A monitoring method for films of arbitrary thickness

Abstract

The present invention discloses a method for monitoring films of arbitrary thickness. Thin-film design software is used to simulate and calculate the reflectance spectrum curves of each film layer at various wavelengths within the monitoring band as the thickness increases. From each spectrum curve, the reflectance extreme value for each film layer when it reaches the correct thickness is determined. A suitable extreme wavelength is then selected for each film layer as its thickness monitoring wavelength. Finally, the thickness monitoring wavelength for each film layer can be used to monitor multilayer films of arbitrary thickness using the traditional reflectance extreme value method. The monitoring band is preferably 400 to 900 nm; the extreme value can be either a maximum or a minimum; A single monitoring film is required for thickness monitoring of films of arbitrary thickness; and both reflectance extreme value monitoring and transmission extreme value monitoring can be used. This method has significant practical value for thickness monitoring of films of arbitrary thickness and for device manufacturing.

Description

Technical Field

[0001] The invention relates to a monitoring method for a film system of arbitrary thickness, belonging to thin film manufacturing technology. Background Art

[0002] In the preparation of optical thin films, monitoring the film's refractive index (n) and optical thickness (nd) are the two most important steps.

[0003] The working principle of optical films is based on the interference phenomenon of light in the film, such as Figure 1 As shown, the interference intensity varies with the optical thickness of films with different refractive indices. Interference is strongest when the optical thickness is λ0 / 4 or an odd multiple of λ0 / 4; it is zero when the optical thickness is zero or an even multiple of λ0 / 4. More specifically, when the optical thickness is an odd multiple of λ0 / 4, if the film refractive index is greater than that of the substrate (e.g., 1.52), the reflectivity reaches a maximum, and vice versa. When the optical thickness is zero or an even multiple of λ0 / 4, if the film refractive index is greater than that of the substrate, the reflectivity reaches a minimum, and vice versa. The method of monitoring film thickness using the maximum or minimum reflection (transmission) values obtained through interference during thin film deposition is called the extreme value method. This extreme value method was proposed by Polster as early as 1952, and to this day, nearly all multilayer coating machines are equipped with this extreme value monitoring device, making it the most convenient and widely used film thickness monitoring system.

[0004] The monitoring principle of the extreme value method can be simply illustrated as follows: if a monochromator is used to select a monitoring wavelength of λ0 = 600nm, then for a high-refractive-index film with n = 2, when the first reflection maximum appears, its optical thickness nd = λ0 / 4 = 150nm, and its geometric thickness d = 150nm / 2 = 75nm; when the second reflection minimum appears, nd = λ0 / 2 = 300nm, d = 150nm; when the third reflection maximum appears, nd = 3λ0 / 4 = 450nm, d = 225nm; and so on. Similarly, the corresponding film thickness can be obtained for low-refractive-index films. This means that by selecting the monitoring wavelength and the number of extreme values, single-layer films of various thicknesses can be arbitrarily monitored, even regular multilayer films composed of λ0 / 4 and λ0 / 4 layers. Because the optical thickness of regular multilayer films is λ0 / 4 or λ0 / 4 times the film layer, a single monitoring wavelength can be simply selected to deposit the entire multilayer film on the same monitoring film. This gives the extreme value method a unique advantage: the optical thickness error that inevitably occurs when monitoring each thin film layer can be automatically compensated. That is, the deficiency (or over-positive) of the previous film layer can be automatically compensated by the over-positive (or under-positive) of the next film layer. This compensation mechanism makes the final cumulative optical thickness error of the entire film system almost equal to zero, enabling the extreme value method to achieve excellent optical properties. In particular, in the preparation of ultra-narrowband filters for wavelength division multiplexing, extremely high wavelength positioning accuracy can be achieved, which is unattainable by any other monitoring method.

[0005] However, with the in-depth research and widespread application of automated thin film optimization design, it is no exaggeration to say that virtually any complex thin film system can be designed. Unfortunately, automated design results in the refractive index of all layers in the final optimized film system often being arbitrary, and the film thickness being non-regular, arbitrary, that is, not λ0 / 4 or λ0 / 4 times the film layer. Due to the limited film materials and the monitoring technology for non-regular arbitrary thickness, many multilayer films with excellent design performance cannot be manufactured. Currently, non-regular arbitrary thickness film systems are achieved by replacing monitoring chips. Even in high-end coating machines, a single monitoring chip is generally required to monitor the thickness of each film layer. This not only prolongs the preparation time of each film layer due to the replacement of the monitoring chip, but also greatly increases the wear of the monitoring chip. More importantly, the extremely critical automatic film thickness compensation mechanism is lost. Summary of the Invention

[0006] The present invention proposes a monitoring method for a film system of arbitrary thickness. For a film system of arbitrary thickness that cannot be directly monitored by the extreme value method, a new extreme wavelength is found through thin film software simulation calculation, and a suitable extreme monitoring wavelength is selected. Then, the extreme monitoring wavelength of each layer of the film is changed to complete the thickness monitoring of the entire film system of arbitrary thickness, thereby realizing the use of the extreme value method to monitor irregular film systems of arbitrary thickness. Although this method requires changing the monitoring wavelength, the entire irregular multi-layer film can be plated on a monitoring piece, thus overcoming the defects brought about by using a monitoring piece to monitor the thickness of a layer of film. More importantly, this method can still retain the automatic thickness compensation mechanism of the extreme value method.

