CN101323971A - A kind of method that utilizes buffer layer to prepare high-quality ZnO film - Google Patents

Abstract

The invention relates to a method for preparing a high-quality ZnO thin film by using a buffer layer. The main steps of the method are: putting the silicon substrate into the reaction chamber of a hot wire chemical vapor deposition device as a deposition substrate after pretreatment; mixing reaction of hydrogen and acetone The diamond film was nucleated and grown in gas, and then the silicon substrate was soaked and etched with a mixed solution of HNO ₃ and HF to form a self-supporting diamond film; then a ZnO film was prepared on the self-supporting diamond film by DC magnetron sputtering. The ZnO buffer layer was sputter-deposited in an Ar, _O2 atmosphere, followed by the deposition of the ZnO main layer. The invention has simplified manufacturing process and low cost, and is conducive to promoting large-scale application of high-quality ZnO thin film devices. The grain size of the ZnO film is small, the crystal quality of the film is high, and the surface roughness is low.

Description

A kind of method that utilizes buffer layer to prepare high-quality ZnO film

technical field

The invention relates to a method for preparing a high-quality ZnO thin film by using a buffer layer, and belongs to the field of inorganic non-metallic material manufacturing technology.

Background technique

With the development of the third-generation communication technology, the frequency of use of surface acoustic wave (SAW) devices has been increasing, from the initial few MHz to the current few GHz, such as the personal communication service system applied to 1.9GHz, the wireless of 2.4GHz Local area network system and wireless multi-terminal transmission system higher than 5GHz. The continuous development of these high-frequency application systems has significantly increased the market demand for high-frequency surface acoustic wave devices. However, traditional SAW materials have a low sound velocity, usually lower than 4000m/s, and only devices with relatively low frequencies can be prepared.

Diamond has the fastest sound wave propagation speed among all materials. Therefore, under the same technological level, diamond materials can produce devices with higher frequencies. At the same time, diamond has many excellent electrical, optical, thermal and mechanical properties, as well as high radiation resistance and physical and chemical stability. Such as low dielectric constant (5.7), high breakdown voltage (10 ⁷ V/cm), high electron and hole mobility (1800cm ² /V·S and 1200cm ² /V·S respectively), high thermal conductivity ( 20W/cm·K). These unique properties enable the device to work safely and stably without any protection even in harsh environments such as high temperature and strong radiation. Therefore, surface acoustic wave devices based on diamond have received more and more attention. However, since diamond itself is not a piezoelectric material, it cannot excite and receive surface waves, so it is necessary to deposit a piezoelectric film on it to form a multilayer thin film SAW device.

As a piezoelectric material, ZnO has a strong electromechanical coupling coefficient, making it widely used in ultrasonic transducers, Bragg deflectors, spectrum analyzers, high-frequency filters, high-speed optical switches and micromachines. . These devices are also widely used in civil and military fields such as wavelength division multiplexing of high-capacity and high-speed optical fiber communication, optical fiber phase modulation, anti-radar dynamic frequency measurement, electronic interception, satellite mobile communication, and parallel optical information processing. With the development of communication technology and the increase of communication traffic, the traffic at lower frequencies has become saturated, which makes the communication frequency develop to high frequency. At the same time, mobile communication also requires a higher frequency. In the frequency range above 1.5GHz, high-frequency filters with low loss become one of the most critical components of mobile communication systems. ZnO is the material of choice for making such high-frequency surface acoustic wave devices.

At present, the preparation of SAW devices with ZnO/diamond structure is still in the research and development stage. The diamond film used in SAW devices must have high nucleation density, low surface roughness and high repeatability. Especially for high-frequency SAW devices, a very flat diamond surface is required to etch submicron IDT electrode patterns, so as to maximize It is necessary to increase the frequency of the device and reduce the insertion loss.

At present, there are mainly the following methods to solve the flatness problem of diamond film: 1. Mechanical polishing of the diamond surface. This method is currently being used, but polishing is difficult and time-consuming; 2. Growth of nano-polycrystalline diamond (NCD). This method has certain feasibility, but the surface impedance is not suitable for directly fabricating IDT on the diamond surface.

It can be seen from this that the traditional method is based on the premise of using the growth surface of the diamond film. However, the test found that the nucleation surface of the diamond film, that is, the contact surface between the diamond and the Si substrate, well maps the surface morphology of the Si substrate. The diamond grain size of the nucleation surface of the self-supporting diamond film grown on a mirror Si substrate is still at the nanometer level of the nucleation stage, so the surface roughness is quite low. Therefore, using the self-supporting diamond film nucleation surface as a platform for growing ZnO thin films can reach a high standard in terms of surface roughness and film quality, and ensure the film thickness to meet the requirements of SAW devices for diamond film substrates.

