HIGH SPEED DRY ETCHING METHOD

Abstract: A high speed dry etching method using a magnetron discharge is provided in which a reactive gas whose molecules contain at least fluorine atom and hydrogen atom, such as CHF₃, is introduced and ionized by the magnetron discharge. A sample, for exmaple, a silicon wafer covered with a silicon dioxide film, is subjected to the ionized gas in order to be etched.

Description: High speed dry etching method This invention relates to a method of high-speed dry etching for use in the manufacture of electrical parts such as semiconductor integrated circuits. The miniaturization of integrated circuits has made such rapid progress in the last few years that recently very large scale integrated circuits (VLSI's) have been developed with dimensions of 1-2 um. For such minaturization, a method that has attracted attention is the Reactive Ion Etching method or RIE, wherein a reactive gas, such as CF4 or CUR4, is introduced into a vacuum vessel having parallel-plate type electrodes; glow discharge is produced by applying high frequency electric power to the electrodes on one of which the sample to be etched has been placed, and etching is effected by accelerating the positive ions in the plasma by means of the negative DC self-bias (cathode voltage drop) produced at this electrode, irradiating the sample vertically. In this RIE method, however, when etching SiO2 using CF4 + H2 gas, for example, the etching rate was at most 300 - 400 8/mien, which meant that it took twenty to thirty minutes to etch a 1 pm SiO2 film. Etching speed was also slow in the case of AZ and poly-Si, and the method is ill-adpated for mass production. It is known that increasing the operating current is effective, but this brings with it the danger of an increase in the cathode voltage drop, with the risk of ion damage and a decrease in the etching selectivity ratio. Another limitation was that the resist used to mask the surface reached a high temperature and became subject to deformation. The present inventors previously developed a highspeed RIE method for etching monocrystalline Si, SiO2, AQ, Mo, W and poly-Si without any risk of ion damage which is described in full detail in U.S. Patent Application Serial No. 324,663, filed by the same inventors as those of the present invention. In this method a magnetron discharge is used for etching by applying a magnetic field perpendicular to an electric field between the parallel electrodes within a reaction chamber where a reactive gas, CF4 mixed with H2, is introduced for ionization. According the parent application, when a sample is to be etched, a sample pallet made of a nonmagnetic material, for example, carbon or alumina, is placed on a pole piece. The sample is placed on the sample pallet. The inside of the reaction chamber is evacuated by opening a vacuum sealing valve, and an etchant gas is supplied from a gas inlet port. High frequency power is supplied to the cathode. By this power, glow discharge is generated between the electrodes so that a plasma is produced from the etchant gas. A cathode voltage drop (negative current self bias) is induced at the cathode. In this condition, in the vicinity of the discharge gap, a magnetic flux perpendicular to the electric field is induced. Therefore, electrons in the vicinity of the field drift in the direction E x B so that the electrons are trapped in a helical drift track in the vicinity of the discharge gap. The ionizing efficiency of the etchant gas in the vicinity above the discharge gap greatly increases in comparison with the surrounding space. As a result, in comparison with the conventional DC self bias voltage, the present invention requires low power for high plasma density. This principle of operation is the basis of high speed etching. This method enables etching to be done at high rates such as more than 6,000 admin for SiO2. The method, however, has a shortcoming in that the etching selectivity ratio of SiO2/Si (the ratio of the etching rate of SiO2 to that of Si) is so low that detection of the end point of etching is difficult when etching, for example, an SiO2 layer on a Si substrate. Another problem is that in some areas on the surface of the sample to be etched a glow discharge condition arises, which results in accumulation of a polymer film on the surface. It is therefore an object of the invention to provide an improved method of high-speed dry etching for use in the manufacture of products such as semiconductor devices. It is a specific object of the invention to provide an improved method of high-speed dry etching having a high etching selectivity ratio of SiO2/Si for use in the manufacture of semiconductor devices such as integrated circuits. It is another specific object of the invention to provide an improved method of high-speed dry etching without polymer accumulation on the surface of the material to be etched. According to the present invention a method of high-speed dry etching is provided in which a reactive gas, such as CHF3, whose molecules contain atoms of at least fluorine (F) and hydrogen (H), is introduced into a decompression vessel. A magnetron discharge is generated in the area between parallel electrodes in the vessel by applying to the electrodes an electric field and a magnetic field which are perpendicular to each other, thereby ionizing the reactive gas through the reaction between electrons in drift motion and the gas molecules. The sample to be etched is then subjected to the reactive ions thus generated. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a perspective view showing an apparatus for dry etching used in an embodiment of this invention; Fig. 2 is a schematic diagram of the apparatus shown in Fig. 1; Fig. 3 is a perspective view of a magnet used in the apparatus shown in Fig. 2; Fig. 4 is a graph showing etching characteristics of silicon dioxide (SiO2) in the method according to the present invention; Fig. 5 is a graph showing etching characteristics of silicon dioxide (SiO2) in the conventional method using CF4 gas mixed with H2 gas for comparison with the method of the present invention; Fig. 6 is a plan view showing another example of a magnet assembly which may be used in the method according to the present invention; and Fig. 7 is a sectional view taken on the line 7 - 7 of Fig. 6. Figs. 1 and 2 show an apparatus used in the preferred embodiment of the method according to the present invention. A decompression vessel 11, made of stainless steel, is provided having a cover plate 12, also made of stainless steel, which seals an opening in the vessel 11. The vessel 11 has an exhaust outlet 13 in a side wall connected to an exhaust system including an exhaust pump 14 (such as diffusion pump), and a driving motor 15. The vessel 11 also has a gas inlet 16 in a side wall connected to a gas supply (not shown). Within the vessel 11, a cathode plate 17, made of copper, is provided in parallel relationship with the cover plate 12 of the vessel 11. This cathode plate 17 is supported by a shield wall 18, made of an insulating material such as teflon, forming a shield room 19 having a bottom wall l9a fixed on the bottom of the decompression vessel 11. The shield room 19 is provided with several holes 20 in the shield wall 18 for communicating with the rest of the decompression vessel 11. Electrically conductive pipes 21 are provided for supplying cooling fluid, such as water, through a conduit network passageway flowpass 17a formed in the cathode plate 17 and for applying thereto a high frequency voltage from an RF power source 22 through an impedance matching circuit 23. The pipes 21 are connected at one end to the cathode plate 17 and at the other end to the matching circuit 23 and a fluid supply source (not shown) outside the decompression vessel 11. The pipes 21 are insulated by an insulator 24 from the bottom of the vessel 11. The cover plate 12 is electrically connected to ground 25 and forms the anode. On the cathode plate 17 there is provided a non-magnetized carbon plate 26 on which a sample 27 to be etched is mounted. A magnet assembly 28 is installed under the cathode plate 17 in the shield room 19 which prevents the electric discharge from entering. The assembly 28 comprises, as shown in Fig. 3, first and second samarium-cobalt (Sm-Co) permanent magnets 29 and 30 each having a rectangular cross section, one of which (29) is surrounded by the other one (30). The first and second magnets 29 and 30 are mounted on a ferromagnetic substrate 31 such as soft iron which forms part of a magnetic circuit between the first and second permanent magnets. Fig. 2 includes a cross sectional view of these magnets taken along line I-I of Fig. 3. As shown in Fig. 2, the magnetic flux B flowing from the north pole of the first magnet 29 into the surrounding south pole of the second magnet 30 is parallel to the anode plate 12 and the cathode plate 17, within the area between the plates. This magnetic flux B corresponds to the air gap between the first magnet 29 and the second magnet 30. The substrate 31 supporting the magnets 29 and 30 is coupled to a rotating screw rod 33 by a nut 31a fixed on the lower side of the substrate 31. The rod 33 is rotatably received by journals 18a in the shield walls 18 and a journal lla in the side wall of vessel 11. The screw rod 33 is driven by a motor 34 which alternately rotates the rod 33 clockwise and counterclockwise so that the magnets 29 and 30 move back and forth along the horizontal screw rod 33. The substrate 31 supporting the magnets 29 and 30 are supported by arms 35 guided along the horizontal guide rod 36 fixed on the bottom wall 19a of the shield room 19. In Fig. 1, 37 is a window in the cover 12 of the vessel 11 and 38 is a handle on the cover. There are also provided in the side wall of the vessel another window 39 and a vacuum gauge 40 for measuring the pressure in the decompression vessel 11. The etching process is carried out using this apparatus as follows. The decompression chamber 11 is exhausted to less than 10-4 Torr by means of the exhaust mechanism comprising exhaust outlet 