[0007] The purpose of the present invention is to propose a monitoring method for film systems of arbitrary thickness. By using simulation calculations of thin film design software, the extreme wavelength and extreme monitoring wavelength of each layer of film can be re-found, so that the traditional extreme value method can be used to monitor film systems of arbitrary thickness. This has important practical significance for the thickness monitoring of irregular film systems and their actual preparation.

[0008] To achieve the above object, the present invention is conceived as follows:

[0009] The method of replacing a monitoring plate with each film layer has successfully achieved thickness monitoring of arbitrary thickness films. Although this method is not very advanced, it is still an important progress because it has achieved the monitoring and manufacturing of arbitrary thickness films. However, it also paid a considerable price: (1) because the monitoring plate needs to be replaced for each film layer, the preparation time is extended; (2) the loss of monitoring plates is greatly increased. If the number of layers of an arbitrary thickness film system is 50, 50 monitoring plates will be consumed, which greatly increases the cost; (3) the original extreme value method of film thickness monitoring system must undergo hardware modification, and a complex monitoring plate storage and conversion system needs to be added; (4) the film thickness automatic compensation mechanism of extreme value monitoring is lost; (5) the monitoring of each layer of the arbitrary thickness film system is based on the pre-calculated theoretical reflectivity fixed value as the criterion for thickness monitoring, so it is called the fixed value method. However, the fixed value of the fixed value method depends on the refractive index of the film layer. If the refractive index of the film layer actually prepared deviates from the design value, it will inevitably introduce thickness monitoring errors; (6) Finally, new film nucleation errors will be introduced. The so-called thin film nucleation error is as follows: a new monitoring film is replaced for each layer of film, but the film forming conditions of the new monitoring film without film and the optical device that has been coated are different. The coating on the new monitoring film without film needs to go through a process of nucleation-nucleus expansion-becoming a continuous film before the film thickness begins to increase, while the film thickness of the coated device is directly increased from the beginning. The difference in film thickness between the two can be controlled below 5% for oxide hard film, but it can reach more than 20% for soft film with a small cohesion coefficient such as ZnS. This nucleation error causes the film thickness on the formally coated optical device to be too positive.

[0010] Based on an in-depth analysis of the above-mentioned monitoring methods, the present invention proposes a new method for monitoring arbitrary-thickness films. The thin-film software TFC is used to simulate the actual plating process of each layer of the arbitrary-thickness film system to be monitored. This involves dividing each thinner layer into Q = 10 sublayers or each thicker layer into Q = 20 sublayers (Q can be larger depending on the accuracy requirements). The reflectivity spectrum curves are then calculated as Q increases, and the new extreme wavelength for each layer of film when it reaches the correct thickness of Q = 10 or Q = 20 is found. To ensure the new extreme wavelength is correct, Q generally needs to be calculated to 11 or 21. That is, after Q is calculated to the correct thickness of 10 or 20 for each layer of film, 1 to 2 additional reflectivity spectrum curves must be calculated. After finding the new extreme wavelength for each layer of film, the appropriate thickness monitoring wavelength can be selected. There are usually three situations for monitoring wavelengths: one is to obtain a unique reflection extreme wavelength within the monitoring band as the thickness monitoring wavelength of the film layer of any thickness; the second is that the film layer can obtain multiple reflection extreme wavelengths at the same time within the monitoring band, and the most appropriate reflection extreme wavelength is selected as the thickness monitoring wavelength; the third is that no reflection extreme value appears within the monitoring band. At this time, it is necessary to find a wavelength that is more sensitive to reflectivity changes as the thickness of the film layer increases based on the reflection characteristics simulated by computer as the thickness monitoring wavelength, and directly perform thickness monitoring based on the theoretical reflectivity constant calculated at the monitoring wavelength.

[0011] Once the monitoring wavelengths for each layer of a film system of arbitrary thickness are determined, actual fabrication can begin. During fabrication, by sequentially changing the predetermined thickness monitoring wavelengths for each layer, the thickness of the entire film system of arbitrary thickness can be monitored using a single monitoring plate using the extreme value method (supplemented by the fixed value method for individual layers or individual layers). Although this method requires changing the monitoring wavelength, since the entire film system of arbitrary thickness can be deposited on a single monitoring plate, it overcomes the shortcomings of the existing technology of using a single monitoring plate to monitor the thickness of a single layer. These include shortening the fabrication time by eliminating the need to replace the monitoring plate for each layer; only one monitoring plate is consumed to complete the fabrication of a film system of arbitrary thickness, reducing manufacturing costs; and more importantly, eliminating the need for complex hardware modifications to the existing extreme value method film thickness monitoring system. Using a single monitoring plate also retains the automatic thickness error compensation mechanism of the original extreme value method. As with the extreme value method, deviations in the film's refractive index will not introduce optical thickness monitoring errors, nor will they introduce new thin film nucleation errors. The above-mentioned method of re-finding the extreme wavelength and monitoring wavelength of each layer of the film system of arbitrary thickness with the help of computer simulation can realize the thickness monitoring of the film system of arbitrary thickness. This method of monitoring the entire film system of arbitrary thickness only by finding new extreme points and changing the monitoring wavelength of each layer of the film while retaining the main advantages of the extreme value method has important practical significance for the thickness monitoring of irregular film systems and their actual preparation.

