Evaporation method for multi-cavity Fabry-Perot filters

Abstract

This invention relates to a vapor deposition method for multi-cavity Fabry-Perot filters, including a coupling layer deposition step. The optical thickness of the coupling layer is monitored using an optical monitoring method, with the monitoring light frequency controlled to be two or more integer multiples of the fundamental frequency. This vapor deposition method can accurately control the optical thickness of each film layer, including the coupling layer, thereby making the filter curve of the prepared filter closer to the theoretically designed curve. This method is particularly suitable for manufacturing filters with extremely steep rise and fall edges and deep cutoff depths, such as near-infrared filters.

Description

Technical Field

[0001] This invention relates to a vapor deposition method for multi-cavity Fabry-Perot filters. Background Technology

[0002] One commonly used structure in existing narrowband filters is the multi-cavity Fabry-Perot structure, which contains multiple FP cavities (i.e., Fabry-Perot cavities) connected by coupling layers. Filters with this structure are generally fabricated using a vapor deposition process, which suffers from high fabrication difficulty, high cost, and low yield.

[0003] One reason for the low yield is that existing processes use fundamental frequency light for optical monitoring (optical extreme value monitoring) on ​​all layers other than the coupling layer. The monitoring light passes through a monitoring plate in the optical coating, generating a real-time changing light intensity signal. The coating equipment processes and calculates this signal to predict the extreme value location and stops coating that layer at the corresponding extreme value. More importantly, optical monitoring allows for dynamic compensation of the film's optical thickness. When monitoring film thickness using optical monitoring, if a layer is prematurely judged to have reached its extreme value due to signal instability, resulting in under-coating, the optical monitoring method automatically compensates for the under-coated optical thickness of the previous layer when coating the next layer. Conversely, if the previous layer was over-coated, the next layer will automatically under-coat to compensate.

[0004] However, the intensity signal of the coupling layer does not change significantly under the monitoring of the fundamental frequency light, leading to inaccurate determination of the extreme value position. Therefore, the current process uses crystal oscillation method to monitor the film thickness of the coupling layer. However, the crystal oscillation method cannot dynamically compensate for the optical thickness of the film layer like the optical monitoring method, which further exacerbates the film thickness error problem of the filter.

[0005] The aforementioned errors are particularly critical in the manufacture of near-infrared filters. The theoretical filtering curve of a near-infrared filter has extremely steep rising and falling edges. These errors directly prevent the actual filtering curve from maintaining such steep rising and falling edges, thus further affecting the filtering effect of the filter. Summary of the Invention

[0006] The main objective of this application is to provide a vapor deposition method for the coupling layer in a multi-cavity Fabry-Perot filter that can accurately control the optical thickness of the coupling layer.

[0007] To achieve the above objectives, the present invention provides a vapor deposition method for a multi-cavity Fabry-Perot filter, including a coupling layer vapor deposition step, wherein an optical monitoring method is used to monitor the optical thickness of the coupling layer, and the frequency of the monitoring light is controlled to be two or more integer multiples of the fundamental frequency.

[0008] In some embodiments of the present invention, the evaporation of the coupling layer is stopped when the monitored coupling layer passes through two or more extreme values ​​of light intensity signals.

[0009] In some embodiments of the present invention, a step is included in which the optical thickness of the coupling layer is compensated during the deposition of the next film layer after the coupling layer has been deposited.

[0010] In some embodiments of the present invention, the optical monitoring method is also used to monitor the optical thickness of other films, which include one or more of the following: reflective film stacks, cavity layers, pre-coated layers, and anti-reflection layers.

[0011] In some embodiments of the present invention, the multi-cavity Fabry-Perot filter is used for filtering near-infrared light.

[0012] The vapor deposition method of the present invention can accurately control the optical thickness of each film layer, including the coupling layer, thereby making the filter curve of the prepared filter closer to the theoretical design curve. The above method is particularly suitable for manufacturing filters with extremely steep rising and falling edges and deep cutoff depths, such as near-infrared filters. Attached Figure Description

[0013] Figure 1 An existing coating apparatus is shown.

[0014] Figure 2 The optical monitoring curves of the evaporation process of the multi-cavity FP filter in Comparative Example 1 are shown.

[0015] Figure 3 The optical monitoring curves are for the evaporation process of the multi-cavity FP filter in Example 1.

[0016] Figure 4 The optical monitoring curves for the evaporation process of the multi-cavity FP filter in Comparative Example 2 are shown.

[0017] Figure 5 This is a schematic diagram comparing the theoretical and measured filter curves of the multi-cavity FP filter in Comparative Example 2.

[0018] Figure 6 This is the optical monitoring curve of the evaporation process of the multi-cavity FP filter in Example 2.

