Apparatus and method for igniting plasma in a process module

Abstract: The invention provides apparatus and methods for improving systems that expose samples to reactive plasmas, and more particularly for igniting plasma within a process module. The systems are of the type which have an electrode pair (12, 14) and a radiofrequency generator (16) connected to one electrode (14). Gas is injected between the electrodes where it is ionized and transformed into a plasma. The invention includes (i) ignition means (62) for ionizing gas, e.g., silane, between electrodes which are separated by a small gap of less than approximately one centimeter; and (ii) a radiofrequency energy generator (16) that preferably operates at high frequencies, e.g., 60 MHz, to transform molecules into plasma. Several embodiments of ignition means are taught by the invention, including: an electron source, an ultraviolet source, a second radiofrequency energy generator, and radioactive sources, among others. A process module constructed according to the invention, using high frequency energy and small electrode separations, has a high rate of deposition and a high production yield.

Description: This invention relates to apparatus and methods for exposing samples to reactive plasmas. More particularly, the invention relates to apparatus and methods for igniting plasma within a process module that is adapted for use in Chemical Vapor Deposition (CVD) and in Plasma Enhanced Chemical Vapor Deposition (PECVD) processes. As used herein, a "sample" generically describes a substrate, such as a glass panel or a silicon wafer, which is suitable for deposition techniques, for example by PECVD. The sample typically has one surface that is treated to receive one or more film depositions. Vacuum deposition systems which deposit semiconducting or insulating films onto samples are well-known and are utilized in a wide range of scientific fields. Complex PECVD systems have been developed, for example, to manufacture devices such as thin film transistors (TFTs), liquid crystal displays (LCDs), flat panel displays (FPDs), solar cells, photodetectors, and integrated circuit structures. Typically, these systems include one or more evacuated process modules that are used to expose the sample to reactive plasmas. Such a process module typically includes first and second electrodes within a chamber and spaced apart to form a gap therebetween: one electrode is electrically grounded to the chamber, and the other electrode is connected to a radiofrequency (RF) source which generates a radiofrequency discharge within the gap. The module also has gas inlets and gas outlets which direct selected gases, such as silane, into and through the gap. To deposit a selected film on a sample, the sample is suspended in the gap spaced away from the active RF electrode, e.g., mounted to the grounded electrode, and a selected gas introduced into the inter-electrode gap. When the gas is exposed to the RF field, it ionizes and forms a reactive plasma which deposits a film onto surfaces exposed to the plasma, including the sample surface. The rate at which this film is deposited onto the sample surface is dependent upon several factors, including: the magnitude of the vacuum in the process module; the electrode spacing; the power and the frequency of the RF energy; and the gas flow rate. For practical reasons, many prior vacuum deposition systems utilize excitation frequencies of approximately 13.56MHz. However, it is known to be beneficial to deposit films at higher frequencies, since faster deposition rates are realized. U.S. Pat. No. 4,933,203, for example, discloses the deposition of hydrogenated amorphous silicon at frequencies from 30MHz to 150MHz, realizing a 500% to 1000% increase in deposition rates over those attained with 13.56MHz systems. Such increases correlate to improved manufacturing throughput and efficiencies. Nevertheless, further increases in deposition rates are sought to attain further improvements in manufacturing throughput and efficiency. One known technique for increasing deposition rates is to decrease the spacing between the two chamber electrodes. However, the reduction of the spacing is not without limit. U.S. Pat. No. 4,933,203 discloses, for example, that the spacing has a practical lower limit of ten millimeters because of certain phenomena. These phenomena include difficulties associated with igniting plasma at small electrode spacings. US 4 617 079 discloses an apparatus according to the preamble of claim 1. It is one object of this invention to provide an improved apparatus and an improved method for depositing films onto samples from a vapor. This object is achieved by an apparatus according to claim 1 and a method according to claim 17. resp. Further developments of the invention are given in the dependent claims. It is provided an PECVD apparatus and method which increase the deposition rate of films deposited on samples exposed to reactive plasmas. There are provided vacuum deposition systems that expose samples to reactive plasmas and attain a relatively high production yield. There are provided such apparatus and methods for exposing samples to reactive plasma and which are relatively low in cost and reliable in operation. There are provided apparatus and methods of the above character and which also can etch sample surfaces. The ionizing element ignites the plasma within the electrode gap and the radiofrequency energy generator afterwards transforms molecules within the module into plasma by generating RF energy at a primary frequency that is preferably 60 MHz. The module preferably comprises a gas that is injected between the electrodes. The gas is of a type that is suitable for ionization and that is suitable for transformation into a plasma when exposed to radiofrequency energy at the primary frequency. In accord with the invention, the gas is generally selected from gases such as silane (SiH 4 ), disilane (Si 2 H 6 ), hydrogen (H 2 ), ammonia (NH 3 ), phosphine (PH 3 ), nitrogen (N 2 ), nitrogen trifluoride (NF 3 ), helium (He), argon (Ar), carbon tetrafluoride (CF 4 ), hexafluoroethane (C 2 F 6 ), oxygen (O 2 ), nitrous oxide (N 2 O), methane (CH 4 ), borane (BH 3 ), diborane (B 2 H 6 ), chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), hydrogen chloride (HCl), carbon tetrachloride (CCl 4 ), hydrogen bromide (HBr), carbon dichloride difluoride (CCl 2 F 2 ), boron trichloride (BCl 3 ), silicon tetrachloride (SiCl 4 ), boron tribromide (BBr 3 ), chlorotrifluoride (ClF 3 ), fluorine (F 2 ), and mixtures thereof. To inject a gas between the electrodes, the improvements thus include, in other aspects, gas inlets and gas outlets, and associated pump mechanisms. The pump mechanisms pump gas through the gas inlet and into the gap, and extract gas and other molecules through the gas outlet as process waste. In another aspect, an electron source with a filament is provided as the ionizing element. The electron source is arranged to inject electrons between the electrodes at an energy that is preferably at least 300eV to ionize