[0012] Specifically, the technical solution adopted by the present invention is:

[0013] A method for monitoring films of arbitrary thickness. For a newly designed film system of arbitrary thickness, since the optical thickness of each layer is a non-regular multilayer film that is not λ0 / 4 or a multiple of λ0 / 4, the thickness cannot be monitored using the traditional extreme value method. Therefore, before the first actual preparation, it is necessary to first use the commercial thin film design software TFC to simulate the actual evaporation process, calculate the reflectivity spectrum curve within the monitoring band for each layer as the thickness increases, and find the reflectivity extreme value when each layer reaches the correct thickness. Then, a suitable extreme value wavelength is selected for each layer as its thickness monitoring wavelength. Finally, the thickness monitoring wavelength of each layer can be used to monitor the preparation of multilayer films of arbitrary thickness with the help of the traditional reflection extreme value method. The extreme value can be a maximum value or a minimum value. Irregular thickness monitoring of film systems of arbitrary thickness is just like the traditional extreme value method, requiring only one monitoring film.

[0014] Thickness monitoring wavelengths can be specifically divided into the following three situations: the first situation is that within the monitoring band, the film layer can obtain a unique reflection extreme wavelength as the thickness monitoring wavelength of the film layer of any thickness; the second situation is the most common. If the film layer is thick enough or the thickness of the film deposited on the monitoring piece is large enough, then the film layer can simultaneously obtain multiple reflection extreme wavelengths within the monitoring band. According to the characteristics of the extreme value and the extreme wavelength, a most suitable reflection extreme wavelength is selected as the thickness monitoring wavelength; the third situation is rare. If the film layer is thin enough or the thickness of the film deposited on the monitoring piece is small enough, no reflection extreme will appear within the monitoring band. This situation often occurs in the first few layers of the film system. At this time, it is necessary to find a wavelength that is more sensitive to reflectivity changes as the thickness of the film layer increases based on the changes in reflection characteristics simulated by computer simulation as the thickness monitoring wavelength, and directly perform thickness monitoring based on the theoretical reflectivity calculated at the monitoring wavelength, that is, fixed value monitoring.

[0015] Furthermore, a monitoring method for a film system of arbitrary thickness is characterized by comprising the following steps:

[0016] 1) Use thin film design software to design the film system. If the designed film system is a non-regular film system, use the thin film design software to simulate and calculate the reflectivity spectrum curve of each layer in the non-regular film system at different wavelengths within the monitoring band at different thicknesses, and obtain the reflectivity spectrum curve of each layer of the film;

[0017] 2) Find the reflectivity extreme value of each film when it reaches the correct thickness from each reflectivity spectrum curve of each film layer, then select a suitable extreme value from the reflectivity extreme value as the film thickness monitoring wavelength, and monitor the manufacturing of the designed film system based on the film thickness monitoring wavelength;

[0018] When the reflectivity extreme value of each film layer when it reaches the correct thickness cannot be found in each reflectivity spectrum curve of each film layer, the manufacturing of the film layer designed by fixed value monitoring and control is adopted.

[0019] Furthermore, the designed film system is a non-regular film system, that is, a film system that is not a quarter multiple. The designed film system is a multi-layer anti-reflection film.

[0020] Furthermore, the monitoring band range is determined according to the spectral sensitivity of the film thickness monitoring system. The monitoring band range of the present invention is preferably 400 to 900 nm.

[0021] Furthermore, any film of any thickness is a transparent dielectric film within the monitoring band, excluding strong absorption films.

[0022] Furthermore, the reflectivity spectrum curve has the monitoring band range as the horizontal coordinate and the reflectivity as the vertical coordinate.

[0023] Furthermore, the different thicknesses mentioned above refer to dividing each designed film into Q equal parts according to thickness, and calculating the reflectivity spectrum curves at various wavelengths within the monitoring band at thicknesses from 1 equal part to Q+1 equal parts. Q ranges from 10 to 30, with 10 or 20 being generally selected.

[0024] Furthermore, in step 2), the reflectivity extreme value of each film layer when the film layer reaches the correct thickness is found from each reflectivity spectrum curve of each film layer, specifically including:

[0025] Compare the reflectivity spectrum curve at Q equal thickness of each film with the reflectivity spectrum curves of each film except the reflectivity spectrum curve at Q equal thickness to find the extreme reflectivity value.

