MXPA01005589A - Method and apparatus for low energy electron enhanced etching of substrates - Google Patents

Abstract

A method of low-damage, anisotropic etching of substrates including mounting the substrate upon the anode in a DC plasma reactor and subjecting the substrate to a plasma of low-energy electrons and a species reactive with the substrate. An apparatus for conducting low-damage, anisotropic etching including a DC plasma reactor, a permeable wall hollow cold cathode, an anode, and means for mounting the substrate upon the anode.

Description

METHOD AND APPARATUS FOR CHEMICAL SUBSTRATE ATTACK, IMPROVED WITH LOW ENERGY ELECTRONS FIELD OF THE INVENTION The present invention relates in general to the preparation of chemically attacked substrates. More particularly, the present invention relates to an improved process for anisotropic chemical attack with low damage of substrates such as semiconductors and anisotropically chemically attacked substrates, improved BACKGROUND OF THE INVENTION The chemical attack in dry is an absolutely critical process in the manufacture of all the elements at micrometric and nanometric scale on electronic and opto-electronic devices of high speed. In short, the manufacture of such microcircuits and devices involves the following process. A substrate of some semiconductor or metal is selected and a pattern is placed on it, the pattern has open areas on it. The coating structure that contains the pattern is a few times Ref: 130297 called a mask. The chemistry of the etching or chemical attack is then carried out through the open areas, which means that indeed some of the underlying material exposed through the open areas is dissolved and removed, so that the pattern is transferred to the underlying layers. Then, the mask is removed and what remains below is the original substrate, but now the substrate has been transferred to it. The process is similar to silk embossing or stenciling of a pattern on a material. The resulting pattern has a three-dimensional structure. In the first days of the manufacture of integrated circuits, most of the chemical attack was carried out using a wet chemical process that is very similar to conventional photochemistry. For example, to record an arrangement of channels or notches in a slice or silicon wafer, the water is first placed in an oxidizing, high temperature environment, and a layer of silicon dioxide develops on the upper surface of the slice. Then, the oxidized slice is covered with a thin photosensitive layer of gelatinous organic material called a "thermosetting substance". Next, a piece of material analogous to a photographic neqative, called a "photomask," is placed on the substance photo-hardening Ultraviolet light is illuminated through the openings in this photomask. Ultraviolet light changes the solubility of the photocurable substance. Thus, areas of the photocurable substance that have been illuminated with ultraviolet light show a different solubility than areas that have not been exposed to light. Finally, a solvent is used that dissolves only the areas of the photocurable substance that have had their solubility increased by ultraviolet light. At this point, the original pattern on the photomask has been transferred to the photoresist material layer. Some people refer to this layer of photocurable material as a "soft mask". Finally, the slice is immersed in a caustic chemical bath, such as potassium hydroxide (KOH) which records or removes the silicon exposed under the openings in the hard mask. After removing the hard mask, the desired silicon slice with the chemically attacked notches remains. As an alternative to the following, the hard masking step described above with a chemical attack of silicon with KOH, ion implantation or diffusion at high temperature could optionally be used to place doping atoms through the openings in the hard mask. Many other structural and chemical variations using the types of wet processing steps described above are possible and are well known to those skilled in the art. In each case, however, the corresponding process suffers from a problem that is inherent with the associated chemical attack using wet chemicals. In particular, at the same time that the wet chemistry is recording or attacking under the slice, it is also attacking laterally under the mask. Of course, this unwanted lateral chemical attack tends to extend approximately as far as the desired vertical chemical attack. This tendency for the wet chemical attack to proceed equally in all directions without prejudice is called "isotropic chemical attack". The isotropic chemical attack is suitable for the elaboration of a line that is 20 micrometers wide through a film that is 1 micrometer deep. The resulting imprecision at the edges of such an element is a small percentage of the complete structure of the device; and therefore, this does not compromise the operation. However, as smaller and smaller structures are manufactured, the isotropic chemical attack. The industry is moving toward manufacturing structures with so-called sub-micron elements, which are essential for high-speed computer microcircuits, optical structures, and electronic and optoelectronic devices. In other words, the precise transfer of a pattern that is half a micrometer wide in a material that is half a micrometer thick requires absolutely straight vertical side walls, or anisotropic chemical etching. The isotropic chemical attack is inadequate because the associated rounded recess could be a very high percentage of the active device material and could destroy its functioning. Currently, it is thought that the only way to obtain the straight side walls is by a technique called reactive ion chemical attack. Instead of immersing the device in wet chemical products, it is exposed to reactive gases and plasmas. The energy ions formed in the plasma are accelerated in the normal direction to the substrate, where they increase the chemistry of the chemical attack at the bottom of the open area defined by the mask and not on the side walls. In this way, the straight side walls can be loqradas with reactive ion chemical attack.