gas in the gap. In certain aspects, the electron source is spaced selectively at a distance from the gap to select the electron energy entering the gap. Alternatively, the source includes an extraction slit that has a selectively variable positive potential relative to the filament, for extracting electrons from the filament with energies of variable magnitude and greater than 300eV. In still another aspect, an ultraviolet light source is provided as the ionizing element which irradiates ultraviolet radiation between the electrodes to ionize gas in the gap. The ultraviolet light source preferably generates photons with energies that are greater than approximately five electron volts. In certain preferred aspects, the module includes an UV interface that is transmissive to the ultraviolet radiation generated by the UV source, so that the UV source generates photons which irradiate the gap through the UV interface. Such an ultraviolet light source is preferably located in standard atmosphere outside the module, which is evacuated, and is arranged to illuminate the vacuum of the evacuated module through the UV interface. In certain aspects, the ultraviolet light source includes (i) a reflector and an aperture which, in combination, collect the emitted ultraviolet radiation and transmit the radiation in a concentrated direction toward the gap in the module; and (ii) one or more optical elements which collect and collimate the transmitted concentrated radiation from the reflector to the within the gap. In still other aspects according to the invention, there is provided a sparking apparatus as the ionizing element. The sparking apparatus generates a spark or other electric discharge between the electrodes to ionize gas in the gap. Preferably, therefore, the sparking apparatus is isolated from the ground of the process chamber and has an exposed spark gap arranged with a line of sight from the spark gap to between the electrodes, such that an emitted spark at the spark gap injects electrons to within the gap. In yet another aspect of the invention, a second radiofrequency generator is connected to the second electrode. The second radiofrequency generator has a second frequency, preferably 400kHz, that is less than the primary frequency and that generates radiofrequency discharges between the electrodes at the second frequency to ionize gas in the gap. In certain preferred aspects, the second radiofrequency generator pulses the second frequency at a selected rate. It is also preferred, according to the invention, to inhibit the second radiofrequency generator once gas is ionized within the gap. The two radiofrequency generators typically operate in combination to generate a voltage between approximately 1kV and 5kV across the two electrodes. The process module may include a high direct current power source that is connected to the second electrode. Such a direct current power source generates a physical spark between the electrodes and accordingly ionizes gas in the gap. In another aspect, a process module may include evacuation apparatus, e.g., a pump and/or compressor, that evacuates the module selectively to approximately 0,0133 mbar (0.01 Torr). Preferably, such a process module also includes pressurizing apparatus, e.g., a pump and/or compressor, which pressurizes the module selectively. The module may provide for alternately controlling the pressure within the module to selectively pressurize the module for a short duration within which ionization of gas occurs. For example, the module is first evacuated by a pump to approximately 0.0133 mbar (.01 Torr) so that oxidation and sample impurity is kept to a minimum. Thereafter, a deposition or etch gas is injected between the electrodes and the pump pressurizes the module to approximately 0.665 mbar (.5 Torr) so that the gas is ignited by the primary frequency generator. Once gas is ionized, the module is depressurized to approximately 0.133 mbar which is a preferred deposition or etch operational pressure. In still another aspect, the process module may include an x-ray source, or alternatively a radioactive source, as the ionization element. The x-ray or radioactive source is arranged to generate a beam of x-ray or radioactive radiation into the gap to ionize gas therein. Preferably, to enhance user safety, such a source is shielded to confine the x-ray or radioactive radiation substantially to one or more paths extending from the source and through the gap. Further, a plurality of gases which operate in combination with one another may be used as the ionization element. For example, a first primary gas can include silane (SiH 4 ), disilane (Si 2 H 6 ), hydrogen (H 2 ), ammonia (NH 3 ), phosphine (PH 3 ), nitrogen trifluoride (NF 3 ), helium (He), carbon tetrafluoride (CF 4 ), hexafluoroethane (C 2 F 6 ), oxygen (O 2 ), nitrous oxide (N 2 O), methane (CH 4 ), borane (BH 3 ), diborane (B 2 H 6 ), chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), hydrogen chloride (HCl), carbon tetrachloride (CCl 4 ), hydrogen bromide (HBr), carbon dichloride difluoride (CCl 2 F 2 ), boron trichloride (BCl 3 ), silicon tetrachloride (SiCl 4 ), boron tribromide (BBr 3 ), chlorotrifluoride (ClF 3 ), fluorine (F 2 ), and mixtures thereof. A second gas, such as helium (He), argon (Ar), krypton (Kr), nitrogen (N 2 ), xenon (Xe) neon (Ne), and mixtures thereof, is combined with the first gas to form a gaseous mixture that is transformable into plasma by the primary frequency. Preferably, therefore, the mixture of the two gases is ignitable by a radiofrequency source that generates 60MHz discharges. Once the mixture of the two gases is ignited, the second gas is inhibited from flowing into the module, so that deposition onto a sample surface results without influence from the second gas. In still another aspect, a motion actuator, e.g., a linear actuator, may be used as the ionization element which moves at least one of the electrodes selectively to change the dimension of the gap between the electrodes. The motion actuator separates the electrodes to a first dimension, at which the radiofrequency generator, of high primary frequency, ionizes gas injected between the electrodes. Preferably, the motion actuator thereafter reduces the gap spacing to below approximately one centimeter, a desired dimension for high rate plasma deposition. In yet another aspect, the module may include an atomic element as the ionization element and which has an ionization energy of no more than twenty-five electron volts. The atomic element, such as helium or argon, is injected into the gap to ionize gas therein. The invention also provides a sample-processing method for exposing a sample to reactive plasma within a process module having first and second electrodes as defined in claim 18. In particular, the method may include the steps of (i) arranging the first and second electrodes within the module to form a substantially uniform gap separating the electrodes of between approximately one and ten millimeters; (ii) injecting a gas, such as silane (SiH 4 ), disilane (Si 2 H 6 ), hydrogen (H 2 ), ammonia (NH 3 ), phosphine (PH 3 ), nitrogen (N 2 ), nitrogen trifluoride (NF 3 ), helium (He), argon (Ar), carbon tetrafluoride (CF 4 ), carbon tetrachloride (CCl 4 ), hexafluoroethane (C 2 F 6 ), oxygen (O 2 ), nitrous oxide (N 2 O), methane (CH 4 ), borane (BH 3 ), diborane (B 2 H 6 ), and mixtures thereof, into the gap; (iii) ionizing the gas; and (iv) applying radiofrequency energy to the gap which has a primary frequency, e.g., 60MHz, that transforms molecules into plasma. In one aspect, the method includes the step of injecting a second gas into the gap wherein the first and second gases form a gaseous mixture that is transformable into plasma by the primary frequency. Preferably, the second gas is helium (He), argon (Ar), krypton (Kr), nitrogen (N 2 ), xenon (Xe) neon (Ne), or mixtures thereof. In still another aspect, the method according the invention includes the additional steps of (i) injecting into the module an etch gas, such as chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), hydrogen chloride (HCl), carbon tetrachloride (CCl 4 ), hydrogen bromide (HBr), carbon tetrafluoride (CF 4 ), hexafluoroethane (C 2 F 6 ), nitrogen trifluoride (NF 3 ), carbon dichloride difluoride (CCl 2 F 2 ), boron trichloride (BCl 3 ), silicon tetrachloride (SiCl 4 ), oxygen (O 2 ), boron tribromide (BBr 3 ), chlorotrifluoride (ClF 3 ), fluorine (F 2 ), or mixtures thereof; (ii) ionizing the etch gas; and (iii) applying to the gap radiofrequency energy with a primary frequency, e.g., 60MHz, to transform the etch gas into an etch plasma such that surfaces in contact with the etch plasma are etched. This method preferably includes the step of arranging the electrodes to form a substantially uniform gap separating the electrodes of between approximately one and seventy five millimeters during the etching of the surfaces. Other particularities associated with the deposition of certain semiconductor coatings, e.g., amorphous silicon, are described in US-A 4,933,203. Brief Description of the Drawings FIGURE 1 is a schematic side view of a process module constructed in accordance with the invention; FIGURE 2 is a side view of the process module of FIGURE 1 with a pressure pump and linear actuators operable to enable ionization of gas within the module; FIGURE 3 graphically shows the relationship between ionization and electrode potential versus gap spacing and pressure for selected typical gases; FIGURE 4 is a side view of the process module of FIGURE 1 with an electron source operable to enable ionization of gas within the module; FIGURE 5 is a side view of the process module of FIGURE 1 with an ultraviolet source, inside the module, operable to enable ionization of gas within the gap; FIGURE 5A is a side view of the process module of FIGURE 1 with an ultraviolet source, outside the module, operable to enable ionization of gas within the gap; FIGURE 6 is a side view of the process module of FIGURE 1 with a sparking source operable to enable ionization of gas within the module; FIGURE 7 is a side view of the process module of FIGURE 1 with a second RF generator connected to the active electrode and operable to enable ionization of gas within the module; FIGURE 8 is a side view of the process module of FIGURE 1 with a x-ray source embedded within the active electrode and operable to enable ionization of gas within the module; and FIGURES 9 to 16 are views of further embodiments. Fig. 1 schematically illustrates a process module 10a constructed in accordance with the invention. The module 10a includes a first radiofrequency (RF) electrode 12, a second RF electrode 14, and a RF generator 16 interconnected with the second electrode 14. The electrodes are spaced apart at a distance "d" to form a substantially uniform gap 18 between the electrodes 12 and 14. The distance "d" is less than approximately ten millimeters and preferably more than one millimeter. A pressure-tight housing 20 surrounds the electrodes 12 and 14 so that the module 10a may be evacuated selectively, to between approximately 0.0133 and 13.3 mbar (0.01 and 10 Torr), by methods known to those skilled in the art. The module 10a includes gas inlet 24 and gas outlet 26: the inlet 24 is used to inject gas into the module 10a; and the outlet 26 is used to extract gas, particulates, and other molecules from the module 10a. In operation, a gas that is suitable for ionization and for transformation into plasma, e.g., silane, is introduced through the inlet 24 and into the gap 18. The gas is ionized, in accordance with further features of the invention described below, and transformed into plasma by RF energy, which preferably has a primary frequency of 60MHz. The RF energy is generated by the generator 16 and transferred to the gap 18 via the RF connection 28 and electrodes 12 and 14, thereby transforming molecules of the injected gas into a plasma 30. A film is deposited onto surfaces in contact with the plasma 30. Accordingly, a sample 32, e.g., a flat glass or silicon wafer with opposed surfaces, is introduced into the gap 18 in the module 10a to expose the sample 32 to the plasma 30 for deposition of one or more films onto its surface. The sample 32 is typically mounted to the first electrode 12 such that the sample 32 is spaced away from the active RF second electrode 14. The first electrode 12, therefore, is preferably electrically grounded by the grounding connection 34 and ground 36. Fig. 2, 4, 5, 5A, 6, 7, and 8 illustrate similar pressure modules 10 constructed according to the invention, and which show other preferred features of the invention. These figures show the sensing modules 10 in schematic cross-sectional views; yet it is to be understood that such views are to be interpreted in the context of a three-dimensional structure. For example, the modules 10 of Fig. 1, 2, 4, 5, 5A, 6, 7 and 8 are generally rectangular in shape, each having another cross-sectional view that is similar in size, and perpendicular to, the associated illustrated view. Further understanding of a three-dimensional process module may be made with reference to Fig. 2, of U.S. Application Serial No. 08/084415, entitled "Method and Apparatus for Inverting Samples in a Process". Among other factors, high deposition rates result from smaller electrode spacings; yet it is increasingly difficult to ignite plasma at increasingly smaller electrode spacings, particularly under ten millimeters. Therefore, the invention provides for the ionization of gas injected into the gap 18 of this small size, as described below. Fig. 2, for example, illustrates the module 10b connected to a pressure pump 42 at the gas outlet 26; and further including linear actuators 44 connected to the first electrode 12. Either of the structures 42 and 44 are independently operable in conjunction with other elements of the module 10b to ignite gas within the gap 18; or, alternatively, they cooperate together with the other elements of the module 10 to ignite the gas. More particularly, the transformation of gas molecules such as silane, hydrogen, ammonia and nitrogen into a plasma - i.e., the ignition