[0026] Furthermore, in step 2), a suitable extreme value is selected from the reflectivity extreme values as the thickness monitoring wavelength of the film layer, specifically including:

[0027] The first type: if the reflectivity extreme value is one, it is directly used as the thickness monitoring wavelength of the film layer;

[0028] The second method: There are multiple reflectivity extreme values. According to the characteristics of the extreme values and extreme value wavelengths, a most suitable reflectivity extreme value wavelength is selected as the thickness monitoring wavelength.

[0029] Furthermore, this method for monitoring films of arbitrary thickness can be applied to reflective extremes, and can also be easily extended to transmittance extremes based on the law of energy conservation. This method for monitoring films of arbitrary thickness is particularly suitable for monitoring multilayer anti-reflection films of arbitrary thickness.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] For arbitrary-thickness film systems that cannot be monitored using the traditional extreme value method, the commercial thin-film design software TFC can be used to simulate and calculate the reflectivity spectrum curves of each layer at various wavelengths as the thickness increases. The reflectivity extreme value at the correct thickness for each layer can then be identified and the optimal extreme value wavelength selected as the thickness monitoring wavelength. This thickness monitoring wavelength can then be used to monitor multilayer films of arbitrary thickness using the traditional reflectivity extreme value method. This makes non-regular thickness monitoring of arbitrary-thickness film systems similar to the traditional extreme value method, requiring only one monitoring film. This allows thickness monitoring of the entire arbitrary-thickness film system while retaining the key advantages of the extreme value method. The monitoring extreme value can be either a maximum or a minimum, and can be either a reflectance extreme value or a transmittance extreme value. This arbitrary-thickness monitoring method is not only simple and easy to implement, but also reduces costs. It is undoubtedly an ingenious extreme value method developed through computer simulation that is capable of monitoring films of arbitrary thickness.

[0032] Table 1 is provided below for comparison. It is clear from Table 1 that the arbitrary thickness film system monitoring method of the present invention is superior to the arbitrary thickness film system monitoring method of the prior art. It is developed based on the conventional extreme value method and the arbitrary thickness film system monitoring method of the prior art, and has the advantages and characteristics of leveraging their strengths to overcome their weaknesses.

[0033] Table 1

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a calculation curve of reflectivity and film thickness of various refractive index films.

[0036] Figure 2 Is a 15-layer Glass|(HL) 3 HLLH(LH) 3 |Air interference filter thickness monitoring calculation example, H and L represent the high and low refractive index films with λ0 / 4 optical thickness: TiO2 (n=2.3) and MgF2 (n=1.38), respectively, and the spacer layer is LL.

[0037] Figure 3 yes Figure 2 A simplified diagram illustrating the automatic compensation process for the thickness error of the first TiO2 film in the film system shown.

[0038] Figure 4 It is a four-layer narrow visible light range and 850nm dual-band anti-reflection film of arbitrary thickness according to an embodiment of the present invention.

[0039] Figure 5 This invention Figure 4The simulated reflectivity spectrum curve of the first layer of four-layer dual-band anti-reflection coating with arbitrary film thickness.

[0040] Figure 6 This invention Figure 4 The simulated reflectivity spectrum curve of the second layer of the four-layer dual-band anti-reflection film with arbitrary film thickness.

[0041] Figure 7 This invention Figure 4 The simulated reflectivity spectrum curve of the third layer of the four-layer dual-band anti-reflection film with arbitrary film thickness.

[0042] Figure 8 This invention Figure 4 The simulated reflectivity spectrum curve of the fourth layer of the four-layer dual-band anti-reflection film with arbitrary film thickness. DETAILED DESCRIPTION

[0043] The extreme value monitoring of regular film systems with λ0 / 4 or λ0 / 4 times the film thickness is already very mature. Its advantage is that one can simply select one monitoring wavelength to coat the entire multilayer film system on the same monitoring piece, which not only minimizes the preparation cost but also simplifies the process. Figure 2 Is a 15-layer Glass|(HL) 3 HLLH(LH) 3 |Air interference filter thickness monitoring calculation example, H and L represent the high and low refractive index films with λ0 / 4 optical thickness respectively: TiO2(n H =2.3) and MgF2(n L =1.38), the spacer layer is LL, and the monitoring wavelength is λ0=600nm. Figure 2 Obviously, the end points of each film thickness are extreme points. This extreme value method using a monitoring film has an automatic thickness error compensation mechanism. That is, the deficiency (or over-accuracy) of the previous film layer can be automatically compensated by the over-accuracy (or under-accuracy) of the next film layer, thus ensuring that the center wavelength of the filter is accurately positioned at 600nm. Figure 3 yes Figure 2 A brief description of the automatic compensation process for the thickness error of the first TiO2 film in the film system shown: Figure 3 In a), the first TiO2 film terminates exactly at the reflection maximum point, and there is no thickness error compensation when the second MgF2 film is deposited; Figure 3 In b), because the first layer of TiO2 film is less than λ0 / 4 (i.e., the geometric thickness is less than 10nm), it is terminated prematurely before reaching the maximum point. At this time, when the second layer of MgF2 film is plated, the insufficient part of the TiO2 film will be compensated to the maximum point, and then its own λ0 / 4MgF2 will be plated to the minimum. This is equivalent to the first layer of film being composed of insufficient thick TiO2 and compensating thin MgF2 to form λ0 / 4; and in Figure 3 In c), the first TiO2 film produces an overly positive λ0 / 4 (i.e., the geometric thickness is 10nm overly positive), meaning it stops after the maximum value. At this point, when the second MgF2 film is deposited, its own deficiency compensates for the overly positive portion of the TiO2 film, directly continuing the remaining MgF2 to the minimum value. This is equivalent to the second film being composed of the overly positive thin TiO2 and MgF2 together to form λ0 / 4. This is how the thickness error of the first TiO2 film is compensated by the second MgF2 film. Similarly, the error compensation process for the second, third, and so on layers is similar. Note: Figure 2 and Figure 3 The horizontal axis is the physical thickness.