Reactive ion chemical attack provides anisotropic chemical etching. However, the ions are heavy ions like argon or CF3 + and are traveling at a few hundred volts of electrons of kinetic energy. In this way, they carry enough moment or amount of movement to move the atoms of the network from their normal position. This damages the surfaces and frequently, the optical and electrical properties of the substrate have been changed to the detriment. The manufacture of ultra-small electronic and optoelectronic devices requires dry etching processes that give high anisotropy, high selectivity between different materials, and minimal surface damage. Currently, the processes of chemical attack with plasma, increased by ions (for example reactive ion chemical attack (RIE), and RIE increased by electron cyclotron (ECR)) reliably creates nanoscale elements of high proportion between dimensions; however, the damage induced by the chemical attack has become increasingly problematic as the critical dimensions shrink. To minimize damage from chemical attack, the reactive species generated in the plasma must have energies greater than the activation energy of the chemical attack reaction (a fraction of an eV), but less than the energy required for displacement. atomic (3 to 10 eV for semiconductors III-V). Given these limitations, the ion energies available in the chemical attack by reactive ion (approximately 300 eV) and by electron cyclotron resonance plasma (approximately 50 eV) are not ideally suited to manufacture nanoscale devices. What is needed and apparently was not available until the presently described invention, is a method of chemically attacking or cleaning a substrate, which eliminates the damage inflicted by the reactive ion chemical attack, which achieves the anisotropic chemical attack, A method for attack Anisotropic chemical induced by electron impact, from silicon (Si) by hydrogen is discussed in an article of 1982 by S. Veprek and FA Sarott, "Electron-Impact-Induced Anisotropic Etching of Silicon by Hydrogen", Plasma Chemistry and Plasma Processing, Vol. 2, No. 3, p. 233. The authors discuss successful chemical attack speeds of up to 1,000 A / minute with little surface roughness at low temperatures. At higher temperatures a rougher pattern was observed. While their exact methodology is unclear, the authors apparently used an apparatus described in a previous publication by A.P. ebbv and S. Veprek, "Reactivity of Solid Silicon With Hydrogen Under Conditions of a Low Pressure Plasma", Chemical Physics Letters, Vol. 62, No. 1, p. 173 (1978). That publication describes an apparatus that includes a DC halo discharge device with the sample submerged in the positive column. The cathode was a standard hot cathode (heated to between 1500-2000 K) having a tungsten filament coated with thorium oxide. While this technique apparently worked to chemically attack Si (111) with hydrogen, it can not work using other reactive gases such as oxygen, chlorine and fluorine, because the hot filament could be immediately consumed. In addition, the apparatus described by Veprek and Sarott is problematic. Other experiments, reported by Gillis et al. in an article entitled "Chemical Attack Improved by Electron Beam of Low Energetic Si (100) - (2x1) by Molecular Hydrogen", J. Vac. Sci. Technology B 10 (6), Nov./Dic, p. 2729 (1992), focused on Si flooding with low-energy electrons (200-1000 eV) produced by an electron gun. The authors reported the chemical attack at a rate of approximately 100 Á / minute with low damage to the surface of Si. Other documents by the present inventors are: "Enhanced Chemical Attack by Si Low Energy Electrons (100) in Hydrogen / Helium Direct Current Plasma" (Gillis et al., Appl. Phys. Lett., Vol. 66 (19 ), p.2475 (1995)); "Chemical Attack Improved by GaAs Low Enerqia Electrons (100) in a Chlorine / Hydrogen DC Plasma "(Gillis et al., Api. Phys.Lett., Vol. 68 (16), p.2225 (1996)), and" Chemical Attack Enhanced by GaN Low Energy Electrons in a Hydrogen DC Plasma "(to be published in J. Electrochem, Soc. 11/96) These publications are incorporated herein by reference", in their entirety.

BRIEF DESCRIPTION OF THE INVENTION The present invention involves an improved chemical attack with low energy electrons (LE4), as opposed to chemical attack enhanced by reactive ions. The process gives straight side walls, and does not damage the substrate. In contrast to the reactive ions described above, the low energy electrons that are used in the present invention travel at less than about 100 electron volts (eV) of kinetic energy (KE), preferably at least about 20 eV. The mass of electrons is many orders of magnitude smaller than the mass of ions and the electrons do not essentially carry moment or momentum to the surface. Therefore they do not damage the surface. The present invention provides an alternative of low damage to the processes increased by ions, using the chemical attack increased by low energy electrons (LE4), in which the substrate sits on the anode of a DC halo discharge. The energy of the electrons and the negative ions that reach the substrate on the anode of a DC discharge is limited to a value no greater than the ionization potential of the reaction gas; energies above this limit are effectively dissipated by non-elastic collisions in the gas phase. No fundamental limit of this type is imposed on the positive ions produced in the microwave and RF plasmas. In one aspect, the invention involves a process for anisotropic chemical attack, with low damage, of a substrate, which includes the steps of placing the substrate on the anode of a direct current plasma reactor and clamping the substrate to a plasma that it includes low energy electrons and a gaseous species that is reactive with the substrate. The substrate can be a group IV semiconductor, a group III-V semiconductor, a group II-VI semiconductor, an oxide, a nitride, a metal or an alloy or mixture of the above. The reactive species can be any that reacts with the substrate and volatilizes within the temperature and flow of the device. The typical reactive species that is going to be used is hydrogen, halogen, compounds interhalocene, hydrogen halides, and volatile organic compounds. The low energy electrons are generated using a cold cathode. A voltage of approximately 0.5-2 kV is applied between the cathode and the anode, generating a halo discharge in which the electrons having a kinetic energy of less than about 100 eV, or preferably less than about 20 eV, reach the anode. In still another aspect, the invention is an apparatus for conducting the low energy anisotropic chemical attack of a substrate. The apparatus includes a chamber formed by a pair of connected tubes, at an angle one of the other. The camera is, of course, capable of being placed under a vacuum. A cathode is in a tube and an anode is in the other, so that the anode is placed at a. angle of about 30 ° to 120 ° or, preferably about 90 ° of the cathode. The cathode is a cold cathode, capable of generating a halo discharge of DC in which low energy electrons having a kinetic energy less than 100 eV, or preferably less than about 20 eV, reach the anode. The cathode has an end that can be connected to a power supply that is preferably located outside the tube. The anode has means for mounting the substrate on East. Preferably, the anode and the substrate are surrounded by an electrical shield to prevent the plasma from "jumping" to alternative electrodes. The shield includes an opening, the size and shape of which determine the area of the sample exposed to the plasma. The apparatus further includes means for introducing the reactive gas into the sealed chamber. In a preferred embodiment, the cold cathode is a hollow cathode formed with permeable, mesh, or perforated walls, generally referred to as permeable, instead of the typical solid walls. The cathode, in a preferred embodiment, can be cylindrically shaped with a side wall of a permeable conductive material, such as stainless steel mesh, and having an end that is open or closed, and an open end. The cathode is connected to a cathode mounting post and to the power supply. The cathode may comprise a plurality of nested or nested side walls, each connected to the power supply. The use of this cathode allows the generation of a large flow of low energy electrons at low pressure and temperature. An advantage of the method and apparatus of the present invention is that the anisotropic, submicron, low damage chemical attack of a substrate is achieved.