13, exhaust pump 14 and motor 15. The shield room 19 enclosing the magnet assembly 28 is also evacuated through the air holes 20 in the side wall 18. When a pressure of less than 10-4 Torr has been reaches, a reactive halogen gas such as CHF3, which has been mixed with H2 gas, is introduced from the gas inlet 16 so that the pressure is controlled to a level of 10-4 to 10-2 Torr. When high-frequency power (13.56 MHz and about 1.6 W/cm2 power density, for example) is applied from the RF power source 22 to the cathode plate 17, a glow discharge is generated in the area 10 between the electrodes 12 and 17. At the same time, a magnetron discharge is produced in the area 32 above the cathode plate 17 by the orthogonally intersecting electric and magnetic fields E x B. More specifically, because of the crossing electric field and magnetic flux, the former, E, due to the self-bias voltage and the latter, B, due to the magnets 29 and 30, free electrons in the neighborhood of the magnets begin to move in cycloidal paths and collide with molecules of the introduced reaction gas resulting in a high efficiency of ionization of the gas. This ionization efficiency is about 10-30 % which is ten times as high as the efficiency in a conventional glow discharge. Since the magnetron discharge is generated only in the limited area 32 along the air gap between the first and second magnets, 29 and 30, the magnet assembly 28 is moved to scan the surface of the cathode plate 17. A uniform magnetron discharge is thus generated over the sample 27 mounted on the cathode plate 17. The ionized reactive gas thus generated by the plasma discharge contacts the sample 27 on the cathode plate 17 and performs high-speed etching. Fig. 4 shows the result of etching of silicon (Si) and silicon dioxide (Si62) when CHF3 is used, with H2 added, as the introduced gas. The sample 27 was a wafer of Si on which an SiO2 film was formed. The amount of high frequency electric power from the source 22 was 300 W, the CHF3 flow rate 20 Sccm, and the etching pressure 0.01 Torr. The horizontal axis X of Fig. 4 represents the flow rate (in Sccm) of H2 gas and the vertical axes Y1-Y3 represent etching rate (8/mien), etching selectivity ratio of SiO2/Si, and polymer accumulation rate respectively. Curves (a) and (b) represent the etching rate of SiO2 and Si respectively, and curve (c) represents the selectivity ratio of Sio2/Si. For experimental purposes, these values were measured with the magnet assembly 28 stationary. As the graph shows, the SiO2 etch rate (a) was as high as about 5,800 admin when H2 gas was not added but the Si etching rate (b) was also as high as about 500 admin and the SiO2/Si selectivity ratio was as low as about 12. SiO2 etching rate (a) slowly decreased as H2 was added and since the Si etching rate (b) was relatively low and decreased more slowly than the SiO2 selectivity etching, the SiO2/Si selectivity ratio (c) increased very rapidly. For example, the Si etching rate fell to about 10 admin at an H2 flow rate of 20 Sccm, and a very high value of over 40 was obtained for the selectivity ratio (c). The Si etching rate (b) continued decreasing below zero when H2 was added at flow rate over 20 Sccm. This means that a polymer film accumulated on the surface of the Si wafer instead of Si being etched away. In the preferred embodiment, therefore, the H2 flow rata should be less than 20 Sccm to etch the wafer. Fig. 5 shows the result of etching of Si and SiO2 when CF4 was used, with H2 added, as the introduced gas in accordance with our previous invention disclosed in U.S. Patent Application Serail No. 324,663, filed November 24, 1981, for the purpose of comparison with the etching characteristics in the present invention. The axes X, Y1 to Y3 and curves (a) to (c) correspond to those in Fig. 4. The CF4 flow rate was also 20 Sccm; and other conditions such as the etching pressure and power of the RF source were the same as those in Fig. 4. As the graph shows, the SiO2 etching rate (a) decreased from about 7,500 A/min as H2 was added, and since the Si etching rate (b) also decreased, the Sio2/Si selectivity ratio (c) increased very slowly. For example, the selectively ratio is below 10 at an H2 flow rate of 30 Sccm, and even Si was etched to the extent of 700-800 A. This necessitates an accurate and highly reliable method of detection of the end point of etching. On the contrary, the method according to the present invention, using a gas whose molecules contain atoms of at least F and H, makes it easier to detect the end point of etching since the Si etching rate (b) in Fig. 4 is much lower than that in Fig. 5 and the SiO2/Si selectivity ratio (c) in Fig. 4 is much higher than that in Fig. 5. This is advantageous when etching SiO2 with a Si mask over the SiO2 and when carrying out high-speed and accurate etching. In particular, time is gained for detection of the etching end point when selectively removing a SiO2 film laminated on to a Si water, using a resist pattern as a mask. In Fig. 5, curve (d) shows the etching rate of Si in