[0044] The extreme value method has an obvious disadvantage: the reflectivity change ΔR caused by the film thickness increment Δnd near the extreme point is close to zero, or in other words, the derivative of the reflectivity with respect to the optical thickness at the extreme point is equal to zero, which will lead to an increase in the thickness monitoring error. This is an inherent defect of the extreme value method monitoring principle. It is precisely because the extreme value method's automatic thickness error compensation mechanism basically eliminates this defect of the extreme value method itself that it shows strong vitality.

[0045] Unfortunately, the automatic compensation of thickness error by the extreme value method when monitoring regular film systems will turn into an obstacle when monitoring irregular arbitrary thickness film systems. A simple example can be used to understand this: Assume that the optical thickness of the first TiO2 film is n H d H =100nm, and the optical thickness of the second MgF2 film is n L d L =120nm, which is the simplest irregular arbitrary thickness double-layer film system. To monitor the thickness of the first TiO2 film, the monitoring wavelength λ0 = 400nm can be selected. The first reflection maximum is reached at λ0 / 4 = 400 / 4 = 100nm, making the first layer monitoring very convenient. However, the problem is that when the second MgF2 layer is subsequently deposited, only 100nm can be monitored. Due to the lack of extreme value criteria, 120nm cannot be obtained, resulting in a 20nm shortfall in the thickness of the second MgF2 film. So someone adjusted the monitoring wavelength λ'0 for the second MgF2 film to 120nm x 4 = 480nm. However, once the monitoring wavelength λ'0 reaches 480nm, the thickness of the first TiO2 film is less than one-quarter of λ'0. Therefore, when depositing the second MgF2 layer, it must first automatically compensate for the thickness of the first TiO2 film to 120nm, and then deposit its own λ'0 / 4MgF2 to 120nm. This, in turn, causes the thickness of the first TiO2 film to be 20nm too thick. This is why existing technology requires replacing a monitoring chip with each film when monitoring a film system of arbitrary thickness. The purpose is precisely to interrupt the automatic compensation mechanism between film layers.

[0046] The method of replacing a monitoring plate with each film layer has successfully achieved thickness monitoring of arbitrary thickness films. Although this method is not very advanced, it is still an important progress because it has achieved the monitoring and manufacturing of arbitrary thickness films. However, it also paid a considerable price: (1) because the monitoring plate needs to be replaced for each film layer, the preparation time is extended; (2) the loss of monitoring plates is greatly increased. If the number of layers of an arbitrary thickness film system is 50, 50 monitoring plates will be consumed, which greatly increases the cost; (3) the original extreme value method of film thickness monitoring system must undergo hardware modification, and a complex monitoring plate storage and conversion system needs to be added; (4) the film thickness automatic compensation mechanism of extreme value monitoring is lost; (5) the monitoring of each layer of the arbitrary thickness film system is based on the pre-calculated theoretical reflectivity fixed value as the criterion for thickness monitoring, so it is called the fixed value method. However, the fixed value of the fixed value method depends on the refractive index of the film layer. If the refractive index of the film layer actually prepared deviates from the design value, it will inevitably introduce thickness monitoring errors; (6) Finally, new film nucleation errors will be introduced. The so-called thin film nucleation error is as follows: a new monitoring film is replaced for each layer of film, but the film forming conditions of the new monitoring film without film and the optical device that has been coated are different. The coating on the new monitoring film without film needs to go through a process of nucleation-nucleus expansion-becoming a continuous film before the film thickness begins to increase, while the film thickness of the coated device is directly increased from the beginning. The difference in film thickness between the two can be controlled below 5% for oxide hard film, but it can reach more than 20% for soft film with a small cohesion coefficient such as ZnS. This nucleation error causes the film thickness on the formally coated optical device to be too positive.