Yet another advantage of the method and apparatus of the present invention is that it is applicable to a variety of substrates using a variety of reactive species. Yet another advantage of the method and apparatus of the present invention is that the apparatus is clearly simple to assemble and operate. Another advantage of the method and apparatus of the present invention is that the permeable hollow wall cathode can generate a higher flow of low energy electrons at a lower pressure. Other features and advantages of the method and apparatus of the present invention will become apparent to a person of ordinary skill in the art upon examination of the drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention can be better understood with reference to the following drawings. The drawings are not necessarily to scale, rather being the emphasis to clearly illustrate the principles of the present invention. Figure 1 is a top plan view of the apparatus of the present invention.

Figure 2 is a perspective view of a hollow wall cathode of the present invention. Figure 3 is a schematic view of the apparatus illustrating three preferred embodiments of the low energy electrons cleaning and etching apparatus of Figure 1; Figure 4 is a schematic view of an illustrative apparatus illustrating three preferred embodiments of the improved low-energy electrons cleaning and etching apparatus of the Figure 1; Figure 5 is a detailed schematic view of a support apparatus or sample of the low energy electrons cleaning and etching apparatus of Figure 1; and Figure 5a is a detailed schematic view of an alternative embodiment of the sample support apparatus of Figure 5.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY A preferred embodiment of a suitable etching apparatus for practicing this camera. The end plate 15 is attached to one end of the section 14 containing the cathode. The end plate 21 is preferably made of glass and serves as an observation gate for the section 14 containing the cathode. The end plate 22 is also preferably made of glass and serves as an observation gate for the sample, as described below. The end plate 17 seals the other end of the section 16 containing the anode. The third section 18 provides access to the chamber for a pair of pump members. A gate valve 24 is connected to an ultrahigh vacuum (UHV) pump station (not shown) and a throttle valve 28 is connected to a rotary vane pump (not shown). A barocell barometer 26 periodically checks the pressure in the system. The UHV pump must be capable of achieving and maintaining a vacuum base below approximately 10"8 Torr above the chamber.The rotary wall valve must be able to achieve and maintain a vacuum of approximately 0.025 to 2 Torr during the process of chemical attack. Of course, a pump can be used if it is capable of achieving and regulating the gaps listed above. The cathode 32 is mounted in the section 14 that contains the cathode. The cathode 32 is preferably a cold cathode, sometimes referred to as a field emitting cathode, which does work without the application of heat. The cathode is activated using an external power source (not shown) that applies a direct current (DC) voltage between the cathode and the anode. Because the chamber contains a gas (described below), the chamber functions as a DC halo discharge tube or DC plasma reactor. The cathode can be a standard cold cathode as are known in the art. Preferably, however, the cathode is a hollow permeable wall cathode of the present invention, more clearly illustrated in Figure 2. The cathode 32 includes a tubular wall 36 made of a permeable, mesh or perforated conductive material, such as mesh stainless steel. The wall can have a variety of shapes, the cathode shown in the Fiqura 2 is designed to fit in a cylindrical cavity such as the section 14 that contains the cathode of the chamber 12 of the apparatus shown in Figure 1. The cathode has two ends , a first open end 38 of face to the section containing the anode of the chamber, and a second end 40. The second end may be opened or closed with an end piece (not shown). The end piece can be a solid piece of material or made from the same mesh material. Other materials such as aluminum may be preferred for the cathode material and the cathode may be coated with gold or other conductive material. Different materials may be preferred for different reactive gases. An important aspect is that the material must not react with or interfere with the reactive gas. A mounting post 42 is coupled to the cathode to mount the cathode inside the chamber. The mounting post can be coupled to the end piece as described below. However, the mounting post can be coupled to the wall 36 or alternative mounting means can be employed. The mounting post 42 is a metal rod having one end coupled to the cathode and a second end coupled to a coupler 43. Preferably, the second end is screwed into the coupler 43. The coupler connects the mounting post 42 to an interfacial connection 44 high voltage, which passes through the end wall 15 and is connected to the external power supply. Of course, the point where the interfacial connection 44 passes through the wall Extreme Í5 is kept air-tight. The method for connecting the cathode to the power supply, and the high voltage interfacial connection, are standard in the art. A glass tube 45 encloses the mounting post 42 and the coupler 43 and protects the metal mounting post from the electrons. The length of the mounting post 42 is adjustable, so that the cathode 32 can be moved closer to or further away from the anode. This can be done by replacing the post 42 with a pole of another length or by using a telescopic type post. The cathode used in the present invention may be a hollow, single-walled, hollow cathode, or may include a plurality of similarly nested or nested structures having all permeable walls, as shown in FIG. Figure 2. The plasma current is further increased with such a design. A cathode according to the present invention can be processed as follows. A length of stainless steel mesh is surrounded around a mandrel having the desired shape (such as cylindrical). The mesh length is cut to size, so that it has two attached edges that run longitudinally of the mandrel. The two edges are welded by points or otherwise fastened together. One end of the mesh tube shaped wire, is formed into a cone by hand and coupled to the first end of the mounting post by welding or folding. The mandrel is removed. The permeable, nested or nested hollow wall cathode, shown in Figure 2, can be made by forming a group of cylinders of varying diameters. A solid stainless plate is welded to the mounting post. The cylindrical walls are concentrically welded to the other side of the plate, starting with the smaller diameter cylinder. While the specific forms for making a cathode according to the invention are described, it should be understood that alternative forms are anticipated. For example, the cathode could have two open ends and the cathode mounting post could be coupled along the length of the cathode wall. The cathode currently described allows the operating voltage of the apparatus to be decreased by at least 2 times due to its greater surface area of electron emission. The permeable walls allow the free flow of plasma and improve the operating parameters