JPH06151421A - Formation of silicon nitride thin film - Google Patents

Abstract

PURPOSE:To form a silicon nitride the film, which has a low internal stress and is superior in chemical resistance, by a method wherein helium or neon are mixed in nitrogen gas plasma to activate the plasma. CONSTITUTION:A silicon wafer and a borosilicate glass are installed on a substrate holder 2 of a high-frequency magnetron sputtering device as a substrate 3. Moreover, a single crystal silicon film is installed on a high-frequency electrode 6 as a sputtering target 4. After the interior of a vacuum container 1 is evacuated to increase the degree of vacuum, helium and nitrogen bombs 15 and 16 are opened to introduce gas of a prescribed mixing ratio. A high-frequency voltage is applied to the electrode 6 to generate plasma and after an elapse of a prescribed time, a shutter 5 is shut to finish a film-forming process. There is no need to perform a post-treatment on a formed silicon nitride thin film, an internal stress of the thin film can be held very low and it becomes possible to form a silicon nitride thin film, which is superior in chemical resistance and is high in quality.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a high quality silicon nitride thin film having a small internal stress and excellent chemical resistance by a reactive sputtering method.

[0002]

2. Description of the Related Art A silicon nitride thin film has been examined mainly for a thin film forming method based on a plasma process from the viewpoint of having excellent physical properties as an insulating material or an optical material in a semiconductor (particularly gallium-arsenic based semiconductor) process. It was Among them, a forming method by a chemical vapor deposition method (CVD method), which is a high temperature process, has been widely taken up because of its advantages such as controllability of deposition rate and composition of a thin film.
On the other hand, the reactive sputtering method has an excellent feature that a high-purity thin film can be formed by a low temperature process, and therefore research and development are actively promoted. However, the silicon nitride thin films formed by these plasma processes are often warped or distorted due to internal stress, and in extreme cases, the thin films are often cracked or destroyed, especially as a structure. This becomes a serious problem when considering the use of thin films. As a method of relaxing the internal stress of the silicon nitride thin film, there are known methods such as increasing the temperature of the substrate to be deposited and annealing the deposited thin film. It is not a practically effective means because it easily causes problems. As a typical known example of a conventional method for forming a silicon nitride thin film, for example, “silicon nitride film by reactive sputtering” [SMHu and LVGregor: “SiliconNitri
de Films by Reactive Sputtering ”, Journal of Elec
trochemical Society, Vol.114, No.8, pp.826〜833 (196
7)].

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems in the prior art,
To provide a method for forming a high-quality silicon nitride thin film having low internal stress and excellent chemical resistance in a method of sputtering silicon in a nitrogen gas plasma and depositing a silicon nitride thin film by a so-called reactive sputtering method. is there.

[0004]

Most of the silicon nitride thin films formed by the reactive sputtering method exhibit a compressive stress due to volume expansion. It is considered that this is mainly due to unreacted nitrogen being taken into the thin film or an unordered film molecular structure resulting from incomplete formation of a silicon-nitrogen bond. Therefore, it is the focus of the present invention to activate the nitrogen plasma to increase the reactivity, and in order to achieve this, by mixing helium or neon, which has a large internal energy in the plasma excited state, with the nitrogen gas, It activates plasma and increases reactivity. In general, a large amount of highly active neutral species excited in a high energy state such as a metastable excited state or a Ludberg state exists in plasma. These excited species have large internal energies, and their relaxation to the ground state is a forbidden transition process, so that they have a long lifetime. Therefore, excitation by energy transfer is possible through collision with these high-energy particles. Here, the ionization energy of the nitrogen molecule of the energy acceptor is 15.58 eV, the dissociation energy is 9.81 eV, and the internal energy in the metastable excited state of the energy donor helium is about 20.
eV, that of neon is about 17 eV. Therefore, the internal energy states of helium and neon in these metastable excited states are high, and ionization and decomposition excitation of nitrogen molecules can be performed through the energy transfer process. Nitrogen molecular ions are a particularly important plasma-excited species in reactive sputtering, and are used for chemical adsorption and sputtering on a silicon target, as well as generation of highly reactive nitrogen atoms through interaction with a silicon solid surface, such as silicon nitride. It is responsible for the basic process of forming the skeleton. In the conventionally used argon or a noble gas that is heavier than that, the internal energy in the metastable excited state is smaller than the ionization energy of the nitrogen molecule, and the effect of causing the ionization of the nitrogen molecule cannot be expected. Furthermore, helium and neon atoms have a smaller mass than nitrogen molecules, and mixing these into nitrogen gas increases the thermal conductivity. From this, it is expected that the thermal equilibrium state on the surface where the film molecules are formed is stabilized and the thermal fluctuation is rapidly relaxed. This effect is effective for relaxing the internal stress of the thin film. On the other hand, in the case of argon or a noble gas that is heavier than that, the thermal conductivity is decreased, and the above effect cannot be expected. The promotion of generation of highly reactive nitrogen species and the increase of thermal conductivity in these plasmas are the basic factors in forming a strong silicon nitride molecular skeleton, which reduces the stress of the thin film and improves the chemical resistance. Can be measured.