[0047] Based on an in-depth analysis of the above-mentioned monitoring methods, the present invention proposes a new method for monitoring arbitrary-thickness films. The thin-film software TFC is used to simulate the actual plating process of each layer of the arbitrary-thickness film system to be monitored. This involves dividing each thinner layer into Q = 10 sublayers or each thicker layer into Q = 20 sublayers (Q can be larger depending on the accuracy requirements). The reflectivity spectrum curves are then calculated as Q increases, and the new extreme wavelength for each layer of film when it reaches the correct thickness of Q = 10 or Q = 20 is found. To ensure the new extreme wavelength is correct, Q generally needs to be calculated to 11 or 21. That is, after Q is calculated to the correct thickness of 10 or 20 for each layer of film, 1 to 2 additional reflectivity spectrum curves must be calculated. After finding the new extreme wavelength for each layer of film, the appropriate thickness monitoring wavelength can be selected. There are usually three situations for monitoring wavelengths: one is to obtain a unique reflection extreme wavelength within the monitoring band as the thickness monitoring wavelength of the film layer of any thickness; the second is that the film layer can obtain multiple reflection extreme wavelengths at the same time within the monitoring band, and the most appropriate reflection extreme wavelength is selected as the thickness monitoring wavelength; the third is that no reflection extreme value appears within the monitoring band. At this time, it is necessary to find a wavelength that is more sensitive to reflectivity changes as the thickness of the film layer increases based on the reflection characteristics simulated by computer as the thickness monitoring wavelength, and directly perform thickness monitoring based on the theoretical reflectivity constant calculated at the monitoring wavelength.

[0048] Once the monitoring wavelengths for each layer of a film system of arbitrary thickness are determined, actual manufacturing can begin. During manufacturing, by sequentially changing the predetermined thickness monitoring wavelengths for each layer, the thickness of the entire film system of arbitrary thickness can be monitored using a single monitoring plate using the extreme value method (supplemented by the fixed value method for individual layers or individual layers). Although this method requires changing the monitoring wavelength, since the entire film system of arbitrary thickness can be plated on a single monitoring plate, it overcomes the shortcomings of the existing technology of using a single monitoring plate to monitor the thickness of a single layer of film. These include shortening the preparation time by not having to replace the monitoring plate for each layer; only one monitoring plate is consumed to complete the preparation of a film system of arbitrary thickness, reducing manufacturing costs; and more importantly, eliminating the need for complex hardware modifications to the original extreme value method film thickness monitoring system; using a single monitoring plate for monitoring also retains the original extreme value method's automatic thickness error compensation mechanism; and, like the extreme value method, film layer refractive index deviations will not introduce optical thickness monitoring errors; nor will new film nucleation errors be introduced. The above-mentioned method of re-finding the extreme wavelength and monitoring wavelength of each layer of the film system of arbitrary thickness with the help of computer simulation can realize the thickness monitoring of the film system of arbitrary thickness. This method of monitoring the entire film system of arbitrary thickness only by finding new extreme points and changing the monitoring wavelength of each layer of the film while retaining the main advantages of the extreme value method has important practical significance for the thickness monitoring of irregular film systems and their actual preparation.

[0049] As a specific embodiment, the present invention selects a dual-band anti-reflection film with a relatively small number of layers but a strong representativeness for use in security camera systems. For a typical daytime and nighttime surveillance camera system, the camera lens needs to transmit a narrow visible light band of 450-630nm and near-infrared light of 850nm or 940nm. This active near-infrared camera technology utilizes 850nm or 940nm near-infrared LEDs for supplemental illumination, producing near-infrared images that are invisible to the human eye but can be captured by the camera. Although a faint, dark red light can be detected at close range when used at night, a phenomenon known as "red burst," 850nm LEDs have a higher emission power, not only extending their operating range compared to 940nm LEDs, but also possessing approximately 10 times greater light sensitivity. Therefore, 940nm LEDs, which lack "red burst," are only used in applications requiring the utmost in concealment. This has made 850nm LEDs a mainstream choice in today's security market.

[0050] Figure 4 The spectral reflectance curves of four layers of arbitrary thickness of the 450-630nm narrow visible light region and 850nm dual-band anti-reflection film as an embodiment of the present invention are summarized in Table 2 below as well as the film system structure, the film layer extreme wavelength and the thickness monitoring wavelength.

[0051] Table 2

[0052]

[0053] As can be seen from Table 2, the optical thickness of each film layer in the example film system is irregular and of arbitrary thickness. Although the film system has only four layers, its optical thickness varies greatly, the extreme wavelength distribution is wide, the monitoring wavelength has extreme values and constant values, and it includes first-order and high-order interference monitoring, which is very representative.

