.

[0009]

As described above,
In the process of forming a multilayer film on a basic optical element such as a lens, the film is formed while measuring the film thickness of each layer. However, depending on how to select the film thickness, the measurement error of the film thickness may become large. In the prior art, since the film thickness of each layer was determined without considering such a thing in the design stage, the film thickness error of a specific layer becomes large, and as a result, the optical film obtained as a whole multilayer film is obtained. There is a problem that the characteristics may deviate from the target values. This was not a problem when the number of layers of the multilayer film was about 30 or less, but when the number of layers reaches several tens,
It has become a serious problem.

The present invention has been made in view of the above circumstances, and it is possible to accurately measure the film thickness of each layer constituting a multilayer film, and as a result, it is possible to reliably obtain the target optical characteristics. Optical element design method, optical element manufacturing
The challenge is to provide a method .

[0011]

[Means for Solving the Problems] A first means for solving the above problems has a multilayer film on its surface,
Then, the spectral characteristics of the multilayer film were measured by a spectroscope, and
Based on the results of
For each thin film of an optical element that has a process of measuring the film thickness
It is a method of designing the target thickness, and during the design calculation,
From the accuracy of the spectroscope and the accuracy of the fitting calculation
The thickness measurement accuracy of the thin film that is determined depends on the optical element.
Meets the required thickness accuracy of the thin film determined from optical properties
If there is a thin film that does not exist,
Determined from the accuracy of the spectroscope and the accuracy of the fitting calculation.
The accuracy of the thickness measurement of the thin film is determined by the light of the optical element.
Meet the required thickness accuracy of the thin film determined from the scientific properties
There is a process to change the thickness to a value like this and perform design calculation.
And a method for designing an optical element (claim 1).

As described above, for the film thickness of each layer, the film thickness of the monitor substrate during or after film formation is measured. In the present invention, the accuracy of the spectroscope and F
Thickness of the thin film determined from the accuracy of the fitting calculation
Before the measurement accuracy is determined from the optical properties of the optical element
There was a thin film that did not meet the required thickness accuracy of the thin film.
The target thickness of the thin film, and
Thickness of the thin film determined from the accuracy of the fitting calculation
Before the measurement accuracy is determined from the optical properties of the optical element
Change to a thickness that satisfies the required thickness accuracy of the thin film,
Perform design calculations. Therefore, in the film forming process, since the film thickness of all the layers can be measured with a desired accuracy, each film can be formed into an accurate thickness, and an optical element having desired optical characteristics can be manufactured. can do. The “desired accuracy” does not necessarily have to be the same in all layers, and can be determined, for example, in consideration of the degree of influence on target optical characteristics for each layer.

A second means for solving the above problems is
A method of manufacturing an optical element, characterized by comprising a process of forming a film by using the thickness of each film constituting the multilayer film determined by the first means as a reference value (claim 2).
Is.

"As a reference value" means as a target value of the film thickness of each layer. In the present means, the reference thickness of each film is determined so that the film thickness can be measured with a desired accuracy, so that each film can have an accurate thickness,
An optical element having desired optical characteristics can be manufactured. The "desired accuracy" has the same meaning as described in the first means.

The third means for solving the above-mentioned problems is as follows.
The thickness of each film constituting the multilayer film determined by the first means is used as a reference value to form a film, and the deviation of the thickness of the formed film from the reference value is determined to obtain the target optical A method for manufacturing an optical element, characterized by comprising a process of changing the reference value of the film thickness of at least one layer thereafter so as to minimize the influence on the characteristics. Item 3).

In actual film formation, it is impossible to form a film having the same film thickness as the reference value, and a minute deviation cannot be avoided. Therefore, when a multilayer film having several tens of layers is formed, minute deviations of the layers may be accumulated, and as a result, desired optical characteristics may not be obtained. To prevent this from happening,
If the thickness of the n-th layer is out of the allowable value after the film formation of the n-th layer, the target film-forming thickness of the (n + 1) th layer, or in addition to this, The target film-forming thickness of the upper layer is changed from the initially designed thickness, and as a result, the influence on the intended optical characteristics is minimized.