[0005]

Embodiments of the present invention will be described below in more detail with reference to the drawings. FIG. 1 shows an example of the configuration of a parallel plate 2-pole type high frequency magnetron sputtering apparatus used in this example. As shown in the figure, the high frequency magnetron sputtering apparatus includes a vacuum container 1, a substrate holder 2, a substrate 3,
Sputter target 4, shutter 5, high frequency electrode 6, matching box 7, high frequency power supply 8, oil diffusion pump 9,
Oil rotary pump 10, exhaust system main valve 11, roughing valve 12, oil diffusion pump suction valve 13, mass flow controller 14, helium cylinder 15, nitrogen cylinder 1
6 and the heater 17 and the like. A method for forming a plasma-excited silicon nitride thin film using the above sputtering apparatus will be described. A silicon wafer (1 × 3 × 0.01 cm) and borosilicate glass (Corning # 0211, 1 × 3 × 0.05 cm) are placed on the substrate holder 2 as the substrate 3. Further, as the sputtering target 4, single crystal silicon is placed on the high frequency electrode 6. Oil rotary pump 10
After opening the oil diffusion pump suction valve 13, the heater 17 of the oil diffusion pump 9 is operated to start up the pump. After the suction valve 13 for the oil diffusion pump is closed, the roughing valve 12 is opened to start evacuation.
When the vacuum level rises to about 10 Pa, the roughing valve 12
Is closed, the suction valve 13 for the oil diffusion pump and the exhaust system main valve 11 are opened, and high vacuum exhaust is performed by the oil diffusion pump 9. Baking of the vacuum container 1 is also performed using the heater 17. When the degree of vacuum rises to around 5 × 10 to ⁶ Pa, the helium cylinder 15 and the nitrogen cylinder 16 are opened, and the mass flow controller 14 independently adjusts the mixing ratio to 1 to 50 cc (cm ³ ) / min so as to obtain a predetermined mixing ratio. It is introduced at a flow rate, and the gas pressure inside the vacuum container 1 is adjusted. Here, a high frequency voltage is applied to the high frequency electrode 6 by the high frequency power supply 8 to generate plasma. The condenser in the matching box 7 is adjusted so that a stable plasma state can be obtained. After discharging for a predetermined time, the shutter 5 is closed, application of the high frequency voltage is stopped, and the film forming process is completed. The substrate holder 2 during film formation was kept at 50 ° C., the applied power density was 2.83 W / cm ² , and the average film thickness deposited was about 0.7 μm. Plasma emission spectroscopy was performed during the deposition process to obtain information on the excited states of the plasma particles. Plasma emission radiated from the sapphire window,
It was introduced into a Zerni-Turna type diffraction grating spectroscope through a quartz fiber, and spectroscopic measurement was performed in the wavelength range of 200 to 800 nm. The film stress was calculated from the amount of change in the curvature of the silicon wafer substrate before and after the film deposition. The molecular structure of the thin film was analyzed by a Fourier transform infrared spectrophotometer (JASCO, FT / IR-5M) equipped with a mercury-cadmium-tellurium (MCT) detector. In addition, the optical properties of the thin film are determined by an ultraviolet-visible spectrophotometer (SHIMAZU, UV-1) equipped with a silicon photodiode detector.
60A). FIG. 2 shows the film stress when the gas flow rate ratio of helium and nitrogen is used as a parameter. In all, compressive stress is applied to the substrate due to the volume expansion of the film. The compressive stress when nitrogen alone (He / N ₂ = 0) is 1
It is 32 kg / mm ² . In comparison, the stress is reduced in the vicinity of about 90 kg / mm ² in a small amount added was 0 <He / N2 <1 range helium. Furthermore, the gas flow rate ratio H
e / N ₂ = 11.5 (at this time, the partial pressures of helium and nitrogen are both 1 Pa), and the stress is about 40 kg / m.
There is a region that drops significantly to around m ² . In this region, the film deposition rate is about half that in the case of only nitrogen, and it is considered that a dense molecular skeleton is formed. Further, when the gas flow rate ratio He / N ₂ = 11.5, the compressive stress of the thin film formed by increasing the gas pressure to 3 Pa is 8 kg /
It was confirmed that it could be reduced to mm ² . In addition, neon is used under the same conditions (gas pressure: 2 Pa, gas flow rate ratio: Ne.
/ N ₂ = 11.5), the compressive stress of the thin film formed was about 13 kg / mm ² , and the effect of these rare gases on the film stress reduction could be confirmed. Next, the chemical resistance of the above-mentioned low-stress silicon nitride thin film was investigated by the etching rate in a strong acid and strong alkaline aqueous solution. The etching rate when immersed in a hydrofluoric acid buffer solution (ammonium fluoride: hydrofluoric acid = 6.7: 1) at 20 ° C. for 20 minutes was about 2 nm / minute. Further, when immersed in an 8N potassium hydroxide solution at 70 ° C. for 30 minutes, it showed chemical resistance that could not be detected by a stylus type step gauge (Talystep) having an accuracy of 0.1 μm. As described above, by introducing helium or neon or a mixed gas thereof in the process of forming a silicon nitride thin film by the reactive sputtering method, it is possible to reduce the internal stress of the thin film at a low substrate temperature of 50 ° C. without the need for post-treatment. It has become possible to form a silicon nitride thin film which can be kept extremely low and has excellent chemical resistance.