such as pressure, temperature and plasma stability. While the cathode of the invention is specifically described for use in the attack method Currently described chemical, and the apparatus for it, it should be appreciated that this has utility in other applications in which cold cathodes are used. For example, the cathode of the invention may prove advantageous for use in gas lasers, and in halo discharge illumination such as for advertisements, lighting and decoration. The anode assembly 50 is mounted in the section containing the anode of the chamber. The anode assembly includes an anodic disk 52, upper, solid, connected to a hollow support 53 cylindrically shaped. A cylindrically shaped hollow support tube 54 supports the anode and includes an upper portion 55 made of stainless steel having one end connected to the anodic disc 52 and a second end coupled to an intermediate portion 56 that is made of ceramic and functions as a separation of ceramic The lower portion 57 is made of stainless steel and has an end coupled to the ceramic spacing 56 and the lower end coupled to an end plate 58. A sample heater 59 extends through the end plate 58, the support tube 54 and inside the hollow support 53. In the preferred embodiment, the heater 59 is a resistive cartridge heater that measures approximately 6.35 mm (1/4 inch) in diameter by approximately 12.7 mm (1/2 inch) in length. By heating the hollow support 53 and the anodic disc 52, the heater also heats the sample. In some cases, it may be desirable to cool the sample. This can be done by removing the heat 59 and the flow of anhydrous nitrogen gas, cold, or liquid nitrogen flow through the hollow support tube 54. The support tube 54 exits the chamber 12 through an opening in the end wall 17. The support tube 54 passes through a gas line 60 which is in communication with the chamber 12. Since the line 60 is in communication with the chamber 12 it must also be capable of retaining a vacuum of base to approximately 10 ~ 8 Torr. In this way, all the gates of the pipeline must be air-tight. The inlet gate 61 of the pipe connects the pipe 60 to the chamber 12. The outlet gate 62 allows the passage of the support pipe 54 from the pipe 60. A thermocouple 63 passes through the gate 64, through from the pipe 60, to the chamber 12 and is coupled to the anodic disk 52. The thermocouple measures the temperature of the anodic disk. An electrical guide 65 passes through the gate 64 through the pipe 60, into the chamber 12 and is coupled to the anodic disk 52. This guide can be used to maintain the anode in, for example, the potential to ground. Because the apparatus as described includes the anodic mounting horizontally positioned, the sample must be retained on the anodic disk. Also, it is desirable to cover the anodic disk and a portion of the anodic mount with a dielectric shield to prevent the discharge current from contacting the metallic anodic disk and the support tube. The dielectric shield is also used to retain the sample on the anodic disk. The dielectric shield 70 is made of PYREXO glass and includes a cylindrical wall 72 that encloses a portion of the anodic assembly and a lower portion 74 that covers the anodic disk 52 and the substrate 68. The inferred portion 74 contains an opening that defines the area of Substrate exposed to plasma. The area of the opening must be selected so that the plasma reactor does not suffer from a loading effect during the chemical attack process. For the small reactor described in the present embodiment, an opening having an area of about 0.2 cm 2 or proved useful. For larger reactors, an opening is not necessarily required. Alternatively, an opening can be used which has the ability to be moved relative to the substrate that is attacked chemically This allows different portions of the substrate to be exposed to the plasma at different times. Such an arrangement has the capacity to support large diameter inserts or slices without causing any kind of damaging load effect on the system. The shield is retained in position by appropriate means. For example, the shield can be attached by a number of springs to the end plate 17. Three projection handles are formed at equal spacing along the flange of the wall 72. Three spatially corresponding fasteners, such as screws, are placed on the end plate 17. A spring is then held between each handle and its corresponding screw. The reactive species is supplied through the gas inlet gate 66 to the pipe 60. The leak valve 67 is used to adjust the flow rate of the gaseous reactive species. From the pipe, the gaseous reactive species is able to enter the chamber 12. Of course, the gas can be introduced into the chamber by any regulated means. The reactive species can be molecular or atomic. The complete anode assembly 50 can be removed from the chamber 12 by removing the clamp 20 securing the end wall 17 to the section 16 containing the anode. After the assembly of The anode is removed, the substrate 68 can be mounted on the anodic disk 52. The apparatus includes the means for observing the substrate. These means are preferably a He / Ne laser 80 that passes through the glass plate 22 and illuminates the substrate. A beam splitter 82 separates the incoming light from the reflected one and transmits the reflected light to the observer. In the present invention, the substrate is preferably a group IV semiconductor, a group III-V semiconductor, a group I-VI semiconductor, a metal, a superconductor or a polymer. Also, it is anticipated that alloys or mixtures of the above may be used. Also, oxides and nitrides of the above can be used. Examples of substrates are Si, SiC, GaAs, AlGaAs, AIN, gold, chromium, high Tc superconductors, aluminum, tungsten and platinum. In preferred embodiments, the substrate is Si, GaAs, or GAN. While the present description has employed the term substrate, it is apparent that the substrate comprises any workpiece, a portion of the workpiece, or a layer of the workpiece wherein the etching method of the invention is being used. For example, the substrate that is chemically attacked may be an epitaxial layer of simple crystalline silicon that has been deposited by chemical vapor deposition (CVD) on a raw silicon insert. Alternatively, the substrate that is chemically attacked may be a thermal silicon dioxide film that has been developed on the insert or slice, or a thin film of silicon dioxide deposited on the insert or slice. In addition, the substrate that is chemically attacked may comprise a region of polycrystalline silicon, or a silicide, or a polyurethane. These and other substrates that are applicable to the process of the invention are well known in the art and the appropriate substrates can be readily selected by those skilled in the art. Preferably, the substrate is patterned with a mask of wax confection, by method known in the art. For example, a mask of silicon dioxide (Si02) or silicon nitride can be placed on substrates using chemical vapor deposition enhanced by plasma followed by photolithographic pattern formation and wet chemical attack of the mask in a chemical attack with muffled oxide 1: 6. Other methods of placing masks and other mask materials can also be used. Si02 is particularly preferred because the present selective method for the substrate over SiO2 under the conditions used.

