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PLASMA ETCHING PROCESS FOR MAKING A MICROCIRCUIT DEVICE
REFERENCE TO COPENDING APPLICATION
This application is a continuation of application Ser. No. 466,717 filed May 3, 1974, now abandoned which is a continuation-in-part of U.S. patent application Ser. No. 417,542 filed Nov. 20, 1973, now abandoned.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of making a microcircuit device, and more particularly to a method of making a semiconductor device including a process of patterning electrodes and wiring by plasma etching.
2. Description of the Prior Art
In semiconductor devices such as a transistor, an integrated circuit, a large-scale integrated circuit and so on, electrodes or wiring is mostly formed of aluminum (Al) but sometimes formed of platinum (Pt), gold (Au), titanium (Ti), molybdenum (Mo) or the like, too. For patterning such a metal film material, a liquid etching solution employing an etchant is usually employed. In these processes using tungsten (W), a heated aqueous solution of potassium hydroxide or sodium hydroxide, HN03—HF mixed solution, a heated aqueous solution of hydrogen peroxide or the like may be employed as an etchant. However, when using an etching mask formed of a photoresist material, there are instances where the photoresist material vanishes or peels off during etching, so that accurate patterning is difficult.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a novel method of making a microcircuit device which is free from the aforesaid defect experienced in the prior art and capable of accurate patterning of electrodes and wiring of a microcircuit device in the case of using a reactive metal, such as tungsten, molybdenum, or the like.
In the method of making a microcircuit device according to this invention, the improvement comprises the steps of providing a mask on a metal layer for ultimately forming electrodes and wiring on a substrate, contacting the substrate in a reaction chamber with a gas of a halogen compound maintained in plasma state with a high-frequency electromagnetic field. This halogenates the non-masked or exposed portions of the metal layer on the substrate. The halogenated areas of the metal are removed, as by sublimation, evaporation or washing with water or a suitable solvent thus patterning the electrodes and wiring.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a preferred example of manufacturing apparatus for practicing a method of making a semiconductor device according to this invention;
FIGS. 2 to 5, inclusive, are vertical cross-sectional views of a semiconductor substrate, showing successive steps involved in producing a semiconductor device in accordance with this invention;
FIG. 6 is a graph showing the relationship between the etching time and the etching depth for tungsten metal;
FIG. 7 is a graph showing the relationship between the pressure in a reaction furnace and the etching rate in connection with tungsten;
FIG. 8 is a graph showing the relationship between 5 the pressure in the reaction furnace and the etching rate in the case of using molybdenum;
FIGS. 9A and 9B are graphs showing the distribution of the etching rate in the surface of a water formed of tungsten; and
10 FIG. 10 is a graph showing characteristic curves of variations in the flat band voltage VFB of a tungsten gas MOS diode produced by liquid etching and that produced by the plasma etching method of this invention.
., DESCRIPTION OF THE PREFERRED
FIG. 1 schematically shows an example of apparatus for use with this invention method. Semiconductor substrates 2 are disposed in a reaction chamber 1; the
20 reaction chamber 1 is evacuated by means of a vacuum pump 5 to remove air; a voltage is impressed to electrodes 3a and 3b or a coil connected to a high-frequency, high-tension power source 4; and a halogen compound, for example, dichlorodifluoromethane
25 (Freon-12, CC12F2), contained in a container 7 is supplied into the reaction chamber 1 together with a gas stream of argon (Ar), helium (He) or other inert carrier gas. Volatile halogenated organic compounds may also be supplied alone or mixtures of such compounds
30 may be used. The preferred halogenated materials include perhalogenated organic compounds, such as trichlorofluoromethane, dichlorodifluoromethane, or other volatile organic compounds containing halogen atoms having an atomic number between 9 and 35,
35 especially chlorine and fluorine atoms; however, brominated compounds, such as CHBr3, CH2BR2 or CH3Br, may also be used. While iodide gas materials may be operable, the metal iodides possess high boiling points and do not appear practical. Inorganic halides
40 such as SiCl4 may also be used as a source of reactive halogen atoms. Selection of the halogenated gas depends upon the nature of the reactive metal.
The gas is maintained in a plasma state due to a highfrequency electromagnetic field established in the reac
45 tion chamber 1, and the plasma thus generated reacts with metal, thereby converting the exposed metal to a metal halide.
A suitable reaction chamber may be constructed according to FIG. 1 wherein a quartz tube 105 mm
50 (i.d.) by 310 mm in length is provided with a copper coil of 8 mm wire having a coil diameter of 130 mm. In the examples, a coil of 13 turns was formed over a length of 250 mm. A suitable RF power supply provides a current of 180 raA at 1200 V and a frequency of
55 13.56 MHz. This obtains an etching rate of about 480 A/min at 180 mA for tungsten. In the preferred embodiments of the invention, the reaction chamber is constructed to permit etching rates greater than 250 A/min. Since the plasma state is maintained at more
60 than 10,000° C, the chamber material is selected to withstand reaction conditions.
A suitable range of pressure for maintaining the plasma state is about 0.10 to 0.55 mm Hg (absolute pressure). The feed rate for the halogenated gas is
65 maintained to provide a stoichiometric excess of the desired halogen atom in the reaction chamber. Optimum operating conditions for CC12F2, for instance, were obtained using a constant flow rate of about 2
bp(° c) mp(° c)
cc/min. under standard gas conditions (1 at m and room temp.), where the plasma was maintained at 0.17 mm Hg.