and the creation of a plasma - and, once ignited, the continual maintenance of the plasma within the module 10b are functionally dependent upon module pressure and electrode spacing. Fig. 3, for example, qualitatively illustrates plasma maintenance curves for two typical gases, Gas 1 and Gas 2, as the relation between the voltage differential required between the electrodes to maintain a discharge and the product of pressure and electrode spacing. The product of pressure and electrode spacing represents the horizontal axis. The voltage differential required between the electrodes to maintain a discharge within the gap 18 represents the vertical axis. The shape of these two plasma maintenance curves, Gas 1 and Gas 2, can be understood with reference to the ionization physics within the module 10b. To ignite and maintain a plasma within the module 10b, electrons must be accelerated within the gap 18 to energies in which the collisions of the accelerated electrons with the gas molecules in the gap 18 cause ionization of the gas molecules, i.e., the creation of more electrons and ions. The probability for an electron to cause ionization is directly proportional to the pressure within the module 10b and the distance traveled by the electron in the plasma, as controlled by the electrode spacing. Relative to Gas 1, for example, if the magnitudes of the module pressure and/or electrode spacing are too small, there are not enough electrons created to ignite the plasma, as illustrated in region A, Fig. 3. If, on the other hand, the magnitudes of the module pressure and/or electrode spacing are too great, the electrons are not accelerated to energies sufficient to cause ionization because of inelastic collisions with gas molecules, as illustrated in region B, Fig. 3. Therefore, there is a condition 43 wherein the product of the module pressure and of the electrode spacing is such that the voltage required to ignite the plasma is at a minimum for the illustrated Gas 1. According to the invention, the preferred operating conditions for depositing film from typical Gas 1 occur in region A, Fig. 3. By way of example, a plasma of hydrogen and silane is ignited in one instance at pressures above 0,8 mbar (0.6 Torr); however, below that pressure, no plasma is ignited. Nevertheless, after ignition at 0.8 mbar, the pressure is reduced to as low as 0,133 mbar (0.1 Torr), since the required voltage to maintain a discharge, and hence maintain the plasma, is lower than the voltage required to ignite, and thereby initiate, the plasma. As also illustrated in Fig. 3, the minimum plasma maintenance voltage for Gas 1, i.e. Vmin, relative to the pressure-electrode product, i.e. (pd)min, is gas dependent. Thus, in accordance with the invention, the electrode spacing and/or module pressure are modified as a function of the particular gas within the module 18 to enable ignition, i.e., the transformation of those gas molecules into the plasma 30. These gases are generally classified into two groups: deposition gases and etch gases. The preferred deposition gases used in accordance with the invention are silane (SiH 4 ), disilane (Si 2 H 6 ), hydrogen (H 2 ), ammonia (NH 3 ), phosphine (PH 3 ), nitrogen (N 2 ), nitrogen trifluoride (NF 3 ), helium (He), argon (Ar), carbon tetrafluoride (CF 4 ), hexafluoroethane (C 2 F 6 ), oxygen (O 2 ), nitrous oxide (N 2 O), methane (CH 4 ), borane (BH 3 ), diborane (B 2 H 6 ), and mixtures thereof (the preferred deposition mixtures are silane and hydrogen; silane, ammonia and nitrogen; silane and phosphine; silane and methane; silane and borane; and silane and diborane). The preferred etch gases used in accordance with the invention are chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), hydrogen chloride (HCl), carbon tetrafluoride (CF 4 ), hydrogen bromide (HBr), carbon tetrachloride (CCl 4 ), nitrogen trifluoride (NF 3 ), carbon dichloride difluoride (CCl 2 F 2 ), boron trichloride (BCl 3 ), silicon tetrachloride (SiCl 4 ), oxygen (O 2 ), boron tribromide (BBr 3 ), chlorotrifluoride (ClF 3 ), fluorine (F 2 ), and mixtures thereof (the preferred etch mixtures are carbon tetrafluoride and oxygen; hexafluoride and oxygen; and nitrogen trifluoride and oxygen). With an electrode spacing of less than approximately ten millimeters, the RF generator 16 generally does not provide sufficient voltage to transform gas molecules injected into the gap 18 into plasma, such as illustrated in region A of Fig. 3. In operation, therefore, the pressure pump 42 of Fig. 2 initially operates as a vacuum pump to evacuate the module 10b to approximately 0.0133 mbar (0.01 Torr), which reduces oxidation and other impurities that might form on the heated samples. Selected deposition gas molecules are then injected into the module 10b, and the pressure pump 42 controls the pressure within the module 10b by operation of the throttle 41 to between approximately 0.133 to 1.33 mbar (0.1 to 1 Torr), which represents a pressure range that permits ionization of the gas molecules by the primary RF generator 16. The throttle 41 functions as an orifice within the gas outlet 26 to increase or decrease the pressure in the module selectively: decreasing the orifice of the throttle 41 increases the pressure within the module 10b; while increasing the orifice decreases the module pressure. The pressure pump 42 increases the pressure within the module 10b selectively, such as in accordance with the plasma maintenance curve of Fig. 3, as a function of the gas within the module to enable the transformation of the gas molecules into plasma. Once the plasma is ignited, the pump 42 evacuates, or de-pressurizes, the module 10 to normal pressures, e.g., 0.133 mbar, which are preferred for uniform high rate deposition. Thus, the pressure pump 42 of Fig. 2 is selectively operable to (i) pressurize the module 10b to enable the transformation of gas molecules injected into the gap 18 into plasma and (ii) de-pressurize, e.g., evacuate, the module 10b to enable high rate deposition at preferred conditions. Those skilled in the art can appreciate that the pump 42 is readily constructed as a vacuum pump or combination pump and compressor to provide the operations (i) and (ii). The operation between pressurization and depressurization within the module 10b is controllable, in accordance with the invention; and preferably short in duration, e.g., 1 second, so that gas is ionized and the module 10b returns to normal evacuated pressures without lengthy pressurization of the module 10b. A similar ignition procedure is realized through operation of the linear actuators 44. The actuators 44 are connected to the first electrode 12 to move the electrode 12 and adjust the gap separation "d" selectively. Thus, to ignite the gas within the gap 18, the actuators 44 increase the separation between the electrodes 12 and 14 to a point, for example in accordance with Fig. 3, where the RF generator ignites and transforms the molecules into the plasma 30. Once ionization occurs, the RF generator 16 has sufficient voltage to maintain the plasma 30 within the gap 