At the same time, the piezoelectric film used in SAW devices requires high c-axis orientation, high surface flatness, high resistivity and piezoelectricity, so as to maximize the frequency of the device and reduce the insertion loss. Non-oriented crystal grains and other defects in the film will reduce the performance of the device and increase the insertion loss. For example, the scattering of surface defects will scatter sound waves and cause energy loss; in addition to the influence of the surface, grain boundaries and vacancies will also cause similar scattering and energy loss, and the scattering is particularly serious when the size of the grains is on the same order of magnitude as the wavelength of the sound wave. Therefore, controlling the microstructural properties of the surface and interior of the piezoelectric thin film ZnO is also critical to fabricating high-frequency and low-loss devices and optimizing device performance. In order to improve the performance of the ZnO thin film, the invention introduces a homogeneous buffer layer into the growth process of the ZnO thin film. This thin layer with particularly excellent performance provides relaxation of internal stress and template effect for the growth of the thin film, so that a high-performance ZnO thin film can be prepared.

Contents of the invention

The purpose of the present invention is to provide a method for preparing high-quality ZnO thin film by utilizing buffer layer.

The purpose of the present invention is achieved through the following technical solutions.

A kind of method that utilizes buffer layer to prepare high-quality ZnO film is characterized in that this technology has following process and step:

(1) Preparation of self-supporting diamond film:

Using Si wafer as substrate, diamond film was prepared by hot filament CVD method. Then the Si substrate is removed by chemical ablation to obtain a self-supporting diamond film.

(1) Tantalum wire pretreatment:

Hot wire chemical vapor deposition (HFCVD) uses tantalum wire as a heating source, but the surface of tantalum wire often has oxide layers and other impurities, and tantalum is also easy to volatilize after being heated. If it is used directly, it will cause pollution to the substrate, so Pretreatment of tantalum wire is very necessary. First polish the surface of the tantalum wire with sand, after vacuuming, then pass in hydrogen and acetone to 4-6KPa, where the ratio of hydrogen to acetone is 1:1-5:1, heat to about 420-450°C Treat for 20-30 minutes. This not only removes the oxide layer impurities on the surface of the tantalum wire, but also forms a layer of carbide covering layer on the surface of the tantalum wire to inhibit the volatilization of tantalum in the subsequent process and reduce the introduction of impurities.

(2) Substrate and its pretreatment

In the present invention, the p-type 1cm×1cm silicon wafer is used as the substrate for the growth of the diamond film, and its mirror surface is used as the diamond growth surface, in order to reduce the roughness of the diamond surface. Before growth, the silicon substrate must undergo certain steps of pretreatment, the purpose of which is to remove the silicon surface oxide layer, clean the surface, and appropriately increase the diamond nucleation centers on the substrate surface.

A new method of manual grinding with ultra-fine diamond powder: first place in HF solution for 5-10 minutes to remove the oxide layer on the Si surface, and then manually grind with a mixture of ultra-fine diamond powder and glycerin with a particle size of 100nm for 10-15 minutes. Attention must be paid to precision and uniformity when grinding to avoid forming too deep scratches. After grinding, place the substrate in an acetone solution for 10 to 15 minutes in an ultrasonic bath to remove oil stains on the surface of the substrate, then place it in deionized water for 10 to 15 minutes in an ultrasonic bath, and repeat the above cleaning steps 3 to 5 times until The surface is very clean, and finally the substrate is taken out and dried.

(3) Deposition of diamond film

After the tantalum wire pretreatment and substrate pretreatment are completed, the silicon wafer is placed on the sample stage with the mirror facing up, and then the diamond film is deposited. The operation steps are as follows:

(a) Vacuuming: use a vacuum pump to evacuate the reaction chamber, the hydrogen branch and the acetone branch in sequence, and finally make the background vacuum of the reaction chamber <5Pa;

(b) Adjustment of gas flow rate and air pressure: after vacuuming, turn on the mass flow meter to feed hydrogen and acetone in sequence. Stable at the design value of 0.8 ~ 1.5Pa;

(c) Heating: After the air pressure is stable, turn on the cooling water and the heating voltage, and increase the heating voltage at a rate of about 1/2V/min, finally keep the reaction temperature at 580-620°C, add a bias current of 4A, and nucleate for 30 minutes;

