CN101078105A - Automatic controlling method for increasing AIN medium film reaction direct current sputtering speed - Google Patents

Abstract

The invention relates to an automatic control method for improving the reactive DC sputtering rate of AlN dielectric thin film, which belongs to the technical field of vacuum solid thin film production. The N ₂ input during the sputtering process reacts with the surface of the Al target to produce AlN. The control system sets the N ₂ partial pressure value P _N in the reaction chamber. The control system measures the reaction chamber pressure P _Ar when only Ar Enter N ₂ for reactive sputtering; adjust the N ₂ input flow rate based on the set N ₂ partial pressure. When the actual pressure of the reaction chamber P > P _N + P _Ar , N ₂ is excessive, and the controller reduces the N ₂ input flow rate; the reaction When the chamber pressure P<P _N +P _Ar , N ₂ is insufficient, the controller increases the input flow of N ₂ . The invention also provides an automatic control system for increasing the reactive DC sputtering rate of the AlN dielectric thin film based on _N2 partial pressure control. The invention stabilizes the residual _N2 partial pressure _PN at a relatively low value, not only ensures the purity of AlN in the film, but also has a higher sputtering rate.

Description

An Automatic Control Method for Improving the Rate of Reactive DC Sputtering of AlN Dielectric Thin Films

technical field

The invention relates to an automatic control method for improving the reactive DC sputtering rate of AlN dielectric thin film, which belongs to the technical field of vacuum solid film production, especially the technical field of solar selective absorption coating production.

Background technique

AlN dielectric thin film materials have high thermal and chemical stability, high transmittance to the solar spectrum, and are widely used in the field of solar thermal utilization. For example, Al-AlN selective absorption coatings are widely used in medium and low temperature solar collectors, and AlN dielectric films are required as anti-reflection coatings for the coatings (Yin Zhiqiang, "Sputtered Solar Energy Selective Absorption Coating", Chinese Patent, Applicant : Tsinghua University, application number: 85100142). In high-temperature solar collectors, metal-dielectric thin-film selective absorption coatings composed of refractory metals and AlN dielectrics are expected to be widely used (Q.-C. Zhang, D.R. Mills, and A. Monger, "Thin-film solar selective surface coating”, Australia Patent, Applicant: The University of Sydney, Application number: AU9220346).

In the field of solar thermal utilization, AlN thin films are usually fabricated by sputtering. Sputtering refers to the vacuum coating technology in which a specific gas is input under vacuum conditions, plasma is generated under the action of an electric field and a magnetic field, and ions bombard the target surface, causing target atoms and atomic groups to escape from the surface and adhere to the substrate surface. Depending on the applied electric field, sputtering can be divided into two types: DC sputtering and radio frequency sputtering. Among them, DC sputtering generally uses metal as the target material, and an inert gas, generally Ar, is input into the reaction chamber to generate plasma under the action of a DC electric field, and metal atoms are sputtered out by physical bombardment, which can be used to make metal thin films. For general dielectric targets, since DC sputtering will generate charge accumulation on the target, only radio frequency sputtering can be used. Compared with DC sputtering, the production cost of RF sputtering is higher. For some metal compound dielectric materials, such as AlN, Al ₂ O ₃ , etc., it can also be produced by reactive DC sputtering metal targets. That is, N ₂ , O ₂ and other active gases are added to the inert gas. Under vacuum plasma conditions, on the one hand, N, O and other active gas atoms react with metal atoms (such as Al) on the surface of the target to produce a metal oxynitride medium (such as AlN, Al ₂ O ₃ ); at the same time, the metal oxynitride molecules (such as AlN, Al ₂ O ₃ ) on the surface of the target are sputtered out under the bombardment of Ar and other inert gas ions, thereby forming metal oxides on the surface of the substrate. Compound Dielectric Film.