[0019] Figure 7 This is a schematic diagram comparing the theoretical and measured filtering curves of the multi-cavity FP filter in Example 2. Detailed Implementation

[0020] In this application, the multi-cavity Fabry-Perot filter is also referred to simply as a "multi-cavity FP filter".

[0021] The term "fundamental frequency monitoring light" in this application refers to light whose wavelength is the same as the center wavelength of the designed multi-cavity Fabry-Perot filter, and is hereinafter referred to as "fundamental frequency light".

[0022] The term "frequency-doubled monitoring light" in this application refers to light whose frequency is two or more integer multiples of the fundamental frequency light, and is hereinafter simply referred to as "frequency-doubled light". Accordingly, the frequency of N-fold harmonic light is N times that of the fundamental frequency light (N is an integer greater than 2), and its wavelength is 1 / N of the wavelength of the fundamental frequency light. For example, if the wavelength of the fundamental frequency light is 1550 nm, the wavelength of the second harmonic light is 1 / 2 of 1550 nm, that is, 775 nm.

[0023] When actually controlling the wavelength of the monitoring light, there may be some error compared to the wavelength of the fundamental frequency light and the frequency harmonic light. Light with a wavelength close to that of the fundamental frequency light and the frequency harmonic light can be selected as the monitoring light.

[0024] The vapor deposition apparatus used in the vapor deposition method of this application is known, but the monitoring method for the coupling layer is different from the existing crystal oscillation method. Figure 1 The diagram illustrates one possible vapor deposition apparatus. Inside a vacuum chamber 1, a deposition umbrella 2 is installed. The deposition umbrella is equipped with a clamp 3 for holding a glass substrate, an optical monitoring plate 4, and a crystal oscillator probe 6. An evaporation source 7 is located at the bottom of the vacuum chamber 1. The optical monitoring plate 4 passes through a monitoring optical path, which includes, in sequence, a monitoring light source 5, the optical monitoring plate 4, a reflector 9, a monochromatic instrument 10, and a signal amplifier processor 11. When performing vapor deposition using this apparatus, the abrasive material is evaporated through the evaporation source, and the evaporated abrasive forms a film layer on the glass substrate, the crystal oscillator probe, and the optical monitoring plate. The monitoring light source is adjusted to emit light of a specific wavelength to the optical monitoring plate 4 as monitoring light. After the monitoring light passes through the optical monitoring plate 4 along the monitoring light path, it is refracted by the reflector 9 and filtered by the monochromator 10. The signal amplification processor 11 processes the light to obtain the real-time optical signal intensity. Then, the extreme value of the optical signal intensity and the optical thickness of the film when the extreme value is reached are calculated in advance based on the obtained optical signal intensity and its change, which serves as the standard for the end of the deposition of a film layer, i.e., the optical monitoring method.

[0025] During the evaporation process of the coupling layer using the multi-cavity Fabry-Perot filter evaporation method of this application, an optical monitoring method is used to monitor the optical thickness of the coupling layer, and the frequency of the monitoring light is controlled to be two or more integer multiples of the fundamental frequency.

[0026] In the original vapor deposition process, fundamental frequency light is generally used to monitor the optical thickness of each film layer except the coupling layer, while the physical thickness of the coupling layer is monitored only by crystal oscillation. This is because, in the optical monitoring of fundamental frequency light, the light intensity signal changes relatively little at the coupling layer (see reference). Figure 2The transmittance curves at layers 22, 44, and 66 in the model show that calculating the extreme values ​​is difficult and prone to error. Therefore, the optical monitoring method using fundamental frequency light is not suitable for accurately stopping the deposition of the coupling layer. However, when the frequency of the monitoring light is adjusted to a frequency-doubled light, the change in light intensity signal at the coupling layer increases significantly (see reference). Figure 3 The transmittance curves at layers 26, 48, and 70 in the image are used to help determine extreme values. Furthermore, a new standard for judgment has emerged. Because frequency-doubled light is used as the monitoring light, the wavelength is shortened to half or less of the original wavelength. Previously, for every 1 / 4 wavelength of optical thickness, two 1 / 4 wavelengths of frequency-doubled light were required in the frequency-doubled light. This means that an extreme value of the light intensity signal will appear once during the coating process. This provides a new standard for judging the stopping of coupling layer deposition, thereby improving the accuracy of the coating.

[0027] By monitoring the optical thickness of the coupling layer using optical monitoring methods, dynamic compensation for the optical thickness of the coupling layer becomes possible. Dynamic compensation of the coupling layer is achieved by compensating for the optical thickness of the coupling layer during the deposition of the next film layer after the previous one has been deposited. Specifically, if the coupling layer is determined to have reached its extreme value earlier, the insufficient optical thickness can be compensated for in the next film layer; if the coupling layer is determined to have reached its extreme value later, the optical thickness of the next film layer is reduced for compensation. This further reduces the deposition error.