(d) Cooling: maintain the process conditions for 80-120 hours, stop feeding acetone, and reduce the voltage at a rate of about 1/3V/min to lower the reaction temperature to room temperature, finally turn off the hydrogen and cooling water, and take out the sample;

(4) Silicon substrate peeling off

In order to obtain a self-supporting diamond film, after the above treatment, the prepared diamond film with a large thickness is placed in a solution with a molar concentration of HF and _HNO3 of 1:1, etched at room temperature for 10 to 15 hours, and the Si After the substrate is dissolved, the required self-supporting diamond film can be obtained;

(2) Preparation of ZnO thin films by DC magnetron sputtering

Generally speaking, it is not easy to form ZnO thin films with good repeatability, few defects, and excellent piezoelectric properties. Non-oriented grains, surface defects, grain boundaries, and vacancies in the thin films will have a negative impact on device performance. The introduction of a ZnO homogeneous buffer layer plays a significant role in alleviating the stress between the film and the substrate and obtaining a film with good crystallinity and excellent performance.

In the present invention, the ZnO thin film is prepared by a DC reactive magnetron sputtering method, and self-supporting diamond is used as a substrate, and the target material is a 99.99% metal zinc target. During the deposition process, hydrogen (Ar) is the sputtering gas, and oxygen (O ₂ ) is the reaction gas.

(1) Surface cleaning of self-supporting diamond film

In order to eliminate the influence of surface dirt and particles on film deposition, first place the diamond substrate in acetone for 10-15 minutes in an ultrasonic bath, then place it in deionized water for 10-15 minutes, and repeat the above cleaning steps 3-5 Repeat until the surface is very clean, and finally take out the substrate and dry it.

(2) Preparation of ZnO buffer layer on self-supporting diamond film

Fix the self-supporting diamond substrate in the magnetron sputtering chamber with the mirror facing down, and then deposit the ZnO thin film. The operation steps are as follows:

(a) Vacuuming: Use a direct-coupled pump to evacuate the reaction chamber, Ar gas and _O2 gas pipelines, and finally make the background vacuum of the reaction chamber <5Pa; use a molecular pump to evacuate the reaction chamber, and finally make the background vacuum of the reaction chamber <3×10 ^-3 Pa

(b) Start-up: After vacuuming, turn on the Ar mass flowmeter to feed Ar gas, and adjust the flow rate to >150 sccm (standard-state cubic centimeter per minute), and turn on the steady flow source for start-up.

(c) Adjustment of gas flow rate and working air pressure: after the glow area is stabilized, introduce O ₂ gas to adjust the Ar, O ₂ flow ratio Ar:O ₂ between 1:1 and 1:3, and adjust the working air pressure from 0.2 to 1:3. 0.9 Pa.

(d) Adjustment of sputtering power: adjust the sputtering power to 50-100 W, and after the parameters are stabilized, keep the sputtering time for 20-60 minutes to deposit the ZnO buffer layer.

(3) Deposition of ZnO main layer

After the preparation of the ZnO buffer layer, adjust the gas flow to adjust the Ar, _O2 flow ratio Ar: _O2 between 1:2 ~ 1:4, adjust the working pressure 0.3 ~ 0.8Pa, adjust the sputtering power 100 ~ 200W, open the sample The disk rotates, the substrate is heated to 200-250°C, and the deposition time is 3-5 hours.

Compared with the prior art, the present invention has the following significant advantages:

(1) The backside (nucleation surface) of the self-supporting diamond film is used as the deposition surface of the subsequent ZnO film, which can well solve the contradiction between the thickness of the diamond substrate and the surface roughness.

(2) Highly oriented ZnO thin films were prepared by a two-step method. The direct current sputtering method can use metal Zn target, which is easy to manufacture, can achieve high purity, low price, and high film forming rate. The introduction of the ZnO buffer layer is of great significance to the ZnO prepared on the diamond film, and it is possible to obtain a ZnO film with high c-axis orientation, good crystallinity and excellent performance.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the present invention/ZnO homogeneous buffer layer/self-supporting diamond film.

Fig. 2 is X-ray diffraction (XRD) pattern of ZnO film of the present invention

Fig. 3 is ZnO thin film atomic force microscope (AFM) figure of the present invention

Detailed ways

Specific embodiments of the present invention are described in detail below.

Example 1

The technological process and steps of the present embodiment are as follows:

(1) Preparation of self-supporting diamond film:

(1) Tantalum wire pretreatment:

Grind the surface of the tantalum wire with a sandpaper, vacuumize to 5Pa, then pass in hydrogen and acetone at 200sccm and 50sccm respectively, adjust the air pressure to 5KPa, heat to 430°C, and pretreat for 30 minutes.