the regions other than the gaps between the first and second magnets 29 and 30 where the magnetic flux density B (orthogonally crossing the electric field E) is relatively low. In this region a glow discharge occurred instead of magnetron discharge. Curve (d), together with curves (a) through (c), was measured when the magnet assembly 28 was stopped, in order to clarify the phenomena occurring in the discharge areas. It is understood from curve (d) that the Si etching rate was different from area to area on the surface of the wafer, and a polymer film accumulated on the surface when the H2 gas flow rate exceeded about 16 Sccm. From this experimental comparison it is clear that another advantage of the present invention is that polymr accumulation on the surface of the sample is avoided, thereby retaining a high etching rate and high selectivity ratio of the sample to be etched. It should be noted that the scanning of the magnet assembly is useful to obtain a uniform and high quality etching since the magnetron discharge area is concentrated in the gap region of the magnets. The reason why the method according to the present invention is superior to the former method in the selectivity ratio of SiO2/Si and polymer accumulation is not certain. It is, however, supposed from the experiments that the reactive gas whose molecules contain fluorine (F) and hydrogen (H) effectively controls the etching speed of Si. More specifically, in the former magnetron discharge method using CF3 gas there was a very high gas dissociation efficiency, with the result that a large quantity of F radicals are produced by dissociation of CF4 gas, and these act as an etching starter for Si. Thus the F radicals have to be removed from the system. This was achieved by converting them to HF by mixing with a large quantity of H2. A high H2 flow rate, however, decreased the SiO2 etching rate and, thus, high-speed etching could not be achieved. In the method according to the present invention, on the contrary, generation of the F radicals is controlled by the H contained in the reactive gas itself, as well as by the H2 gas added. It seems that the magnetron discharge of the present invention generates almost the same amount of F radicals as those generated in the conventional glow discharge. It should be noted that there will be various modifications within the scope of the invention. The preferred embodiment described above has dealt with the case of a Sino2 film as a sample to be etched; however, a silicon oxide film including phosphorous (P) or boron (B) may be etched using this method, as may be a film of silicon nitride. Again, the results described above are not limited to single-crystal silicon in the Si wafer, but can be achieved also with poly-crystalline or amorphous silicon. Although CHF3 gas was mentioned above, the invention is not restricted to the use of this gas. Any gases whose molecules contain at least hydrogen atom and fluoride atom, such as HF, gas can be used. Again, H2 gas was used to reduce the amount of F radicals, but hydrocarbon gases, such as C2H4, can be used instead of the H2 gas. The magnet assembly with first and second magnets can be replaced by magnets having more complicated configurations as shown in Figs. 6 and 7. A first magnet 41 is bent into a U-shaped and the second magnet 42 is interdigitally engaged with the first magnet 41. This type of magnet assembly provides longer gap region 43 for the magnetron discharge. Although illustrative embodiments of the present invention have been described in detail with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes or modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Claims: Claims: 1. A method of dry etching the surface of a sample in a decompression vessel introduced with a reactive gas wherein ions of the reactive gas is irradiated on the sample to effect the etching, which is characterized in comprising the steps of: introducing into the vessel a reactive gas having molecules comprising atoms of fluorine and hydrogen; generating a magnetron discharge within the vessel to ionize the reactive gas; and exposing the surface of the sample to the ionized gas. 2. The method of claim 1, wherein the vessel contains a pair of spaced electrodes and the step of generating a magnetron discharge comprises applying orthogonal electric and magnetic fields to the electrodes. 3. The method of claim 2, wherein the reactive gas is CHF3. 4. The method of claim 3, wherein the CHF3 gas is mixed with H2 gas. 5. The method of claim 2, wherein the magnetic field is generated by a magnet assembly provided close to one of the electrodes in the decompression vessel. 6. The method of claim 5, wherein the magnet assembly is reciprocally moved along the surface of the electrode. 7. The method of claim 2, 3, or 4, wherein the sample is a silicon wafer having a surface of which a silicon dioxide film is formed.

Inventors: HORIIKE, YASUHIRO, OKANO, HARUO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1982
Main classification: H01L21
IPC codes: H01L21, H01J37