The substrate can be chemically attacked before being subjected to the chemical attack process currently described, to eliminate the native surface oxides. This chemical attack can be performed by methods known in the art such as immersing the substrate with a 10% HF solution for about 5 seconds. Other chemical products that can be used for chemical attack are hydrochloric acid is ammonium hydroxide. As an illustration, in use, the substrate, preferably masked and pre-attacked, is mounted on the anode. The UHV pumping station is activated to achieve a pressure in the chamber of approximately 10"8 Torr.The pressure is maintained for approximately 2 hours at a temperature of approximately 150 ° in order to degas the system and remove all H20. In order to remove the residual Si02 or other oxides from the surface of the substrate, the substrate can be annealed by heating approximately 200 ° C in pure H2 plasma for about 10 minutes.The gate valve is closed and the butterfly valve is open , and the leak valve is opened so that a pressure of approximately 0.025 to 2.0 Torr is maintained in the chamber.The temperature of the sample is regulated to the desired degree.The voltage within the range about 0.5 to 2 kV is applied between the cathode and the anode to produce an electron current density of up to about 0.5 A / cm2. At the same time, the reactive gas is fed to the portion containing the anode of the device at a flow of about 5-60 sccm. After the chemical attack is completed, the energy to the cathode is stopped, the flow of qas is stopped, and the vacuum is eliminated. The substrate is removed and is then available for the evaluation of the etching speed and the anisotropy rate and the additional procedure, if desired. It should be noted that the above detailed description describes a preferred embodiment of the invention and is not intended to describe all possible embodiments of the invention. For example, it is considered useful to increase the scale of the device. In this case, the operating parameters could probably change. In the selection of the camera or the size of the exposed sample, the laws of scale change established by the principle of similarity of the halo discharge physics must be followed, in order to select the new values of the energy, the pressure and geometry, to maintain the correct current densities. Further, Gas flow rates must be chosen to avoid loading effects.EXAMPLE 1 Si samples (100) were chemically attacked in 10% HF before loading in the chamber. The chemical etching cycle for each sample comprised three steps: (1) degassing for two hours at 150 ° C in UHV (approximately 10"8 Torr); (2) annealing for 10 minutes, at 200 ° C in pure H20 plasma to remove any thin residual oxide layer, (3) acid attack by a selected type at the selected process pressure of composition H2 / He and discharge stream, maintaining the sample temperature at 60 ° C, the which is known to be the temperature for the highest reaction rate between Si (100) and the hydrogen atoms.Silicon hot chips or plates downstream of the anode increased the reaction rate by facilitating the removal of the reaction products By thermal decomposition of silane on the plates, the H2 / He composition was in the range of 100% H2 to 10% H2.The pressure was in the range of approximately 0.03 to 2.0 Torr. apr Approximately 0.1 to 0.5 A / cm2. The gas flow was approximately 5 at 50 sccm. Voltages within the 0.5 to 2.0 kV range applied between the cathode and the anode produced charge current density in halo up to 0.5 A / cm2. The integrated load that passes through the sample was recorded for the subsequent calculation of the reaction yield. The chemically attacked element, defined by the aperture in the dielectric shield above the sample, was examined by profilometry in order to determine the rate of chemical attack and estimate the volume of the chemically attacked element. This volume was multiplied by the Si density to determine the number of Si atoms removed.