FIG. 3 shows the gas flow rate ratio He / N ₂ = 11.5.
3 shows an infrared spectroscopy spectrum measured by a transmission method of a silicon nitride thin film showing the lowest stress formed under the condition of. 8
Strong absorption due to stretching vibration of the Si—N bond is observed around 80 cm ⁻¹ , and it can be confirmed that the film molecule is composed of the silicon nitride skeleton. Furthermore, observed absorption caused while very weak 3320 cm ~ ¹ on the stretching vibration of N-H bonds and 2180 cm ~ ¹ absorption due to stretching vibration of Si-H bonds found in the amorphous structure Suggests. Furthermore, from the interference pattern appearing in the UV-visible spectrum measured by the transmission method, 200 to 1100n
The refractive index of the silicon nitride thin film in the wavelength region of m was calculated to be 2.198. This result also supports the film structure of amorphous silicon nitride.

Plasma emission spectroscopy provides information about the excited state of plasma particles without disturbing the plasma state. When helium is used, B ³ Π _g- A ³ Σ _u +
A progression band characteristic of the excited molecular species due to the transition and the B ² Σ _u + −X ² Σ _g + transition of the nitrogen molecular ion was observed together with the helium emission line. In addition, the emission line of excited atomic species is 746.9 nm for nitrogen atoms and 2 for silicon atoms.
It was observed at 88.1 nm. The nitrogen molecule ions and nitrogen atoms confirmed here are generated by the ionization and dissociation excitation process of the nitrogen molecules, and depend largely on the activity of nitrogen plasma. Further, in the case of silicon, a sputtering process of nitrogen molecular species having a large mass, particularly nitrogen molecular ions is an important generation process. Furthermore, nitrogen molecule ions have extremely high reactivity with silicon due to active nitrogen atoms generated through chemisorption, and become central particles responsible for the formation of a silicon nitride skeleton. Therefore, the emission intensity of nitrogen molecule ions, nitrogen atoms, and silicon standardized by the emission intensity of nitrogen molecules is an important index for knowing the plasma activity and reactivity. Furthermore, it will be useful information when considering the molecular structure and physical properties of the obtained thin film. The result is shown in FIG. It can be seen that all emission relative intensities tend to increase with the He / N ₂ gas flow ratio and the presence of helium promotes activation of the nitrogen plasma. The emission relative intensity of nitrogen molecular ions increases in a substantially proportional relationship with the gas flow rate ratio, but the slope of the straight line changes when He / N ₂ = 11.5.
From this point, in the region where the gas flow rate ratio is large, the production amount of nitrogen molecular ions decreases with respect to the introduction amount of helium. Further, the emission relative intensity of nitrogen atoms does not show a significant difference up to the above gas flow rate ratio as compared with the case below that. H above