[0028] In addition, optical monitoring using monitoring light at two or more times the fundamental frequency can also be used to monitor the optical thickness of other layers in the filter. These other layers include one or more of the following: reflective film stacks, cavity layers, and pre-coated layers. When these coatings are detected using monitoring light at multiples of the fundamental frequency, the optical thickness of each quarter-wavelength of the monitoring light is reduced by half, thus halving the error in the optical thickness of these layers, making the filter as a whole closer to the theoretical design value.

[0029] The present invention will be further described below with reference to specific embodiments, in which:

[0030] The letter H represents a high-refractive-index film with an optical thickness of 1 / 4 of the wavelength of the monitoring light; the letter L represents a low-refractive-index film with an optical thickness of 1 / 4 of the wavelength of the monitoring light.

[0031] The number after the symbol ^ indicates the number of times the film layer was repeatedly deposited. For example, (HL)^5 means that H and L were deposited in sequence 5 times, i.e., HLHLHLHLHL.

[0032] The numbers before H and L indicate the number of 1 / 4 wavelengths of monitoring light that H and L have passed through. For example, 2H indicates that the optical thickness of the high refractive index film layer has passed through two 1 / 4 wavelengths of monitoring light.

[0033] In the embodiments, the film material used for evaporating high refractive index films is Ta2O5, and the film material used for evaporating low refractive index films is SiO2.

[0034] First, taking a multi-cavity FP filter with a center wavelength of 1550nm and a bandwidth of 5nm as an example, the optical thickness of the uncoupled layer was monitored using existing fundamental frequency light and the physical thickness of the coupling layer was monitored by crystal oscillation (Comparative Example 1); and the thickness of each film layer of the multi-cavity FP filter was monitored using second harmonic light (Example 1).

[0035] Comparative Example 1

[0036] The membrane structure is: Glass / (HL)^54H(LH)^5L(HL)^54H(LH)^5L(HL)^54H(LH)^5L0.3H1.3L / Air

[0037] The wavelength of the baseband monitoring light is 1550nm.

[0038] Its optical monitoring curve is as follows Figure 2 As shown in the figure, this film stack has a total of 68 layers. Layers 11, 33, and 55 are cavity layers, layers 22, 44, and 66 are coupling layers, and the last two layers, 67 and 68, are antireflection layers, which are irregular thickness layers. As can be seen from the figure, due to the very small changes in light intensity within the coupling layers, extreme values ​​cannot be easily determined. The physical thickness must be monitored using a crystal oscillation method, making dynamic compensation impossible.

[0039] Example 1

[0040] The membrane structure is: Glass / HLHL(2H2L)^58H(2L2H)^52L(2H2L)^58H(2L2H)^52L(2H2L)^58H(2L2H)^52L0.6H2.6L / Air

[0041] The wavelength of the second harmonic monitoring light is 787.3nm.

[0042] Its optical monitoring curve is as follows Figure 3 As shown in the figure, the first four HLHL layers are pre-coated layers designed to increase the amplitude of light intensity signal changes in the subsequent films. Films with gentle curves no longer appear in the figure; the transmittance changes of each film layer, including the coupling layer, are increased, making the determination of extreme values ​​more accurate.

[0043] Comparative Example 1, which uses fundamental frequency optical monitoring, and Example 1, which uses second harmonic optical monitoring, are compared. Figure 1 and Figure 2 It can be known that:

[0044] 1. The light intensity signal changes of each film layer, including the coupling layer, are very significant, and there is no film layer whose extreme value cannot be determined.

[0045] 2. Since the monitoring light near the second harmonic is selected, its wavelength value is close to half of the fundamental frequency, so the optical thickness of each 1 / 4 is also reduced by half, thus reducing the error value introduced when judging the extreme value by half.

[0046] 3. After using second-harmonic light monitoring, the light signal transition at the end of the film layer is relatively steep, unlike the relatively gentle transition during fundamental frequency control. This characteristic further reduces the judgment error.

[0047] Based on the above analysis, the error in the thickness of each layer of a multi-cavity FP filter controlled by the frequency doubling method is much smaller than that of the fundamental frequency method. This means that the performance of the multi-cavity FP filter controlled by the frequency doubling method is closer to the theoretical design value.