In this way, in the present specification, the feedforward is to change the target value of the film thickness including the (n + 1) th layer and the subsequent layers according to the actually measured film thickness of the nth layer. It is called control. If feed-forward control is performed, the target film thickness of each layer may be corrected to a value different from the designed film thickness.However, since this correction amount is generally small, this correction causes the film thickness measurement to become impossible. There is no entry.

In the present means, the film thickness target value after that is changed so that the final target optical characteristics are obtained in the film forming stage, so that an optical element having the target optical characteristics can be stably manufactured. be able to.

[0019]

[0020]

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an embodiment of the present invention will be described. Prior to that, an example of a film thickness measuring method used in this embodiment will be described. This is a method of measuring the spectral transmittance of the monitor substrate and measuring the film thickness based on the spectral transmittance of the device as shown in FIG. 2, the principle of which will be described with reference to FIG. In FIG. 3, (a) shows a state in which the multilayer thin film M is formed on the monitor substrate 5. The multilayer thin film M is in a state in which n layers M1 to Mn are formed, and the thickness dn of the thin film Mn of the nth layer is measured in this state. At this time, what is measured is the spectral transmittance determined by the entire multilayer thin film M and the monitor substrate 5. That is, assuming that the spectral transmittance at this time is Tn, Tn is

[0022]

## EQU1 ## Tn = Tn (λ, d1, d2, ... d (n-1), dn, a1, a2, ... a (n-1), an) (1). Here, di (i = 1 to n) is the thickness of the i-th layer, and ai (i = 1 to n) is a value determined by the material of the i-th layer. Strictly speaking, the thickness and material of the monitor substrate 5 are also related, but this is constant (1)
Ignored in the formula.

When measuring the thickness of the nth layer, d1 to d (n-1) and a1 to an are known in the equation (1). Here, assuming that the target value of dn is dn ₀ ,
Calculate the spectral transmittance by substituting ₀ . It is assumed that the result is, for example, a curve indicated by A (solid line) in FIG.
On the other hand, assuming that dn becomes dn ₀ + Δ and dn ₀ −Δ, these values are substituted into the equation (1) to calculate the spectral transmittance. The results are B (broken line) and C in FIG. 3 (b), respectively.
As indicated by (dashed line), the curve deviates from the curve indicated by A in correspondence with the values of Δ and −Δ.

Thus, dn is ± m from the target value dn ₀
Calculate the spectral transmittance curve obtained by shifting Δ (m is an appropriate integer) in advance, select the one that is closest to the actually measured spectral transmittance curve, and measure the corresponding film thickness. The film thickness. Specifically, assuming that the calculated spectral transmittance is T (dn ₀ + iΔ, λ) (i = −m to m) and the measured spectral transmittance is Ta (λ),

[0025]

[Equation 2]

## EQU1 ## is obtained for each i, and i having the smallest value of S is adopted. However, the integration range of the formula (2) is within the measurement range of the spectral transmittance, and the fitting calculation (in this specification, "fitting calculation" is a comparison between the calculation amount determined by a certain parameter and the actually observed amount. And a parameter that gives the amount of calculation that is closest to the observed amount is adopted. The method of comparison is the method of least squares, the method of obtaining the parameter that gives the minimum sum of absolute differences, and the cross-correlation coefficient. The typical method is to use the one that obtains the maximum value and adopts the maximum value). In addition, it goes without saying that the integrated value is actually obtained by numerical calculation using a sample value instead of the equation (2).

On the premise of such a film thickness measuring method, an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a flowchart showing an example of an embodiment of the present invention. First, in step S1, the film thickness of each layer of the multilayer film is determined using a conventional method so as to obtain desired optical characteristics (reflectance, transmittance, phase characteristics, etc.). Next, the process proceeds to step S2, and it is determined whether there is a layer in which the film thickness cannot be measured with desired accuracy. For example, in FIG.
As described above, when the film thickness is measured using the spectral transmittance, it is assumed that the film thickness measurement accuracy required for a certain layer is ± 1 nm. The spectral transmittance at the design thickness of that layer is T
(Λ), the spectral transmittance when the layer thickness changes by +1 nm from the design value is T ′ (λ), and the spectral transmittance when the layer thickness changes by −1 nm from the design value is T ″. If (λ), all of them can be calculated.
At this time,