[0054] Since the optical thickness of each layer of the example film system is an arbitrary non-λ0 / 4 film, the traditional extreme value method cannot be used to monitor the thickness at the monitoring wavelength λ0. Therefore, before the first actual preparation, it is necessary to first use the commercial thin film design software TFC to simulate the actual evaporation process and calculate the reflectivity spectrum curve within the monitoring band of each layer as the thickness increases. Figure 5 、 Figure 6 、 Figure 7 and Figure 8The following are the reflectivity spectral curves for the first, second, third, and fourth layers of the film system simulated for this example. For thinner films, the optical thickness can be divided equally into Q = 10 sublayers. A gradual increase in Q from 0 to 10 indicates that the film thickness during actual evaporation increases from 0 to the correct thickness at Q = 10. Q can be divided into more sublayers depending on the required accuracy, typically set to Q = 20. For films with higher accuracy requirements and greater thickness, Q can be even higher. However, a higher Q value results in too many and too densely calculated reflectivity spectral curves, which can complicate finding the extreme wavelength. For this reason, Q is often set to 10 or 20, and reflectivity spectral curves typically only display a few curves near the correct thickness; identifying the extreme wavelength is sufficient. To ensure accurate determination of the extreme wavelength and avoid misjudgment, one or two additional reflectivity spectral curves are calculated after calculating the correct thickness of each film layer, 10 or 20. Here, if only one additional curve is calculated, the result is Q = 11 or 21.

[0055] Then, the extreme wavelength of each layer of film can be found from the simulated reflectance spectrum curve, and the monitoring wavelength can be determined based on the extreme wavelength. Figure 5 This is the reflectivity spectrum curve calculated by simulation for the first layer of MgF2 film in this embodiment. Figure 5 It can be seen that the first layer of MgF2 film has two extreme wavelengths: 422.6nm (maximum) and 845.2nm (minimum). Because this is the first layer of film on the monitoring film, it is very simple to determine the extreme value. An experienced person can know without simulation calculations that as long as the optical thickness of the first layer of MgF2 film is 211.3nm×4=845.2nm, it is a first-order extreme value ( Figure 5 The correct thickness of 211.3 nm is obtained when the reflection (R) changes from a maximum value to a minimum value (shown by the thin solid line in the middle). Figure 5 In the three reflectivity curves of Q=9, 10 and 11 calculated by simulation, Q=10 is indeed the minimum value at the wavelength of 845.2nm. Or 211.3nm×2=422.6nm is the secondary minimum value ( Figure 5 The correct thickness of 211.3 nm is obtained when the reflection (R) changes from maximum to minimum and then to maximum. Figure 5 Among the three reflectivity curves of Q=9, 10 and 11, Q=10 is the maximum value at a wavelength of 422.6nm. Figure 5 If the full reflectivity spectrum curve is plotted for Q values ranging from 0 to 11, the evolution of the secondary sub-maxima becomes clear. Compared to the two extreme wavelengths of the first MgF2 film, the secondary sub-maxima wavelength of 422.6 nm is superior to the primary sub-maxima wavelength of 845.2 nm. This is because the secondary sub-maxima reflectivity change is greater for the same thickness change and is easier to interpret. Therefore, 422.6 nm is the preferred monitoring wavelength for the first MgF2 film.

[0056] Figure 6 This is the reflectivity spectrum curve calculated by simulation for the second layer of Al2O3 film in this embodiment. Figure 6 It can be seen that the second Al2O3 film is relatively thin, so the extreme value has not yet appeared completely. Although there seems to be an extreme value at a wavelength of 420nm, the two reflection curves of Q=11 and Q=10 are basically glued together, so there is actually no extreme value. In this case, only fixed value monitoring can be used. If the wavelength is 650nm, the reflection fixed value will increase from 1.9% (see Figure 5 ) rose to 9.9% (see Figure 6 ) evaporation is terminated. This fixed-value monitoring method has a drawback: if the actual refractive index of the deposited Al2O3 film deviates from the theoretical design value of 1.62, the fixed value of 9.9% will change, introducing a certain thickness error. However, extreme value monitoring does not introduce this error. This method is based not on the fixed reflectivity but on the extreme point, namely nd = λ0 / 4 or λ0 / 4 times the thickness. Therefore, even if the film refractive index n deviates from the theoretical design value, the product of the refractive index n and the physical thickness d remains unchanged.

[0057] Figure 7 This is the reflectivity spectrum curve simulated and calculated for the third layer of the ZrO2 / Y2O3 mixed film in this embodiment. In order to improve the evaporation characteristics of ZrO2 and make the refractive index more stable, the third layer of the film uses a ZrO2 / Y2O3 mixed film with a mixing ratio of 1:1. Figure 7 It can be seen that because the third layer of ZrO2 / Y2O3 mixed film is thicker, there are 5 extreme values in the monitoring band range of 400-900nm: 430 (maximum), 484 (minimum), 562 (minimum), 658 (maximum) and 792 (minimum), respectively. These extreme values can only be found through simulation calculations. Among these 5 extreme wavelengths, judging from the change in reflectivity signal, it is appropriate to choose 430 (maximum), 658 (maximum) and 792 (minimum) as monitoring wavelengths, but considering the influence of refractive index dispersion, it is more reasonable to choose 430nm, which is closer to the first layer monitoring wavelength of 422.6nm, so 430nm is preferred as the monitoring wavelength (indicated by a thick straight line). When the monitoring wavelength is 430nm, the change pattern of the reflection signal is: maximum → minimum → maximum → minimum → maximum → minimum → maximum → minimum → maximum, which is a six-level high-order extreme value monitoring. This six-level extreme value change process requires Figure 7 The full reflectivity spectrum curve of Q from 0 to 21 can be seen in the figure, but it is easy for experienced thin film workers to estimate it from the optical thickness of the third layer of ZrO2 / Y2O3 mixed film and the monitoring wavelength, so it is omitted here. Figure 7 There are too many and too dense reflectance spectrum curves.