From the point of e / N ₂ = 11.5, it increases to He / N ₂ = 20, and then becomes constant again. Considering the ratio of these active nitrogen species generated based on the amount of introduced helium (gas flow rate ratio), in the region of He / N ₂ <11.5, nitrogen molecules are excited during the process of excitation of nitrogen molecules by helium. It can be understood that the ions are preferentially generated. He
When / N ₂ = 11.5, the relative abundance of the nitrogen molecule ions in the plasma becomes maximum, which is the optimum condition for forming the silicon nitride skeleton. It is considered that the silicon nitride thin film formed under this plasma condition has a strong molecular structure, has a low compressive stress, and exhibits excellent chemical resistance.

[0008]

According to the method for forming a silicon nitride thin film of the present invention, a high quality silicon nitride thin film having low internal stress and excellent chemical resistance can be obtained without the need for post-treatment at low temperature. Further, the method for forming a silicon nitride thin film of the present invention,
It can be incorporated into the LSI process, and can be expected to be used in a wide range of fields such as structural materials, optical materials, and insulating materials. In particular, it is suitably used as a structural material that forms the basis in the field of micromachining, where research has accelerated in recent years. Further, when applied as a mask material for X-ray lithography, the silicon nitride thin film having low internal stress of the present invention is extremely effective.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a structure of a plasma-excited silicon nitride thin film forming apparatus exemplified in an example of the present invention.

FIG. 2 is a diagram showing the relationship between the internal stress and the gas flow rate ratio of the silicon nitride thin film exemplified in the example of the present invention.

FIG. 3 shows He / N ₂ = 11.
5 is a diagram showing an infrared spectroscopy spectrum of a silicon nitride thin film formed under the condition of the gas flow rate ratio of 5. FIG.

FIG. 4 is a diagram showing the relationship between the emission relative intensities of nitrogen molecule ions, nitrogen atoms and silicon atoms normalized by the nitrogen molecules exemplified in the examples of the present invention, and the gas flow rate ratio.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Vacuum container 2 ... Substrate holder 3 ... Substrate 4 ... Sputter target 5 ... Shutter 6 ... High frequency electrode 7 ... Matching box 8 ... High frequency power source 9 ... Oil diffusion pump 10 ... Oil rotary pump 11 ... Exhaust system main valve 12 ... Roughing Valve 13 ... Suction valve for oil diffusion pump 14 ... Mass flow controller 15 ... Helium cylinder 16 ... Nitrogen cylinder 17 ... Heater

Claims (1)

[Claims] 1. A method for forming a silicon nitride thin film by a reactive sputtering method by sputtering silicon in a nitrogen gas plasma, wherein helium, neon, or a mixed gas thereof is introduced into the nitrogen gas plasma to bring about a plasma excited state. A method for forming a silicon nitride thin film, which comprises depositing a silicon nitride thin film with an increased reactivity.