[0028]

[Equation 3]

The value of is determined, and it is determined whether or not this value is, for example, a predetermined multiple or less of the measurement accuracy of the measuring instrument. However, (3),
The integral range of Eq. (4) is the range in which the fitting calculation is performed within the measuring range of the measuring instrument. If this value is less than the specified value of the measurement accuracy of the measuring device, it is ± 1 nm when the film thickness is measured.
It is determined that the measurement cannot be performed with the accuracy within the range, and the layer is determined to be the layer whose thickness cannot be measured with the desired accuracy. A person skilled in the art can appropriately determine how much the "predetermined multiple" should be, depending on the method of approximate calculation when performing integral calculation.

If there is no such layer, the processing is terminated assuming that the value of the obtained film thickness is appropriate.
If there is even one such layer, the process proceeds to step S3, and the film thickness of the bottom layer of such layers is changed to a film thickness capable of measuring the film thickness. That is, while changing the film thickness of the lowermost layer from the current design value in an increasing direction and a decreasing direction alternately, the amount of change is increased by a predetermined amount, and the amount obtained by the equations (3) and (4) is desired. This is repeated until it is determined that the film thickness can be accurately measured.

When the film thickness of the lowermost layer is changed in this way, the process proceeds to step S4, and it is checked whether the target optical characteristics obtained in the entire multilayer film are within the allowable range. If the desired optical characteristics are within the allowable range, the process proceeds to step S2, and it is checked again whether or not there is a layer whose film thickness cannot be measured with desired accuracy.

When it is determined in step S4 that the desired optical characteristics are not within the allowable range, the process proceeds to step S5, in which the film thickness above the layer whose film thickness has been changed is changed to obtain the desired optical characteristics. Get the characteristics. For this calculation, the film thickness determination method that has been conventionally used is used.

As a result of this work, it is checked in step S6 whether or not the desired optical characteristics are finally obtained. If the desired optical characteristics are not finally obtained, the process proceeds to step S7 to change the basic design conditions. That is, the number of layers is increased or the material of the film is changed. Then, the process proceeds to step S1 and the work is restarted from the beginning. If the desired optical characteristics are obtained in step S6, the process returns to step S2 again to check whether there is a layer in which the film thickness cannot be measured with the desired accuracy. When the process shown in FIG. 1 is completed, a multilayer film can be obtained in which the layer thicknesses of all the layers can be measured and desired optical characteristics can be obtained as the entire multilayer film.

After the film thickness of each layer is determined by the process described above, the film forming process is repeated by the method described in the section of the prior art so that the designed film thickness of each layer can be obtained. First, a multilayer film may be formed. At that time, in addition to the film formation using sputtering described in the section of the related art, film formation by CVD, film formation by vacuum vapor deposition, film formation by ion beam assist, or film formation by ion plating may be performed. .

By the way, in the film forming method described in the section of the prior art, since minute deviations from the designed film thickness that occur during the film formation of each layer are accumulated, it is possible to continue the film formation as it is. Characteristics may not be obtained. In order to prevent such a situation from occurring, it is preferable to perform feedforward control.

That is, after forming the nth layer, n
When the thickness of the layer is out of the allowable value, the target film-forming thickness of the (n + 1) th layer is changed from the initially designed thickness accordingly. How much is changed is, for example, (n +
2) Even if the design film thickness after the second layer is not changed, the value is changed so that the final target optical characteristics can be obtained. If feedforward control is performed, the target film thickness of each layer may be corrected to a value different from the designed film thickness, but the amount of this correction is small, so this correction will not enter a region where film thickness measurement is impossible. Absent.

In the above description, the optical characteristics of the multilayer film were measured by the transmittance to determine the film thickness.
When measuring the optical characteristics by reflectance to determine the film thickness,
Needless to say, this also includes the case where the reflectance and the transmittance are measured and the film thickness is determined by combining them, and these are mainly determined by whether the required spectral characteristics are the reflectance characteristics or the transmittance characteristics. It is determined.