18; thus the actuators 44 reposition the electrode 12 to a gap separation "d" of less than approximately one centimeter to increase the deposition rate. Linear actuators 44 preferably reside within the housing 22 so that the module 10b is evacuated without required mechanical motion through the sealed module housing wall 20. Therefore, the actuators 44 have electrical components (not shown), including wires and electrical feed-through fittings, which extend from the actuators 44 and through the housing 20 for communication to outside of the module 10. Alternatively, although not preferred, the actuators 44 physically extend through the evacuated module housing 20 through the use of mechanical seals (not shown). In accordance with the invention, the pump 42 and the actuators 44 preferably operate simultaneously with the RF generator 16 to ionize gas. The required voltage to transform molecules into plasma decreases more rapidly by simultaneously increasing module pressure and electrode spacing. Therefore, the undesirable time duration away from ideal deposition conditions, e.g., 0.133 mbar and 1-9mm gap separation, is decreased. The invention thus provides the pump 42 and the actuators 44 to aid in the ionization of gas injected between the electrodes. The generator 16 cannot generally ionize gas at low pressures, e.g., 0.133 mbar, and at small gap separations, e.g., 1-9mm, without the further features of the invention, e.g., the pump 42, the actuators 44, and other practices according to the invention taught below. The module 10b has a total fixed capacitance that is dependent, in part, upon the module geometry, e.g., the electrode gap spacing. Thus, for example, at the high frequency of the first RF generator 16, the total fixed module capacitance has a relatively low associated impedance. This translates to a low voltage differential between the electrodes 12 and 14 for a given power. For ignition purposes, only voltage is important; hence the requirement for additional features according to the invention, such as the pump 42 and actuators 44. Fig. 4 shows a module 10c with the addition of an electron source 60, in accordance with the invention. The electron source 60 has a hot filament 62, operating at a low voltage and at approximately 10 amps, and emits a beam of electrons 64 through an extraction slit 66 and into the gap 18. The slit 66 is approximately ten millimeters in length and one millimeter in height, and is parallel to the gap 18. The filament 62 is at a negative potential relative to the grounded extraction slit 66 to "extract" the electrons from the filament 62 surface. Accordingly, gas injected into the gap 18 is ionized at a selective time by applying a positive voltage to the slit 66 once the filament 62 is emitting electrons. Preferably, the electrons 64 emitted by the filament 62 have energies of approximately 300eV when the filament is positioned at approximately 70mm or less from the gap 18. If the filament 62 is positioned nearer to the gap 18, less energy is acceptable. However, if the filament 62 is positioned further from the gap 18, more energy is required. This phenomenon occurs because the emitted electrons 64 lose energy during travel to the gap 18, such as through collisions with other molecules in the module 10c. Additionally, different gases within the module 10c require different ionization energies. Therefore, the invention provides for differing electron illumination energies by changing the negative potential of the filament 62. This potential is adjusted by changing the voltage between the filament 62 and slit 66 to alter the energy of the emitted electrons. Those skilled in the art will appreciate that other electron sources fall within the scope of the invention. For example, electron sources exist which are manufactured without the filament 62 of Fig. 4, and which are also suitable for igniting the gas within the gap 18. The probability of ionizing gas within the gap 18 is at a maximum when electrons with energies between approximately 50eV and 100eV collide with the gas molecules in the gap 18. Therefore, other electron sources which are operable to generate electrons with energies between approximately 50eV as 100eV within the gap 18 are also suitable for ionizing gas therein in accord with the invention. Fig. 5 and 5A illustrate one preferred practice of the invention in which an ultraviolet (UV) source 70, 72 illuminates and ignites gas within the gap 18 with UV photons: Fig. 5 shows the UV source 70, 72 within the evacuated module 10d; while Fig. 5A shows the UV source 70, 72 outside the module 10e, in standard atmosphere. More particularly, Fig. 5 illustrates the module 10d with a UV lamp 70 that emits UV photons. A reflector 72 surrounds the lamp 70 such that emitted photons are collected and are emitted only through the aperture 74, which directs the UV photon beam 76 generally towards the gap 18. The beam 76 is substantially collected and collimated by UV transmitting optics 78 such that the beam 76 illuminates the gap 18 without significant loss of intensity. Similarly, Fig. 5A shows a UV source 70, reflector 72, aperture 74, beam 76, and optics 78, and additionally shows an optical interface 79 that transmits UV radiation and which preferably withstands operational pressure differentials. The interface 79 is required because the UV source 70 of Fig. 5A is positioned outside the process module 10e, e.g., exterior to the pressure-sealing housing 20'. UV energy cannot transmit through materials which typically form the housing 20', e.g., aluminum, and thus a glass-like interface 79 is required. Preferably, the photon eneggies emitted by the UV source 70, 72 of Fig. 5 and 5A are greater than approximately 5 eV. Fig. 6 shows another preferred practice of the invention in which a sparking source 80 generates a spark to ignite the gas within the gap 18. The sparking source has a power supply 82 connected to a spark gap 84, and is preferably electrically isolated from the electrodes 12 and 14. In operation, the power supply 82 applies a voltage across the gap 84 selectively to generate a spark. The sparking source is arranged within the module 10f such that generated sparks are introduced into the gap 18, thus ionizing gas therein. Preferably, therefore, the gap 84 has a line of sight 86 directly to the gap 18. A spark is generally induced by a voltage differential across the gap 84 in the kilovolt range and thus generates electrons with high energies. These electrons emit in all directions and many collide with the electrodes 12 and 14, which in turn release secondary electrons that ignite the gas within the gap 18. These secondary electrons include electrons with maximum ionization probabilities for a given gas, e.g., those electrons with energies between approximately 50eV and 100eV. Fig. 7 shows another preferred practice of the invention for igniting gas within a process module 10g. In Fig. 7, the process module 10g has a second RF generator 90 connected to the second electrode 14 via RF line 92. The second RF generator 90 has a second frequency, preferably 