[0058] Figure 8This is the reflectivity spectrum curve calculated by simulation for the fourth layer of MgF2 film in this embodiment. Figure 8 As can be seen, the fourth MgF2 film also has two extreme wavelengths: 524nm (minimum) and 592nm (also a minimum). Considering the reflection signal variation and material dispersion, 524nm is the preferred monitoring wavelength, where the reflection signal changes from maximum to minimum.

[0059] Once the monitoring wavelengths for each layer of a film system of arbitrary thickness are determined, experimental preparation can begin. During preparation, by sequentially changing the thickness monitoring wavelength and interference level for each layer as listed in Table 2, the thickness of the entire arbitrary thickness film system can be monitored using a single monitoring plate using the extreme value method (the second Al2O3 layer requires the fixed value method). Although this method requires changing the monitoring wavelength, since the entire arbitrary thickness film system can be deposited on a single monitoring plate, it overcomes the drawbacks of using a single monitoring plate to monitor the thickness of a single layer. This eliminates the need to replace the monitoring plate for each layer, significantly reducing preparation time. A film system of arbitrary thickness only consumes one monitoring plate, significantly reducing manufacturing costs. More importantly, there is no need to modify the hardware of the existing extreme value method film thickness monitoring system. Using a single monitoring plate retains the automatic thickness error compensation mechanism of the original extreme value method. If the refractive index of the actual film layer deviates from the theoretical design value, there will be no thickness monitoring error caused by changes in the fixed value, nor will there be any film nucleation error caused by replacing the monitoring plate. This has important practical value for the film thickness monitoring and device manufacturing of arbitrary thickness films, especially the monitoring and manufacturing of arbitrary thickness multilayer anti-reflection films.

Claims

1. A monitoring method for a film system of arbitrary thickness, characterized in that: The following steps are involved: 1) Use thin film design software to design the film system. If the designed film system is a non-regular film system, use the thin film design software to simulate and calculate the reflectivity spectrum curve of each layer of the non-regular film system at different wavelengths within the monitoring band at different thicknesses, and obtain the reflectivity spectrum curve of each layer of the film; The designed membrane system is a non-regular membrane system, that is, a membrane system that is not a quarter multiple; 2) Find the reflectivity extreme value of each film when it reaches the correct thickness from each reflectivity spectrum curve of each film layer, then select a suitable extreme value from the reflectivity extreme values as the film thickness monitoring wavelength, and monitor the manufacturing of the designed film system based on the film thickness monitoring wavelength; When the reflectivity extreme value when the correct thickness is reached cannot be found in the reflectivity spectrum curve of each layer of film, the manufacturing of the designed film system is controlled by using fixed value monitoring; In step 2), a suitable extreme value is selected from the reflectivity extreme value as the thickness monitoring wavelength of the film layer, specifically including: The first type: if the reflectivity extreme value is one, it is directly used as the thickness monitoring wavelength of the film layer; The second type: There are multiple reflectivity extremes. According to the characteristics of the reflectivity extremes and extreme wavelengths, a reflectivity extreme wavelength is selected as the thickness monitoring wavelength.

2. The monitoring method of a film system of any thickness according to claim 1, characterized in that: In step 1), the designed film is a multilayer anti-reflection film.

3. The monitoring method of a film system of any thickness according to claim 1, characterized in that: In step 1), the designed film is a transparent dielectric film in the monitoring band.

4. The monitoring method of a film system of any thickness according to claim 1, characterized in that: In step 1), the monitoring band range is 400~900nm.

5. The monitoring method of a film system of any thickness according to claim 1, characterized in that: In step 1), the reflectivity spectrum curve has the monitoring band range as the horizontal axis and the reflectivity as the vertical axis.

6. The monitoring method of a film system of any thickness according to claim 1, characterized in that: In step 1), the different thicknesses refer to dividing each designed film into Q equal parts according to thickness, and calculating the reflectivity spectrum curves of each wavelength within the monitoring band range at thicknesses of 1 equal part to Q+1 equal parts.

7. The monitoring method of a film system of any thickness according to claim 6, characterized in that: In step 1), Q is 10~30.

8. The method for monitoring a film system of any thickness according to claim 1, wherein: In step 2), the reflectivity extreme value of each film layer when it reaches the correct thickness is found from the reflectivity spectrum curves of each film layer, specifically including: Compare the reflectivity spectrum curves of each film at Q equal thickness to find the reflectivity extreme value.

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