EXAMPLE 2 The GaAs substrates (100) were modeled with Si02 masks by plasma vapor deposition increased by plasma from an Si02 layer of 3000 Á, followed by photolithographic modeling and wet chemical attack of Si02 in a buffered oxide chemical attack 1: 6 The substrates were chemically attacked at room temperature in 100% Cl2, 100% H2, and various Cl2 / H2 mixtures. The process pressures were from 25 to 75 mTorr. Gas flows from 10 to 30 sccm for chlorine and hydrogen were maintained. After LE4, substrates were examined by profilometry to determine the etching rate, scanning electron microscopy (SEM) to determine the surface morphology and anisotropy of the chemical attack, and by scattered energy X-ray spectral analysis, for estimate the surface composition.

EXAMPLE 3 The substrates were one micron thick GaN films developed by metal-organic molecular beam epitaxy (MOMBE) on a 50 nanometer buffer or compensating layer of AIN on a Si (100) substrate. The substrates were modeled by deposition of a 0.2 micrometer Si02 film by plasma enhanced chemical vapor deposition, in which the characteristics of the test elements from 3 micrometers to 50 micrometers wide were defined by a standard photolithographic technique. Prior to LE4, the substrates were submerged for five seconds in 10% HF solution and then immediately mounted on the anode of the etching device. The current density was kept constant at approximately 300 mA / cm2 DC. The temperature of the sample was in the range of 75 ° C to 250 ° C. The typical process pressure was approximately 0.20 Torr with a hydrogen gas flow rate 60 sccm.

RESULTS Yes (100) At rates of chemical attack above 2000 Á / minute, yields approximating 0.02 silicon atoms / electron were achieved, with a resulting mean square quadratic roughness (RMS) of 2-3 Á. The most effective chemical attack was noticed at intermediate electron energies, which corresponds to the pressure region of 0.6 - 1.2 Torr for 100% H2, 1.0-1.6 Torr for 50% H2 / 50% He, and 1.2-1.4 Torr for 10% H2 / 90% He . The above results can be explained by estimating the energy of the incident electrons, as a function of the composition and pressure of the gas, and by comparing the contribution of the various electron-induced processes such as the changes in the energy of the electrons. Although the cross section for electrically enhanced etching of Si to produce silane (SiH4) is not yet known, it is reasonably expected to reach the threshold at a few eV. He Decomposition threshold stimulated by silane electrons, is as low as 10 eV. Therefore, at moderate electron energy, the net result is the competition between the enhanced chemical attack by electrons from the silicon surface and the redeposition of silicon from the electron-induced decomposition of the chemical attack product. For 100% H2, the reaction efficiency decreases with the energy of the incident electrons exceeding 10 eV, where the dissociation of the silane begins (pressure <0.6 Torr) and redeposition of the chemically attacked material takes place. At the same time, at low electron energies, < 5 eV (pressure> 1.2 Torr), the yield decreases due to the decrease in the cross section of the reaction stimulated by electrons. The dilution of the process gas by helium displaces the equilibrium towards higher energies. The dissociation of the silane at electron energy above 10 eV causes not only a decrease in the performance of the reaction, but also a change in the quality of the attacked surface. All samples attacked at lower pressure (high electron energy) show surface roughness of approximately 300 nm and obvious signs of redeposition. In contrast, at higher pressure / lower electron energy, the surfaces chemically attacked were similar to mirror in appearance with a mean square Roughness (RMS) of 2-3 A. It is believed that oriented silicon (111) and oriented silicon (110) could be attacked chemically with similarly attractive results. In addition, it is anticipated that polycrystalline silicon, amorphous silicon, and silicides could be chemically attacked using the process of the invention. Typical applications in silicon-based technology include the modeling of submicron elements on integrated circuits, and the chemical attack of critical elements on silicon sensors and micromachines.

GaAs (100) Samples chemically attacked in pure chlorine at room temperature showed high rates of chemical attack but extremely harsh surfaces. This result, observed initially in the improved chemistry with ions in the Cl2 plasma, is attributed to group III chloride residues caused by its low vapor pressure. The constitution of the residue is anticipated by the addition of hydrogen to the gas of chemical attack, which eliminates the element of group III as the most volatile hydride.