400kHz, which is less than the preferred primary frequency of 60MHz. The second RF generator 90 is selectively operable to apply RF energy at the lower frequency to the gap 18. The second RF generator 90 is energized only for a short duration, at a time sufficient to ionize gas within the gap 18. Alternatively, the generator 90 is repetitively pulsed at approximately once per second to ionize the gas. In operation, the primary 60MHz energy combines with the lower 400kHz energy to ignite and ionize the gas. Each generator 16 and 90 provides approximately equal energy, e.g., .05W/cm 2 to 1W/cm 2 within the gap 18; yet, as stated earlier, the high frequency of the first RF generator 16 lacks sufficient voltage to ionize gas within a gap 18 spaced at less than approximately one centimeter. The lower frequency generator 90, on the other hand, provides relatively high voltage across the electrodes. At lower RF frequencies, such as at 400kHz, the module capacitance has a much higher associated impedance and greater peak-to-peak voltage as compared to a 60MHz frequency. A high voltage generates high electron energies with a corresponding increase in ionization probability. Accordingly, the generators 90 and 16 preferably combine to generate a peak-to-peak voltage between approximately 1 kV and 5 kV in the gap 18. One advantage of the second RF generator 90 is that, unlike other ignition techniques described herein, no internal modifications to the module 10 are needed to ignite the gas. The generator 90 is external to the housing 20 and connects with the second electrode 14 through the RF line 92. In one practice of the invention, the line 92 is coaxial with the RF line 28, to reduce the required cabling, which pierces the evacuated housing 20 at a pressure-tight RF feed-through fitting. In one other practice of the invention, the generator 90 is not an RF generator; but is instead a high DC power source. As stated above, voltage is an important ionization parameter. Thus, DC source 90, in this alternative embodiment, generates a spark in the gap selectively to ionize gas therein. In operation, the DC source 90 combines with the 60MHz energy to ignite and ionize the gas. After the gas is ionized and molecules have been transformed into plasma, the DC power source 90 is turned off. Fig. 8 shows a process module 10h constructed according to a further practice of the invention and which includes an x-ray source 100 and shielding 102. The x-ray source 100 and shielding 102 are constructed by methods known in the art to generate a beam of x-ray radiation 104 that extends through at least part of the gap 18. As illustrated, the x-ray source 100 is preferably embedded within one of the electrodes 12 and 14', here shown imbedded in electrode 14'. The shielding 102 confines the x-ray radiation to the desired beam 104, and thereby protects users of the process module 10h from the unwanted x-rays. Alternative to an x-ray source, the source 100 can be a radioactive source in accordance with the invention. As above, a shielding 102 protects users of the module 10 and guides the emitted radioactive particles along a path that includes the gap 18. X-ray and other radioactive radiation provide the energy required to ionize gases within the gap 18. Once ionized, the source 100 is preferably covered, via an automatic mechanical shutter (not shown), since the RF generator 16 has sufficient power to maintain plasma once the gas is ionized. Those skilled in the art can appreciate that the source 100, x-ray or radioactive, may be positioned elsewhere to ionize gas within the gap 18. However, it is a convenient packaging to locate the source 100 within one of the electrodes 12 and 14', since both x-rays and radioactive energy propagate through the electrode material. It is to be understood that the apparatus described in connection with Fig. 1-8 are illustrative rather than limiting, and that additions and modifications will be apparent to those skilled in the art without departing from the scope of the claims. Accordingly, variations for ionizing gas within the gap of a PECVD process module exist in accordance with the invention, other than those specifically described in connection with Fig. 1-8. For example, with reference to Fig. 1, the inlet 24 is used in one practice of the invention to inject combinations of gases, rather than a single gas for ionization. Alternatively, a second inlet 24 is installed into the housing wall 20 to inject additional gases. These additional gases can act as the catalyst to ignite the primary gas from which the plasma 30 is derived. More particularly, by injecting a second gas comprising helium, xenon, krypton, nitrogen, argon, neon, or other atomic elements with a primary gas, e.g., silane, electrons liberated by ionization of the molecules of the second gas cause further ionization of the primary gas molecules, e.g., silane, and enable the transformation of molecules into a plasma. Accordingly, these second gases are injected for a short duration through the inlet 24 until the gas combination ignites, whereinafter the second gas is inhibited from flowing through the module so that a deposition from a "pure", e.g., silane-based plasma results. Preferably, the injected atomic elements have ionization energies which are less than approximately 25eV. Additionally, those skilled in the art can appreciate that the invention as described herein is also applicable for etching surfaces within a process module, for example to "clean" the module. Etch gases such as chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), hydrogen chloride (HCl), nitrogen trifluoride (NF 3 ), carbon tetrachloride (CCl 4 ), hydrogen bromide (HBr), carbon tetrafluoride (CF 4 ), carbon dichloride difluoride (CCl 2 F 2 ), boron trichloride (BCl 3 ), silicon tetrachloride (SiCl 4 ), boron tribromide (BBr 3 ), chlorotrifluoride (ClF 3 ), and fluorine (F 2 ) are typically mixed with oxygen (O 2 ) to form the preferred gaseous mixture that is ignited according to the further features of the invention. For etch purposes, the electrode spacing is generally spaced to between approximately 1 and 75 millimeters. Fig. 9 to 16 present further details of preferred constructions of process modules that (i) ignite gas and (ii) expose samples to plasma, in accordance with the invention. Figure 9 is a fragmentary and detailed view of a construction of an electron source 100 and filament 102 that is similar to the source 60 of FIGURE 4. Similar to the module 10 of FIGURE 4, portions of a first grounded electrode 104 and second RF electrode 106 are illustrated in FIGURE 9. Also shown in FIGURE 9 are a sample 108 and outlet baffle 119. The source 100 has a support 113, an extraction aperture 112, e.g., a slit, insulator 114, and a grounded aperture 116. The filament 102 has a line of sight which extends underneath the outlet baffle 119, in a 1.5mm wide and 100mm long path 117, and between the electrodes 104 and 106. As in FIGURE 4, the hot filament 102 emits electrons 118 through the aperture 112 and into the gap 120 when a positive potential