Sputtering method

Abstract

A sputtering method of forming a thin film by allowing a target material to react with a gas includes narrowing down film deposition conditions from an existing period of nitrogen radicals by focusing on a nitriding process in thin-film forming processes when the thin film is formed by pulsing a waveform of electric current from a DC power supply at the time of generating plasma and applying the electric current to the target material.

Claims

1. A sputtering method of forming an amorphous nitride film, comprising: determining pulsed electric current application conditions based on a pre-measured duration of nitrogen radicals generated from nitrogen gas by application of pulsed voltage; providing a target material in a chamber; supplying a nitrogen gas into the chamber; generating a plasma by pulsing a waveform of electric current from a DC power supply with the nitrogen gas present in the chamber; and applying the pulsed electric current to the target material to form the amorphous nitride film, wherein the pulsed electric current application conditions include a pulse frequency of the waveform of the pulsed electric current within a range of 10 kHz to 50 kHz inclusive, a ratio of a period in which a voltage is applied in one pulse period within a range of 0.1% to 30% inclusive, a period in which the pulse is not applied within a range of 15 microseconds to 45 microseconds inclusive, and a sputtering pressure within a range of 0.1 Pa to 0.3 Pa inclusive.

2. The sputtering method according to claim 1, wherein the pulse frequency is within a range of 20 kHz to 50 kHz inclusive.

3. The sputtering method according to claim 1, wherein the ratio of the period in which the voltage is applied in one pulse period is within a range of 20% to 30% inclusive.

4. The sputtering method according to claim 1, wherein target material is silicon to form the amorphous nitride thin film made of silicon nitride.

5. The sputtering method according to claim 1, wherein the target material contains at least one of tantalum, niobium, chromium, aluminum and titanium.

6. The sputtering method according to claim 1, wherein the chamber is a vacuum chamber, and the target material contains silicon.

7. The sputtering method according to claim 1, further comprising arranging a magnet at a back surface of the target material, and generating a magnetic field on a surface of the target material with the magnet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view showing a schematic structure of a sputtering apparatus according to Embodiment 1 of the present disclosure;

(2) FIG. 2 is a schematic view schematically showing a sputtering process according to Embodiment 1 of the present disclosure;

(3) FIG. 3 shows results of measuring the behavior of plasma species according to Embodiment 1 of the present disclosure;

(4) FIG. 4 is a graph of correlation between duty ratio and film deposition rate according to Embodiment 1 of the present disclosure;

(5) FIG. 5 is a graph of correlation between duty ratio and thin film refractive index according to Embodiment 1 of the present disclosure; and

(6) FIG. 6 is a cross-sectional view showing a schematic structure of a related-art pulse sputtering apparatus.

DESCRIPTION OF EMBODIMENTS

(7) Initially, various aspects in the sputtering method according to the present disclosure will be described.

(8) A sputtering method according to a first aspect of the present disclosure is a sputtering method of an amorphous nitride thin film as a reactive sputtering method of forming a thin film by allowing a target material to react with a gas, which includes the steps of pulsing a waveform of electric current from a DC power supply at the time of generating plasma and applying the pulsed electric current to the target material, in which a pulse frequency of the waveform of the pulsed electric current is 10 kHz or more to 50 kHz of less, a ratio of a period in which a voltage is applied in one pulse period is 0.1% or more to 30% or less, a period in which the pulse is not applied is 15 microseconds or more and a sputtering pressure is 0.1 Pa or more to 0.3 Pa or less.

(9) In the sputtering method according to the first aspect, a high-quality thin film can be provided at a low cost with a simple apparatus structure, and improvement in film deposition rate can be realized while securing active species and reaction time for nitriding reaction. Therefore, the formation of a protective film securing high reliability of electronic components can be realized with high quality and at a low cost.

(10) In the sputtering method according to a second aspect of the present disclosure, in the first aspect, the pulse frequency may be 20 kHz or more to 50 kHz or less.

(11) In the sputtering method according to a third aspect of the present disclosure, in the first or the second aspects, the ratio of the period in which the voltage is applied in one pulse period may be 20% or more to 30% or less.