Where the vapor pressure of the metal halide is high, sublimation or evaporation of the halide takes place following plasma reaction to vapor etch the metal. An inert photoresist material reacts very little with the halogenated organic compound in the plasma state. Even if a small reaction occurs, the photoresist would not be etched away since its vapor pressure is low, compared to the metal halide. Further, silicon (Si) forming the semiconductor substrate 2 and silicon dioxide (Si02) or silicon nitride (Si3N4) forming insulating films are not etched substantially.
The boiling points (bp) and the melting points (mp) of various metal halides are shown in the following tables. The halides of high vapor pressure at room temperature are shown in Table 1 and those of low vapor pressure are shown in Table 2.
WF„ 17.5 2.5
MoF, 35.0 17
ReF, 47.6 18.8
OsF„ 47.5 34.4
IrF, 53 44
A1CI, 180.2 (solid phase) 190
AlBr, 255 99.5
TiCl4 136.4 30
TiF, 285 —
TiBr, 23Q 390
VF, 112 (758 mmHg) —
NbF, 229 75.5
MoCU 268 194
TaCl, 242 217
TeFj 229 97
RuF„ 250 101
In the case of using the halides of high boiling point, removal in vapor phase can be achieved by raising the temperature of the semiconductor 2.
FIGS. 2 to 5 show a sequence of steps involved in the manufacture of a semiconductor device in accordance with one example of this invention. An insulating film 12 as of silicon dioxide (Si02) or silicon nitride (Si3N4) is formed on a silicon semiconductor substrate 11 by a known process and the insulating film 12 is selectively removed to form windows therein. A tungsten (W) layer 13 is then formed uniformly over the entire area of the insulating film 12 and the exposed area of the substrate 11, as by chemical vapor deposition, evaporation, sputtering or the like, as depicted in FIG. 2. The insulating film 12 and the tungsten layer 13 may be formed to desired thicknesses of 2000 to 5000 and about 10,000 A (1/u.) respectively. Optimum thickness for Si02 is about 5000A, for Si3N4 about 3000 A.
Then a photoresist layer 14 is formed over the entire area of the tungsten layer 13. This is followed by patterning of the photoresist layer 14, by which the photoresist layer 14 is removed at selected-locations to form windows therein through which the underlying tungsten layer 13 is exposed, as illustrated in FIG. 3. The photoresist layer 14 serves as a vapor-phase etching mask in the subsequent step of the inventive process.
Next, as described previously with regard to FIG. 1, the semiconductor substrate is placed in the reaction
chamber 1, the aforementioned dichlorodifluoromethane is introduced into the reaction chamber 1 as a gas, and a high-frequency voltage is impressed to the electrodes 3a and 3b or the coil to generate a plasma. The 5 exposed areas of the tungsten layer 13 are halogenated into tungsten hexafluoride (WF6). The tungsten hexafluoride is volatile at room temperature, and hence is sublimited or evaporated. This removal step forms windows to expose therethrough the insulating layer 12 10 at selected locations, as depicted in FIG. 4. In this manner, the vapor etching is achieved.
Thereafter, the photoresist layer 14 remaining on the tungsten layer 13 is removed as illustrated in FIG. 5. In this case, since the photoresist is organic matter, the layer 14 can be removed by introducing oxygen (02) into the reaction chamber 1 in place of the halogenated gas.
Thus, patterning of tungsten, which is chemically 20 stable in most environments and difficult to melt, can be carried out with great accuracy.
Although the foregoing example has been described in connection with tungsten (W) it is possible to effect patterning in evenly deposited films of metals such as 25 aluminum (Al), molybdenum (Mo), titanium (Ti), etc., employed as electrically conductive materials for electrodes or wiring of semiconductor devices. Aluminum is difficult to convert to its fluoride (A1F3) and not easy to etch in this form. Therefore, it is preferred to con30 vert aluminum metal to another halide form, such as the chloride (A1C13).
Further, since halides of gold (Au), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), platinum (Pt), or the like are soluble in water, an aqueous solution of 35 acid or organic liquid or react therewith, it is also possible to etch away the non-masked or exposed metal layer by dissolving the metal halide in a liquid solvent, such as water or the like after converting the metal layer into a halide of the metal as previously described. 40 FIG. 6 shows the relationship between the time for etching tungsten according to the method of this invention described above and the etching depth in the case where the pressure in the reaction chamber 1 shown in FIG. 1 is 0.17 mmHg, a current flowing in the coil 45 supplied with the high-frequency voltage being used as a parameter. For example, where the coil current was 160 mA, the tungsten layer was etched away to a depth of 3500 A in 10 minutes. Further, where the coil cur5Q rent was 120 mA, the tungsten layer was etched away to a depth of 2500 A in 10 minutes. With reduced coil current to lower the intensity of the high-frequency magnetic field, the intensity of plasma is lowered to decrease the etching speed. Further, it has been found 55 that the etching depth increases linearly with respect to time.
FIG. 7 shows the relationship between the pressure (in mm Hg) in the reaction chamber 1 and the etching rate (in A/min) in connection with tungsten, coil cur
60 rent being the parameter. The etching rate tends to drop with increase in pressure and with decrease in the coil current. It is believed that if the pressure is lowered out of the given range, the etching rate rapidly decreases below a certain value.
65 The optimum etching rate can be determined for each reactive metal and mask material, usually above 250 A/min. Higher rates are not always favorable, because organic photoresistant materials may react unde