is applied to the extraction aperture 112. The electrons 118 have electron energies of approximately 300eV to ignite gas within the gap 120. Figure 10 is a fragmentary and detailed view of a construction of an UV lamp 130 that is similar to the UV lamp 70 of FIGURE 5. Similar to the module 10 of FIGURE 5, portions of a first grounded electrode 104 and second RF electrode 106 are illustrated in FIGURE 10. Also shown in FIGURE 10 are a sample 108 and outlet baffle 119. The UV lamp 130 has a support 132 and reflector 136. As in FIGURE 5, the reflector 136 collects and re-directs UV photons 138 toward and into the gap 120 by way of a path 117 underneath the outlet baffle 119. Preferably, as in FIGURE 5, at least one UV transmitting optical component such as optics 139 is arranged within the beam 138 such that the photons are collected and substantially collimated into the gap 120. Figure 11 is a fragmentary and detailed view of a construction of a sparking source 140 that is similar to the sparking source 80 of FIGURE 6. Similar to the module 10 of FIGURE 6, portions of a first grounded electrode 104 and second RF electrode 106 are illustrated in FIGURE 11. Also shown in FIGURE 11 are a sample 108 and outlet baffle 119. The sparking source 140 has a support 142 and a spark gap 144 formed by two electrodes 144a and 144b. As in FIGURE 6, the electrodes 144a and 144b are given a potential difference of approximately 5-10kV to generate a spark at the spark gap 144. The spark generates electrons 146 which travel toward and into the gap 120 by way of a path 117 underneath the outlet baffle 119. Preferably, therefore, and as in FIGURE 6, the spark gap 144 has a direct line of sight to the gap 120. Figure 12 is a detailed view of a construction of a process module 150 constructed in accordance with the invention, and which shows further details of the fragmentary elements illustrated in FIGURES 9 to 11. The module 150 has a housing wall 152 that contains a first grounded electrode 104 and second RF electrode 106. The sample 108 is suspended by the clamps 110 in the gap 120 and away from the second electrode 106. The clamps 110 are driven by motorized assemblies 156. The gap 120 of FIGURE 12 is not at the operational distance of less than one centimeter. Rather, FIGURE 12 illustrates a wider gap 120 that is realized by lifting the first electrode 104 away from the second electrode 106 with a linear actuator 158, similar to the actuators 44 of FIGURE 2. The same linear actuator 158 moves the first electrode 104 closer to the second electrode 106 to reduce the gap 120 to operational distances, as illustrated in FIGURE 13. FIGURE 13 shows the module 150 in an operational mode. The first electrode 104 is moved down onto the sample 108 and clamps 110 and then compressed further to the desired gap spacing of less than one centimeter. Thus the actuator 158 of FIGURES 12 and 13 illustrates one practice for separating the electrodes selectively, as described in connection with FIGURES 2 and 3, to ignite the gas and transform ionized molecules to plasma. FIGURE 14 shows another cross-sectional view of the module 150 illustrated in FIGURES 9 to 13. FIGURE 14 shows many previously described elements, and additionally shows a gas inlet 160 and gas outlet 162, which are similar to the inlet 24 and outlet 26 of FIGURES 1-8. FIGURES 15 and 16 respectively illustrate the inlet 160 and outlet 162 of FIGURE 14 more fully, and additionally show other fragmentary detail of the module 150 illustrated in this Appendix. FIGURE 15 also shows an inlet manifold 164 and inlet baffle 119'; and FIGURE 16 shows an outlet manifold 166 and outlet baffle 119. A dielectric element 168 preferably separates the RF electrode 106 from a base element 170.

Claims: Plasma process module for exposing a sample to reactive plasma, comprising a first radiofrequency electrode (12) in the module, a second radiofrequency electrode (14) in the module and spaced away from said first electrode to form a substantially uniform gap of less than approximately one centimeter between said electrodes, first radiofrequency generation means (16) for transforming molecules of the injected gas into a plasma, said first radiofrequency generation means being interconnected with said second electrode (14) and having a primary frequency, and means (60; 70; 80; 90; 100) for ionizing gas injected between said electrodes, characterized in that said plasma process module is adapted for CVD and PECVD, and said means for ionizing gas is selected from the group consisting essentially of an electron source means (60) arranged to inject electrons between said electrodes, an ultraviolet source means (70; 72) being arranged for irradiating ultraviolet radiation between said electrodes, a sparking means (80) arranged to generate electric discharges between said electrodes a second radiofrequency generator means (90) connected to said second electrode (14), said second radiofrequency generation means (90) having a second frequency that is less than said primary frequency and being arranged selectively to generate radiofrequency discharges between said electrodes at said second frequency and at a power approximately equal to that of the first radiofrequency generation means, a high direct current power source means connected to said second electrode, said direct current power source means being arranged to generate a physical spark between said electrodes, a pressurizing means (42) arranged for pressurizing said module selectively to between approximately 0.0133 and 13.3 mbar (0.01 and 10 Torr), an x-ray means (100) being arranged to illuminate said gap with a beam of x-ray radiation a radioactive source means being arranged to illuminate said gap with a beam of radioactive radiation, a motion means (44) for moving at least one of said electrodes selectively to charge the width of said gap, said motion means separating said electrodes wherein said primary frequency ionizes gas injected between said electrodes, and an atomic element injecting into said gap, said atomic element having an ionization energy of no more than twenty-five electron volts. A process module of claim 1, wherein said first electrode (12) is electrically grounded. A process module of claim 1 or 2, comprising means (24) for injecting gas into said gap, said gas being suitable for ionization and for transformation into plasma when exposed to radiofrequency energy at said primary frequency. A process module of any of claims 1 to 3, comprising evacuation means arranged for evacuating said module selectively to approximately 0.0133 mbar (0.01 Torr). A process module of one of claims 1 to 4, wherein said electron source (60) means has a filament (62) from which it produces electrons, wherein said filament has a negative potential, and wherein said electron source means (60) comprises extraction slit means (66) that is electrically positive relative to said filament, and further comprising second means for adjusting said negative potential selectively to adjust the energy of said electrons. A process module of one of claims 1 to 