(2) Substrate and its pretreatment

A p-type 1cm×1cm silicon wafer is used as the substrate, and its mirror surface is used as the diamond growth surface. Si was first placed in HF solution for 10 minutes, and then manually ground for 15 minutes with a mixture of ultrafine diamond powder and glycerin with a particle size of 100 nm. After grinding, the substrate was cleaned in an ultrasonic bath in acetone solution for 15 minutes, then placed in an ultrasonic bath of deionized water for 15 minutes, and the above cleaning steps were repeated 3 times, and finally the substrate was taken out and dried.

(3) Deposition of diamond film

Place the silicon wafer on the sample stage with the mirror facing up, and then deposit the diamond film. The operation steps are as follows:

(a) Vacuuming: use a vacuum pump to evacuate the reaction chamber, the hydrogen branch and the acetone branch in sequence, and finally make the background vacuum of the reaction chamber 4.5Pa;

(b) Gas flow and air pressure adjustment: after vacuuming, open the mass flow meter in turn to feed hydrogen and acetone, wherein the hydrogen and acetone sulfur content are respectively 200 sccm and 65 sccm, and then the pressure in the reaction chamber is stabilized by adjusting the pressure reducing valve. 1 Pa;

(c) Heating: After the air pressure is stable, turn on the cooling water and the heating voltage, and increase the heating voltage at a rate of 1/2V/min, finally keep the reaction temperature at 620°C, add a bias current of 4A, and nucleate for 30 minutes.

Adjust the flow rate of hydrogen and acetone to 200sccm and 50sccm respectively, the reaction temperature is 680°C, the working pressure is 5Pa, and the deposition time is 120 hours;

(d) Cooling: After maintaining the process conditions for a predetermined time, first stop feeding acetone, and reduce the voltage at a rate of about 1/3V/min to reduce the reaction temperature to room temperature, and finally turn off the hydrogen and cooling water, and take out the sample;

(4) Silicon substrate peeling off

The prepared diamond film with a large thickness is placed in a solution of 1:1 molar concentration of HF and _HNO3 , etched at room temperature for 12 hours, and the required self-supporting diamond film is obtained after the Si substrate is dissolved;

(2) Preparation of ZnO thin films by DC magnetron sputtering:

In the present invention, a ZnO thin film is prepared by a DC reactive magnetron sputtering method, and a self-supporting diamond is used as a substrate, wherein the thickness of the self-supporting diamond is about 400 μm. The purity of the zinc target used is 99.99%, the target diameter is 120 mm, the thickness is 5 mm, and the distance between the target and the substrate is 80 mm. During the reaction deposition process, hydrogen (Ar) is the sputtering gas, oxygen (O ₂ ) is the reaction gas, and the purity is uniform. is 99.99%.

(1) Surface cleaning of self-supporting diamond film

First place the diamond substrate in acetone for 10 minutes in an ultrasonic bath, then place it in deionized water for 15 minutes, repeat the above cleaning steps 3 times until the surface is very clean, and finally take out the substrate and dry it;

(2) Preparation of ZnO buffer layer on self-supporting diamond film

Fix the self-supporting diamond substrate in the magnetron sputtering chamber with the mirror facing down, and then deposit the ZnO thin film. The operation steps are as follows:

(a) Vacuuming: Use a direct-coupled pump to evacuate the reaction chamber, Ar gas and _O2 gas pipelines, and finally make the background vacuum of the reaction chamber 4.5Pa; use a molecular pump to evacuate the reaction chamber, and finally make the background vacuum of the reaction chamber The vacuum is 2×10 ^-3 Pa

(b) Start-up: After vacuuming, open the Ar mass flowmeter to feed Ar gas, adjust the flow rate to 160 sccm, and turn on the steady flow source to start the ignition;

(c) Adjustment of gas flow rate and working air pressure: after the glow area is stabilized, introduce _O2 gas to adjust Ar, the flow ratio of _O2 to Ar: _O2 is 1:1, and adjust the working air pressure to 0.2Pa;

(d) Adjustment of the sputtering power: adjust the sputtering power to 100W, and after the parameters are stabilized, keep the sputtering time for 30 minutes, and deposit the ZnO buffer layer;

(3) Deposition of ZnO main layer

After the preparation of the ZnO buffer layer, adjust the gas flow rate to adjust the Ar, _O2 flow ratio to 1:3, adjust the working pressure to 0.5Pa, adjust the sputtering power to 200W, turn on the sample plate to rotate, heat the substrate to 250 °C, and the deposition time is 3 Hour.