In order to reduce the production cost of the solar selective absorption coating, it is generally hoped to adopt the method of direct current sputtering to produce metal and AlN dielectric thin films. Among them, the metal thin film adopts Ar non-reactive DC sputtering, and the AlN dielectric thin film adopts Ar, N ₂ reactive DC sputtering. Under the traditional process conditions, the sputtering rate of AlN dielectric film is much lower than that of metal film, which becomes the bottleneck of the production of selective absorbing coating. Taking the production process of a medium-low temperature Al-AlN solar selective coating as an example, a 100nm thick Al metal film, a 150nm thick Al-AlN metal-dielectric film and a 100nm thick AlN dielectric film need to be sputtered sequentially on the glass substrate. Under the condition that other conditions remain unchanged and only the flow rate of _N2 is changed, when the sputtering rate of metal Al is 20nm/min, the sputtering rate of metal-medium Al-AlN is 10nm/min, while the sputtering rate of pure AlN medium The firing rate is only 2nm/min. The time for sputtering metal Al, metal-medium Al-AlN and medium AlN is 5, 15 and 50 minutes respectively. In addition, it takes about 30 minutes to evacuate the sputtering reaction chamber, and the time for sputtering medium AlN accounts for 50% of the entire sputtering process. more than half. Therefore, increasing the sputtering rate of the AlN medium is of great significance to improving the production efficiency of the solar selective absorbing coating.

The process parameters of AlN thin film reactive DC sputtering are: sputtering current I, Ar flow rate, N _flow rate, Al target bias voltage U, and reaction chamber pressure P. The measures to improve the reactive DC sputtering of AlN thin films include: increasing the sputtering current I, controlling the flow of Ar and N ₂ , and controlling the bias voltage of the Al target. On the one hand, increasing the sputtering current can effectively increase the reactive sputtering rate of AlN thin films, but the magnitude of the current is limited by the power of the sputtering system. On the other hand, Ar, _N2 flow rates have a greater impact on the film composition and sputtering rate of Al target reactive sputtering. Under the condition that other conditions remain unchanged, when the N ₂ flow rate is small, Al-AlN metal-dielectric film is produced by Ar, N ₂ sputtering Al target. With the increase of _N2 flow rate, the proportion of AlN in the Al-AlN film gradually increased, while the sputtering rate decreased gradually. When the N ₂ flow rate exceeds a certain threshold N _th1 , a pure AlN dielectric film will be sputtered, and the sputtering rate will also have a sharp drop. At this time, when the flow rate of N ₂ is reduced, the sputtering rate does not rise immediately, and a pure AlN dielectric film is still sputtered. When the N ₂ flow rate is reduced to a certain threshold N _th2 (N _th2 <N _th1 ), the sputtering rate rises sharply, and the Al-AlN metal-dielectric thin film is sputtered. This phenomenon is called the hysteresis effect of AlN reactive DC sputtering.

The existence of the hysteresis effect prevents people from adopting the traditional method of presetting the gas flow rate to improve the reactive DC sputtering rate of the AlN dielectric thin film. It is necessary to control the gas flow rate, especially the active gas _N2 flow rate in real time. Through the research, it is found that there is also an obvious hysteresis effect between the nitrogen partial pressure _PN , the Al target bias voltage U, and the active gas N ₂ flow rate. As shown in Figure 1, in the case of only Ar plasma ignition, the N ₂ partial pressure is close to 0, and the Al target bias voltage is relatively high (U ~ 400V). When the _N2 flow rate gradually increases from 0, the Al target bias voltage gradually decreases, while the _N2 partial pressure is still close to 0. When the N ₂ flow increases to the threshold N _th1 , the Al target bias voltage drops rapidly, while the N ₂ partial pressure rises sharply.

It can be seen from Figure 1 that the higher the bias voltage of the Al target, the higher the film deposition rate, so some people control the flow of N ₂ through the bias voltage of the Al target. _The specific feedback _method is: set an optimal bias voltage U ₀ of the Al target in the control system, when the bias voltage _U of the Al target When the voltage U>U ₀ , it is considered that the N ₂ is insufficient, and the N ₂ flow rate is increased. Through the above method, the target voltage is stabilized at the optimal position, thereby increasing the deposition rate of the AlN medium.