4, comprising UV interface means, said UV interface means (79) being transmissive to said ultraviolet radiation and being arranged in optical alignment such that said ultraviolet source means irradiates said gap through said UV interface means. A process module of claim 6, wherein said module is evacuated and wherein said ultraviolet light source means (70; 72) is disposed outside said evacuated module, and said interface means (79) is arranged between the vacuum of said evacuated module and the atmosphere external to said evacuated module. A process module of one of claims 1 to 4, wherein said ultraviolet light source means (70; 72) comprises (i) reflector means (72) having an aperture, said reflector means (72) collecting said ultraviolet radiation and transmitting said radiation through said aperture, and (ii) optics means (78) for collecting and collimating said transmitted radiation from said reflector means, said optics means (78) being transmissive to said ultraviolet radiation. A process module of one of claims 1 to 4, wherein said sparking means (80) has an exposed spark gap, said sparking means (80) being arranged such that said spark gap has a line of sight to between said electrodes (12, 14). A process module of one of claims 1 to 4, wherein said second radiofrequency generation means (90) includes means for pulsing said second frequency at a selected rate. A process module of claim 10, comprising means for inhibiting said second radiofrequency generation means (90) selectively after gas is ionized within said gap. A process module of one of claims 1 to 4, wherein said pressurizing means comprises a de-pressurizing means arranged for de-pressurizing said module selectively, and wherein said ionizing means further comprises a means for alternately controlling said pressurizing means and said de-pressurizing means to selectively pressurize said module for a short duration within which ionization of gas occurs. A process module of one of claims 1 to 4, wherein said x-ray means is embedded substantially within said second electrode . A process module of one of claims 1 to 4, comprising means (102) for shielding said x-ray means (100) such that said x-ray radiation is substantially confined to one or more paths extending from said x-ray means and through said gap. A process module of one of claims 1 to 4, comprising means for shielding said radioactive source means such that said radioactive radiation is substantially confined to one or more paths extending from said radioactive source means and through said gap. A process module of any of claims 1 to 15, wherein said electrodes (12, 14) are bowed adjacent to said gap, thereby forming said gap between non-flat surfaces. A CVD or PECVD method for exposing a sample to reactive plasma within a process module having first and second electrodes (12, 14), comprising the steps of arranging said first and second electrodes (12, 14) within the module to form a substantially uniform gap separating said electrodes of between approximately one and ten millimeters, injecting gas into said gap, said gas being selected from the group consisting essentially of silane (SiH 4 ), disilane (Si 2 H 6 ), hydrogen (H 2 ), ammonia (NH 3 ), phosphine (PH 3 ), nitrogen (N 2 ), nitrogen trifluoride (NF 3 ), helium (He), argon (Ar), carbon tetrafluoride (CF 4 ), hexafluoroethane (C 2 F 6 ), oxygen (O 2 ), nitrous oxide (N 2 O), methane (CH 4 ), borane (BH 3 ), diborane (B 2 H 6 ), chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), hydrogen chloride (HCL), carbon tetrafluoride (CCl 4 ), hydrogen bromide (HBr), carbon dichloride difluoride (CCl 2 F 2 ), boron trichloride (BCL 3 ), silicon tetrachloride (SiCl 4 ), boron tribromide (BBr 3 ), chlorotrifluoride (ClF 3 ), fluorine (F 2 ) and mixtures thereof, and being suitable for ionization and for transformation into plasma when exposed to radiofrequency energy, applying radiofrequency energy to said gap, said radiofrequency energy having a primary frequency which transforms molecules into plasma, and ionizing said gas by one step selected out of injecting electrons between said electrodes, generating ultraviolet radiation with an ultraviolet source and directing said ultraviolet radiation between said electrodes, generating electric discharges between said electrodes, applying a second radiofrequency energy to said gap, said second radiofrequency energy having a second frequency that is less than said primary frequency and being approximately equal to the first radiofrequency energy, connecting a direct current source to said second electrode and generating a physical spark in said gap, evacuating said module selectively to approximately 0.0133 mbar (0.01 Torr) pressuring said module selectively such that said molecules are transformed into plasma by said first radiofrequency energy, generating a beam of x-ray radiation and directing said radiation into said gap generating ionizing particles in said gap from a radioactive source, moving at least one of said electrodes selectively to change the dimension of said gap wherein said primary frequency ionizes gas injected between said electrodes, and introducing an atomic element to said gap, said atomic element having an ionization energy of no more than twenty-five electron volts. A method according to claim 17, wherein said primary frequency is approximately sixty megahertz. A method according to claim 17, wherein the step of injecting electrons further comprises the step of adjusting the energy of said electrons selectively. A method according to claim 17, wherein the step of generating ultraviolet radiation further comprises the steps of providing an UV interface window that is transmissive to said ultraviolet radiation and directing said ultraviolet radiation through said UV interface window to said gap. A method according to claim 17, wherein said second frequency is approximately 400kHz. A method according to claim 17, wherein the step of applying a second radiofrequency further comprises the step of pulsing said second radiofrequency energy selectively. A method according to claim 17, wherein the step of applying a second radiofrequency further comprises the step of inhibiting said second radiofrequency energy selectively after gas is ionized within said gap. A method according to claim 17, wherein the step of generating a beam of x-ray radiation further comprises the step of shielding said x-ray radiation such that said x-ray radiation is substantially confined to one or more paths extending through said gap. A method according to claim 17, wherein the step of generating ionizing particles further comprises the step of shielding said radioactive source such that said ionizing particles are substantially confined to one or more paths extending through said gap.

Inventors: BERRIAN DONALD, MEILING HANS, VANDERPOT JOHN W, WESTENDORP HAN, BERRIAN, DONALD, MEILING, HANS, VANDERPOT, JOHN W., WESTENDORP, HAN
Applicants: TOKYO ELECTRON AMERICA INC, TOKYO ELECTRON AMERICA INC.
Application year: 1995
Main classification: C23C16
IPC codes: H01J37, C23C16, H05H1, C23F4, C23C14