[0038]

EXAMPLE A multilayer film was formed on an optical element according to the present invention. The film formation was performed by the sputtering device shown in FIG. This sputtering apparatus has a light projector 8 and a light receiver 9 and is equipped with an optical monitor capable of measuring the spectral transmittance from 400 nm to 850 nm. The film thickness was measured by fitting from the difference between the measured spectral transmittance and the calculated spectral transmittance at the film thickness. The transmittance value in% was taken every 2 nm from 400 nm to 850 nm, and a fitting calculation was performed using these values. In this case, the number of data is 225.

Next, the film configuration of the multilayer film was designed by using the well-known optimization design method for the multilayer film. This film structure
In order to measure the film thickness with desired accuracy, analysis was performed by the procedure shown in FIG. 1 to obtain a corrected film structure. The judgment of [whether there is a layer whose film thickness cannot be measured with desired accuracy] in step S2 of FIG. 1 was performed as follows.

After the film formation of a certain layer, the calculated value of the spectral transmittance at a certain target film thickness at every 2 nm is changed from T (1) to T (225), and only the film thickness accuracy required for film thickness measurement is obtained. When the film thickness is deviated, the calculated value of the spectral transmittance in the film thickness is set at intervals of 2 nm from T '(1) to T' (225). Then, the absolute value of the difference is obtained corresponding to each wavelength. That is, T (225) -T '(225) is calculated from T (1) -T' (1), and their absolute values are obtained.
Next, take the sum of the 225 numbers obtained and call it A. When the value of A is less than 100%, the deviation of the characteristics is too small, and it is considered that sufficient fitting accuracy cannot be obtained, so the target film thickness of the layer is changed. The value of A is
If it is larger than 100, the fitting thickness can be calculated and the target film thickness is not changed.

In film formation using a sputtering apparatus, a feedforward control method was adopted as a film thickness control method in order to obtain optical characteristics as close as possible to design characteristics.
The film forming procedure of each layer in this feedforward control is shown in FIG.
Shown in. This film forming procedure was repeated from the first layer to the final layer.

In step Al of FIG. 4, the designed spectral transmittance characteristic as the target spectral transmittance characteristic of the multilayer film, the target film thickness of the layer to be film-formed with respect to this characteristic, and various spectra when deviated from the target film thickness The transmittance characteristics and the film thickness for each of these were acquired and stored in the controller.

In step A2, film formation was performed by controlling the time so that the target film thickness was obtained. In order to control the time,
The film formation rate of the film formation target layer was obtained in advance and stored in the control device, and film formation was performed for a time determined from this film formation rate and the target film thickness.

In step A3, the spectral transmittance characteristics of the multilayer film at the stage of completing the film formation of this layer were measured. Step A4
Then, the spectral transmittance characteristics measured in step A3 and the spectral transmittance characteristics acquired in step Al were fitted to determine the film thickness of this layer. The instability of the deposition rate of the sputtering apparatus used in this example is ± 1 to 1.5.
%, It was unavoidable that a slight deviation from the target film thickness.

In step A5, in order to minimize the influence of the minute film thickness deviation generated in this layer on the target spectral transmittance characteristics, the film thickness of each film-formed layer is fixed, Again, the design was performed to obtain the target spectral transmittance characteristics. Specifically, in this example, the respective film thicknesses of the remaining layers not yet formed were determined by the above-described optimization design method for the multilayer film, which is well known to those skilled in the art. In general, the respective film thicknesses of the remaining layers thus determined were different from the film thicknesses determined at the time of step A1 of film formation of the first film formation target layer. However, the thickness of each layer is different from the original design because it is a redesign to minimize the effect on the target spectral transmittance characteristics due to the minute film displacement caused by the film formation of each layer. It didn't make a big difference.

In this way, the process again returns to step A1 of feedforward control for forming the next layer, and the process from step A1 to step A5 is repeated until the final layer is formed. Finished. Using the above procedure, a multilayer film consisting of 28 layers of Nb ₂ O ₅ / SiO ₂ was formed on the quartz glass substrate. This multilayer film was an arbitrary film thickness multilayer film in which 28 layers of Nb ₂ O ₅ layers and SiO ₂ layers were alternately laminated on the substrate, and the film thicknesses of all layers were different. The measured values of the spectral transmittance characteristics of the formed multilayer film are shown by the solid line in FIG. The broken line is the design value. Transmittance 1.0 is 1
Equivalent to 00%. It can be seen that the optical characteristics very close to the designed characteristics are obtained by the designing method and the film forming method of the present invention.