The SEM microphotographs were measured for GaAS samples chemically attacked at room temperature and 50 mTor of total pressure to various gas compositions: a) 30% H2 of a Cl2 solution; b) 50% H2 / 50% of a Cl2 solution; c) 75% H2 / 20% of a Cl2 solution; d) 100% H2. The morphology of the chemically attacked surface dramatically improves as the partial pressure of hydrogen increases: from extremely rough surfaces, cracked sidewalls and extensive residues at a high concentration of Cl2 for very smooth top surfaces at hydrogen concentrations exceeding 75% The RMS surface roughness for several samples chemically etched to > 75% of H2 was determined by an atomic force microscope (AFM) in contact mode and it was found to be 3-5 A (non-attacked samples had the same roughness). LE4 pure hydrogen from GaAs produced smooth surfaces and good anisotropy at the attack speed of 150A / minute. In contrast, electron cyclotron resonance (ECR) in hydrogen plasma did not chemically attack the GaAs reproducibly can cause surface roughness. In addition, the term hydrogen atoms do not chemically attack the GaAs; These eliminate the s as AsH3, but do not attack the atoms of Ga. Clearly, the chemistry increased by electrons in LE4 follows a different reaction mechanism. In the tests, pillars of a high proportion between small dimensions were successfully produced. Its attractive appearance demonstrates the feasibility of chemically attacking nanoscale elements in GaAs per LE4. A good anisotropy for pressure was observed throughout the whole range of the reactant composition.60 mTorr. The anisotropy deteriorates at higher pressure, with the notch (isotropy) that becomes observable at 75 mTorr and with rounded side walls with pronounced recess at 150 mTorr. This tendency is qualitatively explained by the dependency to the pressure of the thickness of the anodic sheet, where the negative particles receive their final acceleration towards the surface of the sample. The chemical attack becomes anisotropic when the component of normal electron velocity at the surface of the sample becomes much larger than the average speed of random movement with which the electrons enter the anode plate from the positive column. Similar arrangements refer to the anisotropy of the ion-enhanced chemical attack to the angular distribution of the velocity vectors of the arriving ions.

The dependence to the speed of the chemical attack on the hydrogen concentration, with the total pressure kept constant at 50 mTorr, was measured. The speed of the chemical attack gradually decreases from 3000 Á / minute to 100% of Cl2 up to 100% Á / minute to 100% of H2 due to the lower reactivity of the hydrogen radicals with GaAs. This allows the practitioner to switch between chemical attack speed and surface quality. By choosing 75% H2 / 25% Cl2, GaAs (100) surfaces can be recorded in LE4 with excellent anisotropy, stoichiometric composition and surface roughness RMS 3-5 Á, at the fully respectable speed of 250 Á / minute, all at room temperature. The temperature dependence of the etching rate for different compositions of the pressurized gas of 50 mTorr was measured for a) 75% H2 / 25% Cl2; b) 100% Cl2 and c) 100% H2. With chlorine present at any concentration, the rate of chemical attack increases by two orders of magnitude between room temperature and 150 ° C, reaching spectacular values greater than 4.5 μm per minute. For pure hydrogen, the rate of chemical attack does not change significantly over the same temperature range.