(12) In the sputtering method according to a fourth aspect of the present disclosure, in any of the first to third aspects, the period in which the pulse is not applied may be 15 microseconds or more to 45 microseconds or less.

(13) In the sputtering method according to a fifth aspect of the present disclosure, in any of the first to fourth aspects, the amorphous nitride thin film may be made of silicon nitride and the target material may be silicon.

(14) In the sputtering method according to sixth aspect of the present disclosure, in any of the first to fifth aspects, the target material may contain a material of at least one kind of tantalum, niobium, chromium, aluminum and titanium.

(15) Hereinafter, Embodiment 1 of the present disclosure will be explained in detail with reference to the attached drawings. Numerals, shapes, structures, steps, the order of steps and so on shown in Embodiment 1 are shown as examples and do not limit the present disclosure. In components in the following embodiments, components not described in independent claims showing the broadest concepts will be explained as arbitrary components.

Embodiment 1

(16) First, a structure of a sputtering apparatus according to Embodiment 1 of the present disclosure will be explained mainly with reference to FIG. 1.

(17) FIG. 1 is a cross-sectional view showing a schematic structure of the sputtering apparatus according to Embodiment 1. The sputtering apparatus according to Embodiment 1 includes a vacuum chamber 1, a vacuum pump 2, a gas supply source 4, a backing plate 8, a DC power supply 20, a pulsing unit 21, a power supply/pulse controller 22 functioning as an example of a controller and a substrate holder 5.

(18) The vacuum chamber 1 has a structure in which the inside of the vacuum chamber 1 is decompressed into a vacuum state by evacuating the vacuum chamber 1 by the vacuum pump 2 connected through a gate valve 3.

(19) The gas supply source 4 can supply a gas necessary for sputtering to the vacuum chamber 1 at a constant rate. As a gas supplied from the gas supply source 4, for example, a gas having a reactivity with a target material such as nitrogen and oxygen or a mixed gas of the gas having the reactivity and a noble gas such as argon or the like can be selected.

(20) The gate valve 3 provided in the vacuum chamber 1 can control the vacuum degree inside the vacuum chamber 1 to a desired gas pressure by changing an aperture of the gate valve 3.

(21) As shown in FIG. 1, a target material 7 as a target material of sputtering is arranged at an upper part in the vacuum chamber 1. The target material 7 is an arbitrary sputtering material, which is for example, an inorganic material such as a metal material or a semiconductor material. In the case of Embodiment 1, the material is Si as an example.

(22) The backing plate 8 is arranged at an upper part in the vacuum chamber 1, supporting the target material 7 so as to face the later-described substrate holder 5.

(23) The DC power supply 20 is electrically connected to the target material 7 through the pulsing unit 21 and the backing plate 8, which can apply a voltage to the target material 7.

(24) The pulsing unit 21 can accumulate DC current generated by the DC power supply 20 in a built-in capacitor or the like and can pulse the current by turning on or off the unit by a built-in semiconductor switching device or the like.

(25) The power supply/pulse controller 22 controls a power value of the DC power supply 20 and a pulse frequency and on/off periods to be instructed to the pulsing unit 21 based on previously inputted setting values.

(26) A magnet 9 and a yoke 10 are arranged on a back surface of the backing plate 8 at the upper part in the vacuum chamber 1, which can generate a magnetic field on a surface of the target material 7.

(27) In FIG. 1, the substrate holder 5 supporting a substrate 6 is arranged at a lower part in the vacuum chamber 1. The substrate holder 5 is arranged under the substrate 6, supporting the substrate 6 so that the surface of the substrate 6 faces the surface of the target member 7 supported by the backing plate 8.

(28) Next, a sputtering method of forming an amorphous nitride thin film using the sputtering apparatus according to Embodiment 1 will be described.