The filter formed with this multilayer film is used as an S / N of an optical measuring instrument such as a spectrophotometer for measuring the transmittance.
It was used to reduce the wavelength dependence of the ratio. The S / N ratio of the optical measuring instrument generally has a wavelength dependency given by the product of the wavelength dependency of the intensity of the light source and the wavelength dependency of the sensitivity of the light receiver. In a certain optical measuring device, it was necessary to keep the accuracy of the transmittance measurement value between different wavelengths constant, so that it was necessary to eliminate the wavelength dependence of the S / N ratio.

In order to meet this need, the characteristics obtained by combining the wavelength dependence of the light source light intensity and the wavelength dependence of the sensitivity of the light receiver are adjusted to be flat, that is, the wavelength dependence of the light source intensity. The filter having the inverse characteristic of the wavelength dependence given by the product of the wavelength dependence of the sensitivity and the sensitivity of the light receiver may be arranged between the light source and the light receiver.

As described above, in this embodiment, the time-controlled film formation is performed in step A2, the spectral transmittance is measured in step A3, and the film thickness is determined by fitting in step A4. A2, step A3, and step A4 may be performed simultaneously instead of pressing the steps as described above. Then,
Spectral characteristics are measured during film formation, and at the same time, the film thickness of the layer to be film-formed is determined by fitting at any time, so if film formation is completed when the target film thickness obtained in step Al is reached. Good. Therefore, it is not necessary to obtain the film formation rate of the film formation target layer in advance, and a film thickness deviation from the target film thickness of the film formation target layer due to instability of the film formation target may be reduced.

[0050]

As described above, according to the present invention,
Since the reference thickness of each film is determined so that the film thickness can be measured with a desired accuracy, each film can be made to have an accurate thickness, and feedforward control can be accurately performed. It is possible to manufacture an optical element having the optical characteristics of

[Brief description of drawings]

FIG. 1 is a flowchart showing an example of an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a method for forming a multilayer film.

FIG. 3 is a diagram showing a principle of a method for measuring a film thickness based on a spectral transmittance.

FIG. 4 is a flowchart showing an example of an embodiment of feedforward control.

FIG. 5 is a diagram showing measured values of optical characteristics of a multilayer film obtained as a result of using the present invention.

[Explanation of symbols]

1 ... vacuum chamber 2 ... Board holder 3 ... Rotary axis 4 ... Optical element 5 ... Monitor board 6 ... Sputtering device 7 ... window 8 ... Floodlight 9 ... Receiver M ... Multilayer thin film M1 to Mn ... thin film

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-3-50501 (JP, A) JP-A-10-186103 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 1/11

Claims (3)

(57) [Claims] 1. A multi-layered film is formed on the surface of
The spectral characteristics of the multilayer film by a spectroscope,
Based on the result, the thin film being formed by the fitting calculation
Of each thin film of an optical element that has a step of measuring the film thickness of
It is a method of designing the elevation,
Determined from the accuracy of the spectroscope and the accuracy of the fitting calculation.
The accuracy of the thickness measurement of the thin film is determined by the light of the optical element.
Meets the required thickness accuracy of the thin film determined from the scientific properties
When there is no thin film, the target thickness of the thin film is
Determined from the accuracy of the spectroscope and the accuracy of the fitting calculation
The accuracy of the thickness measurement of the thin film is
Meet the required thickness accuracy of the thin film determined from the properties
Must have a process to change the thickness and perform design calculation
And a method for designing an optical element. 2. An optical system comprising a process of forming a film using the thickness of each film forming the multilayer film determined by the method of designing an optical element according to claim 1 as a reference value. Device manufacturing method. 3. The thickness of each film constituting the multilayer film determined by the method for designing an optical element according to claim 1 is used as a reference value to form a film, and the reference of the thickness of the formed film is used. To have a process for obtaining the deviation from the value and changing the reference value of the film thickness of at least one layer after that so as to minimize the influence on the intended optical characteristics. And a method for manufacturing an optical element.