However, this method has certain limitations, which are specifically manifested in: under the conditions of other process conditions changing, such as sputtering current, Ar flow, etc., the hysteresis curve of Al target bias voltage U and _N flow F _N will occur Change, the bias voltage control point U ₀ will also change. In the actual production process, it is difficult to ensure that the process conditions are strictly consistent, so there are certain difficulties in the selection of the voltage control point U ₀ . In addition, at present, a mass flow controller (Mass Flow Controller, MFC) is commonly used to control the gas flow. But the response speed of MFC is slow (response time> 1 second), it is difficult to meet the needs of real-time control.

We believe that the hysteresis effect of AlN reactive DC sputtering is determined by the following two factors: 1) the chemical reaction between the active gas and the Al target, and 2) the physical bombardment of the Al target by the inert gas. When the active gas _N2 flow is insufficient, the physical bombardment of the inert gas is dominant, it not only bombards the Al on the surface of the Al target, but also bombards the AlN formed by the reactive gas _N2 on the surface of the Al target, thus sputtering Al-AlN Metal-dielectric films. However, N ₂ is completely captured by the surface atoms of the Al target to generate AlN, so even if the N ₂ flow rate increases, the N ₂ partial pressure remains basically unchanged. When the reaction gas is excessive, the chemical reaction of the reaction gas is dominant, and the AlN produced by the reaction of the reaction gas _N2 and Al covers the entire Al target surface, and the physical bombardment of the inert gas Ar can only sputter out AlN, while the excess _N2 It is manifested as a sharp rise in the pressure of the reaction chamber.

Contents of the invention

Based on the above physical phenomena, the present invention provides an automatic control method based on _N2 partial pressure control to increase the reactive DC sputtering rate of AlN dielectric thin films, wherein the sputtering target material is Al, and the active reaction gas is _N2 . During the sputtering process In , most of the input N ₂ reacts with the surface of the Al target to produce AlN, and the remaining N ₂ appears as an increase in the pressure of the reaction chamber. The method is characterized by:

(1) Set the N ₂ partial pressure value P _N of the reaction chamber through the control system.

(2) Measure the pressure P _Ar of the reaction chamber when only the Ar plasma is ignited through the control system.

(3) Feed in N ₂ for reactive sputtering, and adjust the N ₂ input flow rate based on the set N ₂ partial pressure. When the actual pressure of the reaction chamber is P > P _N + P _Ar , it is considered that N ₂ is excessive, and the controller reduces the input flow of N ₂ ; when the pressure of the reaction chamber is P < P _N + P _Ar , it is considered that N ₂ is insufficient, and the controller increases the N 2 ₂ Enter flow.

The present invention also provides an automatic control system based on _N2 partial pressure control to increase the reaction DC sputtering rate of AlN dielectric thin film. The control system includes air pressure setting and reading, air pressure difference comparison processing, and _N2 flow control Three parts, characterized by:

1. The air pressure setting and reading part can set the value of the _N2 partial pressure _PN before reactive sputtering and during the sputtering process, and convert it into a pseudo-logarithmic voltage signal.

2. The air pressure setting and reading part can pre-read the pressure P _Ar of the reaction chamber when only the inert gas is sputtered, and can automatically generate the set total pressure P = P _N + P _Ar of the reaction chamber, and convert is a pseudo-logarithmic voltage signal.

3. The air pressure difference comparison processing part compares the actual air pressure with the set total air pressure, and converts it into the voltage signal required for the change of _N2 flow.

4. The N ₂ input flow control unit adopts at least one N ₂ flow controller whose response time is less than 10 ^-2 seconds, so as to meet the needs of real-time control.