(29) In the sputtering method according to Embodiment 1, a pulse wave in which the power is intensively applied for a short period of time such as approximately 10 microseconds is utilized as a method of applying the power for generating plasma in the reactive sputtering method. In a process of forming an oxynitride film by using a metal or semiconductor target, high power is momentarily applied by applying the power for generating plasma for a short period of time of approximately 10 microseconds. For example, when the same power is consumed intensively only in a period of 1/100 of a pulse period as compared with DC sputtering using a continuous wave of 100 W, the power of 10000 W that is 100 times of the above is momentarily applied. When the high power is momentarily applied as described above, the nitrogen gas assumed to have relatively high dissociation energy and difficulty dissociating is promoted to be dissociated, and atomic nitrogen having high reactivity or nitrogen in a radical state is generated. Accordingly, nitriding of a thin film is promoted. As a result, a thin film having a high concentration ratio of nitrogen as compared with a thin film obtained by DC sputtering using the continuous wave can be obtained.

(30) Next, important factors constructing the sputtering method of the amorphous nitride thin film according to Embodiment 1 will be explained.

(31) The present inventors have considered that it is important to measure and grasp a period of time concerning generation and disappearance of nitrogen radicals which is a key of the reaction process for determining the pulse application conditions. A purpose of applying the power in the pulse state is to dissociate nitrogen and to increase the energy of sputtered particles sputtered from the target by reducing the period in which a pulse voltage is applied and increasing momentary effective power. A period in which the pulse voltage is not applied is time of migration in which sputtered particles reaching the substrate move to optimum positions and time for reaction with the nitrogen radicals at the same time. That is, in a case where sufficiently nitrided dense thin film is obtained, it is preferable that the period in which the pulse voltage is applied is reduced as short as possible and that the period in which the pulse voltage is not applied is taken sufficiently long. However, a film deposition rate becomes extremely slow in such control of the application period of the pulse voltage, which lacks practicability.

(32) Accordingly, the present inventors have derived actual film deposition conditions, particularly, how long the period in which the pulse voltage is not applied should be secured by previously measuring timing at which nitrogen radicals are generated and how long it takes until the nitrogen radicals disappear.

(33) FIG. 2 is a schematic view showing a sputtering process according to Embodiment 1 of the present disclosure. FIG. 2 schematically shows a state of a plasma space at the moment when the pulse voltage is applied. As the high power is momentarily applied, nitrogen molecules 32 in plasma are decomposed and activated to thereby generate atomic nitrogen, nitrogen radicals 34 and nitrogen ions 33. Among them, the nitrogen ions 33 are allowed to collide with the surface of the target material 7 due to a sheath potential gradient of plasma and sputtered silicon particles 31 as component atoms of the target material 7 are sputtered. Part of the ejected sputtered silicon particles 31 reaches the surface of the substrate 6 or the surface of a thin film already deposited on the substrate while having high energy. After that, the nitrogen radicals 34 collide and diffuse continuously after the pulse signal is turned off and react with silicon atoms reaching on the surface of the substrate 6 or the surface of the thin film already deposited on the substrate to thereby form silicon nitride. A silicon nitride thin film in which silicon atoms 35 and nitrogen atoms 36 are densely arranged is formed on the substrate 6 by the repetition of the above. In the reaction process, the nitrogen radicals 34 are reduced after the pulse signal is turned off due to combination with silicon or recombination with nitrogen radicals to each other, and then disappear. As the nitriding reaction does not occur after the nitrogen radicals disappear, to take the period in which the pulse voltage is not applied only decreases productivity.

(34) Hereinafter, examination results based on measurement results concerning generation and disappearance of nitrogen radicals in the above sputtering process will be shown below.

(35) FIG. 3 shows results obtained by measuring generation and disappearance of nitrogen radicals. Si was used as the target material, the nitrogen gas was used as the gas and the argon gas was used for comparison. As pulse conditions, a frequency was 10 kHz and a duty ratio at which the power is applied was 5%. Temporal changes in light emitting intensity in plural wavelengths by using a gate ICCD camera and a precision spectroscope manufactured by Hamamatsu Photonics K.K. were measured. Here, the duty ratio in the present disclosure indicates a ratio of the period when the pulse is on (On-period) in a period of one pulse period. That is, 5% at the frequency of 10 kHz means 5% in 100 microseconds as one period, which corresponds to 5 microseconds.