[0048] Based on the above conclusions, multi-cavity FP near-infrared filters were fabricated using fundamental frequency light and second-harmonic light as monitoring light during evaporation. The performance requirements for the multi-cavity FP near-infrared filter are: a cutoff depth of 40 dB at 1542 nm when near-infrared light is incident at 0 degrees; high transmittance at 1550 nm; and a transmittance >91%. Since the cutoff position of 1542 nm and the high transmittance position of 1550 nm are very close, a steeper curve edge is required for the transition from cutoff to high transmittance. The following describes the monitoring of the optical thickness of the uncoupled layer using existing fundamental frequency light and the monitoring of the physical thickness of the coupled layer using crystal oscillation (Comparative Example 2); and the monitoring of the thickness of each film layer of the multi-cavity FP filter using second-harmonic light (Example 2).

[0049] Comparative Example 2

[0050] The film structure is: Glass / H3LHL4H(LH)^2L(HL)^32H(LH)^3L(HL)^34H(LH)^3L(HL)^34H(LH)^3L(HL)^34H(LH)^3L(HL )^34H(LH)^3L(HL)^34H(LH)^3L(HL)^34H(LH)^3L(HL)^32H(LH)^3L(HL)^24H(LH)^22L0.336H1.29L / Air.

[0051] The wavelength of the baseband monitoring light is 1565nm.

[0052] The optical signal variation curve controlled by its membrane system is as follows: Figure 4 As shown. The membrane stack has a total of 134 layers, with the last two layers (133 and 134) being irregular antireflective layers. Figure 4There are 10 relatively flat curves, which appear at the coupling layers, specifically layers 10, 24, 38, 52, 66, 80, 94, 108, 122, and 132. The actual filter curves of the multi-cavity FP filter are compared with the theoretically designed filter curves as follows: Figure 5 As shown, at the center wavelength of 1565nm, the theoretical and measured transmittance are quite close, but the steepness deteriorates near 1550nm. The transmittance at 1542nm is below 0.01%, or 40dB, but the transmittance at 1550nm is only about 76%, far from the design requirement of over 91%. Repeating the above experiment yields the same conclusion, indicating that obtaining a very steep curve edge using fundamental frequency monitoring light is somewhat difficult.

[0053] Example 2

[0054] The membrane structure is as follows:

[0055] Glass /

[0056] HLH3L2H6L2H2L4H2L2H2L2H2L2H2L6H2L2H2L

[0057] 4H2L2H2L2H2L2H2L2H2L2H2L2H2L8H2L2H2L2H2L

[0058] 2H2L2H2L2H2L2H2L8H2L2H2L2H2L2H2L2H2L2H6L

[0059] 6H2L8H2L2H2L2H2L2H2L2H2L2H2L2H2L8H2L2H2L

[0060] 2H2L2H2L2H2L2H2L2H2L8H2L2H2L2H6L2H2L2H6L

[0061] 2H2L2H2L8H2L2H2L2H2L2H2L2H2L2H2L2H2L4H2L

[0062] 2H2L2H2L2H2L2H2L2H2L4H2L6H10L2H2L

[0063] 0.6H2.6H / Air.

[0064] The wavelength of the second harmonic monitoring light is 790nm.

[0065] The optical signal variation curve controlled by its membrane system is as follows: Figure 6As shown in the figure, the film stack consists of 138 layers, with the first four layers being pre-coated layers. Layers with gentle curves no longer appear in the figure; the transmittance variation of each layer, including the coupling layer, is larger, making the determination of extreme values ​​more accurate. A comparison between the actual filter curve of the multi-cavity FP filter and the theoretically designed filter curve is shown below. Figure 7 As shown, at the center wavelength of 1565nm, the theoretical and measured transmittance are quite close. The transmittance at 1542nm is less than 0.01%, but the transmittance at 1550nm is 91.9%. Compared with the fundamental frequency light monitoring, its transmittance has increased significantly. The edge jitter in the measured curve is significantly closer to the theoretical curve.

[0066] The embodiments described in this invention are for illustrative purposes only and do not constitute a limitation on the scope of the claims. Other substantially equivalent substitutions that can be conceived by those skilled in the art are all within the scope of protection of this invention.

Claims

1. A method of evaporation of a multi-cavity Fabry-Perot filter, comprising an evaporation step of a coupling layer, characterized in that The optical monitoring method is used to monitor the optical thickness of the coupling layer, and the frequency of the monitoring light is controlled to be two times or more integral times of the base frequency. The monitoring of the coupling layer is stopped when the coupling layer passes two or more light intensity signal extreme values. The method further comprises the step of compensating the optical thickness of the coupling layer after the deposition of the coupling layer, and in the deposition of the next film layer. The optical monitoring method is also used to monitor the optical thickness of other film layers, which include one or more film layers of a reflective film stack, a cavity layer, a pre-deposition layer, and an anti-reflection layer. The multi-cavity Fabry-Perot filter is used for filtering near-infrared light.

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