GaN Good anisotropy and high GaN selectivity were observed in relation to the Si substrate. Preliminary results indicated that GaN records faster at elevated temperatures while Si's etching speed is negligible at temperatures of 150 ° C. This was a strong dependence on the temperature of the chemical attack rate of GaN in the hydrogen plasma. In a series of GaN samples chemically etched into a pure hydroquinone plasma at a pressure of 0.20 Torr and 300 mA / cm2 of plasma current density, the etch rate was increased from 70A / minute at 50 ° C to 525 Á / minute at 250 ° C. But, at high temperatures the surface became rich in Ga. For a GaN sample attacked or recorded at 250 ° C for 20 minutes, the Ga spheres were clearly visible on the surface. Samples attacked at temperatures lower than 100 ° C (attack speed or 100 A / minute break) did not reveal any metallic Ga sphere. The Auqer spectra of the samples before and after the chemical attack showed that the chemically attacked samples at higher temperatures were rich in qalium, while at the stoichiometry of the surface attacked chemically at lower temperatures or essentially the same as for the samples not attacked. For these Auger estimates, the relative intensities of line L3M45M45 at 1068 eV and line K123L23 of nitrogen at 384 eV were compared on the samples before and after the chemical attack. The quantitative analysis of Auger was impossible due to the significant roughness inherent in the thick GaN films. By increasing the processing temperature, the rate of chemical attack was greatly increased, but at the expense of the degradation of the surface stoichiometry. However, according to the measurements it is possible to chemically filter the GaN films in hydrogen plasma with a respectable etching rate of 200 Á / minute, while maintaining the surface stoichiometry. In those cases where a higher rate of chemical attack is desirable, it is expected that the speed of LE4 can be significantly improved by the addition of a small percentage of chlorine to the qas mixture of the process. For example, the previous results with LE4 of GaAs show that the best combination of stoichiometry and chemical attack speed is achieved for 25% Cl2 / 75% H2 as a reactive gas composition. It will be obvious to those of skill in the art that many modifications can be made to preferred embodiments of the present invention, as described above, without departing substantially from the principles of the present invention. It is intended that all such modifications be included within the scope of the present invention, as defined in the claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Claims (34)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A process for chemically etching dry, anisotropically, with low damage, a substrate, characterized in that it comprises the steps of: placing a substrate on an anode of a direct current plasma reactor; fastening the substrate to a plasma that includes low energy electrons that have a kinetic energy of less than about 100 eV and at least one species reactive with the substrate; and the generation of electrons with a cold cathode.
2. 2. The process according to claim 1, characterized in that it also comprises the step of selecting the substrate of the group consisting of group III-V semiconductors, group IV semiconductors, group II-VI semiconductors, metals, alloys of the previous ones, superconductors and polymers.
3. 3. The process according to claim 1, characterized in that the substrate comprises a group III-V semiconductor.
4. 4. The process according to claim 3, characterized in that it further comprises the steps of selecting the group III-V semiconductor from the group consisting of GaAs, AlGaAs, GaN and AIN.
5. 5. The process according to claim 1, characterized in that the substrate comprises Si or SiC. The process according to claim 1, characterized in that it further comprises the step of selecting the reactive species of the group consisting of hydrogen, halogens, interhalogen compounds, hydrogen halides, volatile organic compounds and mixtures thereof. 7. The process according to claim 1, characterized in that the reactive species comprises hydrogen and chlorine. 8. The process according to claim 1, characterized in that it further comprises the step of modeling the substrate by chemical screening of the substrate through the openings in a mask. 9. The process according to claim 1, characterized in that the step of fastening The substrate to a plasma is driven at a pressure of approximately 0.03 Torr and approximately 2.0 Torr. 10. The process according to claim 1, characterized in that it also comprises the step of chemically pre-attacking the substrate wet, before placing the substrate on the anode. 11. The process according to claim 1, characterized in that the cold cathode is a hollow cathode with a permeable wall. 12. The process according to claim 1, characterized in that it further includes the steps of degassing the plasma reactor and annealing the substrate before clamping the substrate to a plasma including low energy electrons. The process according to claim 1, characterized in that it further includes the step of placing the anode at an angle of about 30 ° to about 120 ° from the cathode. The process according to claim 1, characterized in that it further comprises the step of selecting the substrate from the group consisting of gold, chromium, high Tc superconductors, aluminum, tungsten, platinum and alloys thereof. 15. A process for dry, anisotropic, low damage chemical attack of a group semiconductor III-V, characterized the process because it comprises the steps of: the provision of a direct current plasma reactor that includes a cathode and an anode; the placement of the semiconductor on the anode of the direct current plasma reactor; the generation of low-energy electrons with a cold cathode; and attaching the semiconductor to a plasma that includes low-energy electrons and a reactive species with the semiconductor. 16. The process according to claim 15, further characterized in that it comprises the step of placing the anode at an angle of about 30 ° to about 120 ° of the anode. 17. The process according to claim 15, characterized in that the cathode is a cold cathode hollow of permeable wall that is capable of generating electrons having a potential energy of about 1 eV to about 20 eV. The process according to claim 15, characterized in that it further comprises the step of modeling the semiconductor by chemical etching of the semiconductor by etching the semiconductor through the openings in a mask. 19. The process according to claim 15, characterized in that it also includes the steps of chemically pre-attacking the semiconductor before placing the semiconductor on the anode and annealing the semiconductor before clamping the semiconductor to the plasma, by clamping the semiconductor to the plasma of H2 to High temperature . • The process according to claim 15, characterized in that the semiconductor is GaAs and the plasma includes electrons having a potential energy below about 20 eV, and a reactive species of about 25% chlorine and about 75% of hydrogen. 21. The process according to claim 15, characterized in that the semiconductor is GaN and because it also includes the step of selecting the molecular species from the group consisting of chlorine, hydrogen and mixtures thereof. 22. The process according to claim 15, characterized in that it also includes the step of placing a dielectric shield on the semiconductor having an opening that defines the area of the semiconductor to be fastened to the plasma. 23. An apparatus for anisotropic chemical attack of low substrate damage, characterized in that the apparatus comprises: a chamber adapted to contain a direct current plasma reactor; a cathode cooled inside the chamber, to generate low-energy electrons within the chamber; an anode inside the chamber, placed at an approximate angle of between 30 ° and 120 ° from the cathode; the means for mounting the substrate on the anode; the means to apply a vacuum to the camera; and the means for supplying a reactive species with the substrate to the chamber. 24. The apparatus according to claim 23, characterized in that the chamber comprises a pair of glass tubes connected at an angle of approximately 90 ° and wherein the cathode is in a tube and the anode is in the second tube. 25. The apparatus according to claim 23, characterized in that the cathode is a hollow cathode with a permeable wall. 26. The apparatus according to claim 25, characterized in that the cathode is in the form of a tube and includes a side wall of material conductor in the form of a mesh connected to a power source, and at least one open end. 27. The apparatus according to claim 23, characterized in that it also includes means for heating the substrate. 28. The apparatus according to claim 23, characterized in that it also includes the means for observing the substrate. 29. The apparatus according to claim 23, characterized in that it further includes a dielectric shield surrounding the substrate, which includes an opening that defines the area of the substrate to be exposed to the plasma. 30. A hollow cathode of a permeable wall, characterized in that it comprises: a tubular wall of a permeable conductive material; and the means for connecting the tubular wall to an energy supply. 31. The cathode according to claim 30, characterized in that the permeable conductive material is selected from the group consisting of stainless steel, aluminum and gold. 32. The process according to claim 1, further characterized in that it comprises the step of pre-attacking chemically and cleaning the substrate before the chemical attack of the substrate using plasma that includes low-energy electrons. 33. The process according to claim 22, characterized in that the opening is movable with respect to the semiconductor. 34. The apparatus according to claim 29, characterized in that the opening is movable with respect to the substrate.