(36) In FIG. 3, the horizontal axis represents time from the moment when the pulse is applied (time after pulse-on) [unit: microsecond] and the vertical axis represents light emitting intensity in respective wavelengths (Intensity [normalized]). In the light emitting intensity in the vertical axis, normalization was performed by setting the maximum point to “1”. In FIG. 3, a bold solid line represents a wavelength indicating a light emission 37 as a first positive of nitrogen, which corresponds to 61.8 nanometers. This is a characteristic light emitted when nitrogen radicals are recombined and returned to nitrogen molecules, and increase and decrease in amount of nitrogen radicals can be evaluated by this light emitting intensity. Similarly, a broken line in FIG. 3 represents a wavelength indicating a light emission 38 of silicon ions, which corresponds to 568.0 nanometers. This indicates an amount of ions sputtered from the target material and ionized in plasma. A thin solid line in FIG. 3 represents a wavelength indicating a light emission 39 of argon ions, which corresponds to 501. 5 nanometers.

(37) The pulse voltage is applied at a point that the horizontal axis=0 and is turned off after 5 seconds. The light emission 38 of silicon ions and the light emission 39 of argon ions exceed the peak and begin to decrease at the same time when the pulse is turned off, which are not capable of being observed after several microseconds pass. On the other hand, it is found that the light emission 37 as the first positive of nitrogen increases for 1 microsecond after the pulse is turned off and decreases after that. It is found that the reduction is gradual and the light emission 37 continues to exist until observation becomes difficult at the vicinity of 50 microseconds. The fact that the light emission 37 as the first positive of the nitrogen exists even after the pulse is turned off indicates that nitrogen radicals exist in the space and nitriding reaction of silicon proceeds on the substrate. In other words, it is preferable to determine the pulse period by referring to the existing period of nitrogen radicals.

(38) According to the above measurement results, it is necessary to secure at least 20 microseconds during which nitrogen radicals become 20%, that is, 15 microseconds after the pulse is turned off as a period in which the pulse voltage is not applied. It is further preferable to secure 50 microseconds during which nitrogen radicals are not observed, that is, 45 microseconds after the pulse is turned off as the period in which the pulse voltage is not applied. Considering control stability of the apparatus, a period of approximately 100 microseconds may be suitable as a long period of one pulse period.

(39) That is, a period of one pulse is set to a range of 20 microseconds or more to 100 microseconds or less. That is, the pulse frequency is set to 10 kHz or more to 50 kHz or less. Further, it is preferable that the period of one pulse is set to a range of 20 microseconds or more to 50 microseconds or less. That is, it is preferable that the pulse frequency is set to 20 kHz or more to 50 kHz or less.

(40) In the sputtering method according to Embodiment 1, at least a period of 15 microseconds or more is necessary as the period in which the pulse is not applied in one pulse period. Then, a period of 15 microseconds or more to 45 microseconds or less is more preferable as the period in which the pulse is not applied in one pulse period.

(41) It is desirable that the ratio of the period in which the power is applied in one period (duty ratio) is a short period of time for achieving the object of applying high power momentarily as described above. However, in a point in which the duty ratio is less than 0.1%, the power is in the middle of rising and the period is insufficient for reaching the set power. From the vicinity of a point in which the duty ratio exceeds 30%, the above-described atomic nitrogen or nitrogen in the radical state formed due to dissociation of the nitrogen gas is reduced, and when the duty radio becomes approximately 50%, nitrogen is in approximately the same state as normal DC sputtering. Therefore, the ratio of the period in which power is applied in one cycle is preferably in a range of 0.1% or more to 30% or less.

(42) Next, experimental results obtained by performing examination of the duty ratio for improving the film deposition rate are shown in FIG. 4. In this experiment, Si was used as the target material and only the nitrogen gas was used as the gas. Conditions of the pulse signal, a frequency was 10 kHz and a duty ratio at which the power is applied was 5% to 30%. A pressure was 0.3 Pa. In the graph of FIG. 4, the horizontal axis represents duty radio [%] and the vertical axis represents film deposition rate [nm/min]. As shown in the graph of FIG. 4, it is found that the film deposition rate is improved by increasing the duty ratio. On the other hand, it is found that it is not so effective when the duty radio is increased to 30 or more according to extrapolation lines in FIG. 4. According to the above, a range from 20% or more to 30% or less in duty ratio is effective concerning the film deposition rate.