The residual _N2 partial pressure _PN is stabilized at a relatively low value by the above method, which not only ensures the purity of AlN in the film, but also has a higher sputtering rate.

Description of drawings

Fig. 1. _N2 partial pressure, Al target bias voltage variation curve with _N2 flow rate.

Fig. 2. Schematic structure diagram of _N2 flow rate and _N2 partial pressure feedback control system.

Detailed ways

The present invention is illustrated below in conjunction with embodiment.

Example 1

On the borosilicate 3.3 glass substrate, the AlN dielectric thin film was fabricated by using Ar, N ₂ reactive DC sputtering Al target. The flow rate of Ar is 50 sccm, and the sputtering current I=30A. The gas pressure P _Ar = 0.4Pa when Ar is only used for glowing, and the nitrogen partial pressure P _N ≥ 0.12Pa when excessive N ₂ is passed, and the sputtering rate is about 1.5nm/min; with the present invention, the nitrogen partial pressure P _N is controlled at 0.03 Pa, the sputtering rate reaches 6nm/min.

Example 2

AlN dielectric thin films were fabricated on stainless steel substrates by using Ar, N ₂ reactive DC sputtering Al targets. The flow rate of Ar is 70 sccm, and the sputtering current I=40A. The gas pressure P _Ar = 0.6Pa when only Ar is passed through, and the nitrogen partial pressure P _N ≥ 0.1Pa when excessive N ₂ is passed through, and the sputtering rate is about 2nm/min; with the present invention, the nitrogen partial pressure P _N is controlled at 0.01Pa , the sputtering rate reaches 8nm/min.

Example 3

On the silicon substrate, the AlN dielectric thin film was fabricated by using Ar, N ₂ reactive direct current sputtering Al target. The Ar flow rate is 50 sccm, and the sputtering current I=20A. The gas pressure P _Ar = 0.4Pa when only Ar is passed through, and the nitrogen partial pressure P _N ≥ 0.12Pa when excessive N ₂ is passed through, and the sputtering rate is about 1nm/min; with the present invention, the nitrogen partial pressure P _N is controlled at 0.02Pa , the sputtering rate reaches 2nm/min.

Claims (2)

1. An automatic control method for improving the reactive DC sputtering rate of AlN dielectric thin films, wherein the sputtering target material is Al, and the active reaction gas is N ₂ . During the sputtering process, most of the input N ₂ reacts with the surface of the Al target to produce AlN , characterized in that the method includes the following steps: (1) Set the _N2 partial pressure value P _N of the reaction chamber through the control system; (2) measure the reaction chamber pressure P _Ar when only passing the Ar plasma to glow by the control system; (3) Feed N ₂ for reactive sputtering, adjust the N ₂ input flow rate based on the set N ₂ partial pressure, when the actual pressure of the reaction chamber P > P _N + P _Ar , it is considered that N ₂ is excessive, and the controller reduces N ₂ The input flow rate; when the reaction chamber pressure P<P _N +P _Ar , it is considered that N ₂ is insufficient, and the controller increases the input flow rate of N ₂ . 2. An automatic control system based on _N2 partial pressure control to improve the reaction DC sputtering rate of AlN dielectric thin film, characterized in that the system sequentially includes air pressure setting and reading, air pressure difference comparison processing, and _N2 flow control. parts, The air pressure setting and reading part, set the value of _N2 partial pressure _PN before reactive sputtering and during sputtering, and convert it into a pseudo-logarithmic voltage signal; at the same time, the air pressure setting and reading part pre- Read the pressure P _Ar of the reaction chamber when only inactive gas is sputtered, and automatically generate the set total pressure P=P _N +P _Ar of the reaction chamber, and convert it into a pseudo-logarithmic voltage signal; The air pressure difference comparison processing part compares the actual air pressure with the set total air pressure, and converts it into the voltage signal required for _N2 flow change; The N ₂ flow control part adopts at least one N ₂ flow controller whose response time is less than 10 ^-2 seconds for real-time control.

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