It can be seen that the prepared ZnO film has good crystalline quality and c-axis orientation by X-ray diffraction (XRD) pattern (accompanying drawing 2); atomic force microscope (AFM) (accompanying drawing 3) test finds that the ZnO film grain size is small , the crystal quality of the film is high and the surface roughness is low.

Claims (1)

1. a kind of method utilizing buffer layer to prepare high-quality ZnO thin film is characterized in that the method has following technological process and step: (1) Preparation of self-supporting diamond film 1) Pretreatment of tantalum wire: In the hot wire chemical vapor deposition method, tantalum wire is used as the heating source. Firstly, the surface of the tantalum wire is polished clean with sand leather. After vacuuming, hydrogen and acetone are introduced to 4-6KPa. The ratio of acetone is 1:1 to 5:1, heated to about 420 to 450°C for 20 to 30 minutes; 2) Substrate pretreatment: first place the p-type 1cm×1cm silicon wafer in HF solution for 5-10 minutes, then manually grind it in a mixture of ultrafine diamond powder and glycerin with a particle size of 100nm for 10-15 minutes, and place Place the silicon wafer in an acetone solution for 10-15 minutes in an ultrasonic bath, then place it in deionized water for 10-15 minutes, repeat the above cleaning steps 3-5 times until the surface is very clean, and finally take out the silicon wafer and dry it; 3) Deposition of diamond film: the pretreated silicon wafer is placed on the sample stage as a substrate, and its mirror surface is upward as a diamond growth surface, and the diamond film is deposited; (a) vacuuming: use a vacuum pump to sequentially vacuum the reaction chamber , the hydrogen branch and the acetone branch are vacuumed, and finally the background vacuum of the reaction chamber is <5Pa; (b) gas flow and air pressure adjustment: turn on the mass flow meter in turn to feed hydrogen and acetone, and the ratio of hydrogen to acetone is 1:1 ~5:1, and then adjust the pressure reducing valve to stabilize the pressure in the reaction chamber at 0.8~1.5Pa; (c) heating: turn on the cooling water and heating voltage switch, and increase the heating voltage at a rate of about 1/2V/min, and finally Keep the reaction temperature at 580-620°C, add a bias current of 4A, and nucleate for 30 minutes; (d) deposit the diamond film for 80-120 hours, stop feeding acetone, and reduce the voltage at a rate of about 1/3V/min to reduce Reaction temperature is up to room temperature, finally shut off hydrogen and cooling water, take out sample; 4) Silicon substrate peeling: the prepared diamond film is placed in a solution of HF and HNO ₃ with a molar concentration of 1:1, etched at room temperature for 10 to 15 hours, and the required self-supporting diamond film is obtained after the Si substrate is dissolved. film; (2) Preparation of ZnO thin films by DC magnetron sputtering 1) Surface cleaning of the self-supporting diamond film: place the diamond substrate in acetone for 10 to 15 minutes in an ultrasonic bath, then place it in deionized water for 10 to 15 minutes, repeat the above cleaning steps 3 to 5 times until the surface Very clean, finally take out the substrate and dry it; 2) Prepare the ZnO buffer layer on the self-supporting diamond film: fix the self-supporting diamond substrate in the magnetron sputtering chamber with the mirror facing down, and then deposit the ZnO film; (a) vacuuming: use a direct pump Vacuumize the reaction chamber, Ar gas and _O2 gas pipelines, and finally make the background vacuum of the reaction chamber <5Pa; use a molecular pump to evacuate the reaction chamber, and finally make the background vacuum of the reaction chamber <3×10 ^-3 Pa; (b ) Ignition: after the vacuuming is completed, introduce Ar gas, and adjust the flow rate > 150 ml per minute under standard conditions, and turn on the steady flow source to start the ignition; (c) gas flow and working pressure adjustment: after the glow area is stable, turn on Enter _O2 gas, adjust the Ar and _O2 flow ratio Ar: _O2 between 1:1~1:3, adjust the working pressure to 0.2~0.9Pa; (d) Adjust the sputtering power: adjust the sputtering power to 50 ~100W, after the parameters are stable, keep the sputtering time for 20~60 minutes, and deposit the ZnO buffer layer; 3) Deposition of the ZnO main layer: after the preparation of the ZnO buffer layer, adjust the gas flow rate to adjust the flow ratio of Ar and _O2 to Ar: _O2 between 1:2 and 1:4, and adjust the working pressure to 0.3 to 0.8Pa. The sputtering power is 100-200W, the sample disk is turned on and rotated, the substrate is heated to 200-250°C, and the deposition time is 3-5 hours.

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