(43) Results obtained by observing refractive indexes of thin films fabricated in the above experiment are shown in FIG. 5. The refractive index is widely used as an index indicating the density of a thin film which is the most effective and can be measured easily. In a case where composition of the thin film is not largely deviated, it can be determined that a film having a higher refractive index is denser. In a case of silicon nitride, films having refractive indexes in the vicinity of 2.02 to 2.03 are preferable.

(44) In FIG. 5, the horizontal axis represents duty ratio [%] and the vertical axis represents refractive index of thin films. In the graph of FIG. 5, all films formed with different sputtering pressures are shown. According to the graph of FIG. 5, films with lower density are formed with increase in duty ratio in a condition in which the sputtering pressure is 0.5 Pa or more. On the other hand, in a condition in which the sputtering pressure is 0.3 Pa or less, it is found that quality is maintained when the duty ratio is increased. Combining the results with the above results of FIG. 4, it is found that the film deposition rate can be improved while maintaining the quality of the density of thin films by increasing the duty ratio in the condition that the sputtering pressure is 0.3 Pa or less. The lowest sputtering pressure in the experimental results was 0.1 Pa. In a condition that the sputtering pressure is lower than 0.1 Pa, plasma discharge became unstable and it was difficult to perform stable sputtering film deposition.

(45) Here, concerning the frequency of the pulse signal in the pulse sputtering, it has been found that plasma discharge becomes extremely unstable on a low frequency side, for example, in a condition that the frequency is lower than 1 kHz, and that one cycle becomes approximately 10 microseconds and it is difficult to decrease the duty ratio to a desired value due to constraints of a power supply device on a high frequency side, for example, in a condition that the frequency exceeds 100 kHz in the examination by the present inventors. Accordingly, it can be considered that a suitable range of the frequency of the pulse signal is 1 kHz or more to 100 kHz or less.

(46) In the sputtering method of the amorphous nitride thin film according to the present disclosure, the target material contains a material of at least one kind of tantalum, niobium, chromium, aluminum and titanium.

(47) As described above, according to the present disclosure, generation and disappearance of nitrogen radicals in plasma as the important factors are grasped on the time base in the formation of the nitride thin film by the sputtering method, thereby realizing improvement in film deposition rate while securing active species and reaction time for nitriding reaction. Therefore, the formation of the protective film securing high reliability of electronic components can be realized with high quality and at a low cost. Accordingly, use of hazardous gases and the detoxifying apparatus or safety measures for them are not necessary, and high-quality nitride protective film can be easily formed also at places other than semiconductor factories.

(48) It is difficult to fabricate the high-density film of the silicon nitride film as the protective film for securing reliability in electronic components and the like, and the film deposition rate is extremely slow in the related-art sputtering method, which are problems on practical use as described above. The present disclosure provides a breakthrough in a point that the behavior of nitrogen radicals as the most important factors for forming silicon nitride is observed and film deposition conditions are determined based on the behavior including the viewpoint of productivity.

(49) In the sputtering method according to the present disclosure, the voltage is applied to the target in the pulse state for generating nitrogen radicals at high concentration, and invisible behavior such as the generation amount and existing time of nitrogen radicals which is extremely important for forming the nitride thin film is measured and grasped in detail, then, the results are reflected on film deposition conditions, thereby positively realizing nitriding reaction which is difficult in normal sputtering and stably forming the nitride thin film having components extremely close to a stoichiometric ratio at high speed by considering parameters concerning the film deposition rate within a range satisfying the above conditions.

(50) The sputtering method according to the present disclosure allows the formation of the protective film formed of the nitride thin film which realizes high reliability suitable for electronic components by the apparatus having a simple structure, therefore, use of hazardous gases and expensive utilities such as the detoxifying apparatus relating to the gases are not necessary, which is effective as a method of forming a highly-functional inorganic protective film.