Method for dry-etching

Abstract: The present invention provides a method for dry-etching a solid surface with a gaseous bismuth halide compound, which permits achivement of a simple and perfect dry-process for manufacturing of electoric devices, quantum devices etc., giving a high reproducibility.

Description: FIELD OF THE INVENTION The present invention relates to a method for dry-etching. More particularly, the present invention relates to a novel method for dry-etching a solid surface such as a silicon surface or a gallium arsenide surface, which is useful as a basic technology for the manufacture of quantum devices. PRIOR ART AND PROBLEMS In the conventional manufacturing process of semiconductor devices, the etching method of a semiconductor crystal surface is popularly adopted as a basic technology for removing unnecessary portions of an insulating film or a metal thin film along a resist pattern at a high accuracy. Accordingly, as the degree of integration of a semiconductor device has increased, however, the pattern width has become smaller each year. With chemical etching (wet etching) using a solution, which has been used as a concrete means of etching, it has now become difficult to cope with the need to form such a high-density pattern. Under these circumstances, general attention is paid to a method known as dry-etching. The dry-etching method conducts etching in a relatively clean atmosphere under vacuum, and is therefore expected to enable the processing of fine quantum devices. For silicon (Si), a representative device material, for example, a dry-etching technique using fluorine and chlorine based halogen gases is known. This dry-etching technology makes it possible, in the case of silicon, to remove unnecessary portions of a fine pattern while keeping controllability on a necessary level, and is now considered as a major technique in practice. In spite of the remarkable progress made in the dry-etching technique, in the case of silicon, it is common practice to remove an oxide film on a silicon surface by the application of wet etching using an etching liquid such as hydrofluoric acid. This is because the oxide film on a silicon surface cannot effectively be removed by conventional dry-etching techniques. Application of wet etching may thus be inevitable. In the case of this wet etching, however, a problem is that the surface undergoes a change in the course of passing silicon through a vacuum after etching. Various means are of course studied to maintain the stability of the surface after removal of the oxide film. Along with the progress toward finer and higher-integration devices, no effective means has not as yet been found to stabilize the silicon surface after etching, but various problems have become more apparent. These problems have also become similar for the other various solid surfaces useful for fine quantum devices. For the purpose of developing a dry-etching technique to form a fine pattern on quantum devices, there is naturally a demand for achieving a perfect dry-etching process in which the removal of an oxide film on a solid surface such as silicon, GaAs and so on is accomplishing by a dry-process. The present circumstances as described above however impose serious restrictions on the achievement of a perfect dry-process. This also exerts an important influence on the practical development of the dry-etching technique itself for the formation of a pattern on the device material surface, and hinders efficient improvement permitting high-accuracy patterning. The present invention was developed in view of the circumstances as described above, and has an object to provide a novel method for etching, which overcomes the restrictions in the conventional etching technique of a solid surface, permits dry-etching at a high reproducibility by simple means, also for an oxide film on a crystal surface, and allows the achievement of a perfect dry-process. SUMMARY OF THE INVENTION The present invention provides, as a means to solve the above-mentioned problems, a method for dry-etching a solid surface with a gaseous bismuth halide compound in an ultra high vacuum. The present invention provides also methods for dry-etching with a gaseous bismuth halide compound which permits: 1) etching a solid crystal surface of a semiconductor, such as silicon, gallium arsenide as compound semiconductor, etc; 2) etching a silicon surface in units of a single atomic layer; 3) selective etching of a silicon surface having a different plane orientation; 4) removal of an oxide film on a silicon surface; and 5) selective etching between a silicon surface and a gallium arsenide surface. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a diagram illustrating the relation between RHEED specular beam intensity and time in the case of the dry-etching of a Si(111) surface and of a GaAs (100) surface by BiF 3 . DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for dry-etching a solid surface simply with a high reproducibility by using a gaseous halide compound of bismuth (Bi) as described above in ultra high vacuum (UHV). Among solid materials, the silicon surface is generally very rich in reactivity. As a result, this is considered to cause contamination by impurities since different substances present on the surface are incorporated to reduce the surface energy. Particularly during growth, removal of these impurities is an important task. On the other hand, a means which positively utilizes this property to actively control surface energy is called a "surfactant". This is observed in all materials, and As and Sb are reported to be effective on silicon. However, the purpose in this case is to facilitate hetero-epitaxy into an unmatched lattice system (alleviating the distortion to a uniform state) and incorporation of the surfactant itself into the film is very large. Recently, Bi has been proposed as a new material exhibiting a surfactant effect. Fabrication of atomically abrupt Si/Ge/Si hetero-interfaces was demonstrated using Bi as a surfactant by Kunihiro Sakamoto et al (Jpn. J. Appl. Phys. Vol. 33 (1994) pp.2307-2310). Bi is characterized in that, although it falls under the Group V family like As and Sb, the large atomic radius permits a remarkable inhibition of incorporation into the film. Furthermore, it was found that heating to 400°C caused desorption of Bi from the surface. With these findings in view, the possibility of using Bi as a carrier carrying halogen atoms to the surface was considered from the point of view of etching. The conventional etching methods may be considered as the accumulation of many trials to carry halogen atoms onto the substrate, and have drawbacks in that accurate control cannot be conducted as compared with a means of epitaxial growth, that large quantities of toxic substances as an etchant are used, and that auxiliary facilities are therefore very complicated and large in scale. The use of Bi has the following marked features: (1) It is possible to reduce surface contamination; (2) As the surface is covered with Bi, it is possible to alleviate the surface energy intrinsic to the solid surface resulting in a smooth surface; and (3) Because introduction is not by means of ionization or in the form of plasma, there is only a little damage to the surface. By using Bi having these advantages as a carrier of halogen atoms, the functions of conventional wet etching can be replaced by dry-etching. The halide of bismuth (Bi) used as an etchant in the method of the present invention can be expressed by a general formula BiX 3 (where X is a halogen atom), and can be selected from the group consisting of various compounds including those based on BiF 3 , BiCl 3 and BiBr 3 . Apart from halogen atoms, oxygen or sulfur atoms may be bonded. In all cases, these bismuth halides would be applied onto the solid surface. It is emphasized that the method of the present invention using a gaseous bismuth halide compound as an etchant realises the etching of atoms on the surface in units of a single atomic layer. A solid surface is etched in an atomic layer-by-layer fashion with a bismuth halide compound. In applying this method to a silicon surface, for example, attention is given to F which has the highest electronegativity among applicable halogen atoms. It has been reported that a clean surface of silicon having a high reactivity can be clearly ground by means of Br. In the present invention, in which Bi reduces the surface energy and is used as a carrier, a material having a high electronegativity is chosen. That material is BiF 3 . It is stable and has a high boiling point of 650°C. (Some reports give a boiling point of 760 or 730°C.) This temperature is within the region in which K-cell control is applicable. As a result, consumption of etchant material is very slight. According to the method of the present invention, it is possible to remove an oxide film on the solid surface, and to remove residual etchant at a relatively low temperature. By using the present method for dry-etching a solid surface, etching accuracy is controlled within units of a single atomic layer. Namely, the etching process is stopped at a desirable single atomic layer in accordance with an intensity of RHEED. The etching process is controlled by stopping the supply of the gaseous bismuth halide compound. A K-cell is preferably used to control the supply of the bismuth halide compound by closing a shutter of the K-cell or opening said shutter. According to the method of the present invention, furthermore, the following functions are available when compared with removal of the oxide film by conventional wet etching using a solution: (1) It is not necessary to conduct waste liquid disposal; (2) Uniform and sophisticated etching is applicable, which corresponds to the current tendency toward finer and three-dimensional devices; (3) It is possible to simplify the production line including the elimination of the drying process; and (4) It is possible to selectively etch, such as between a silicon surface and a gallium arsenide surface. Also in the method of the present invention, the ultra high vacuum (UHV) condition is adopted. However, the ultra high vacuum (UHV) condition required by the dry-process and various other conditions for the introduction and discharge of an etchant gas and heating of the substrate largely vary in accordance with the corresponding conventional methods within the scope of the present invention. The method of the present invention is now described further in detail by means of working examples. Example 1 BiF 3 was supplied as an etchant to Si(111) and a surface oxide film was removed by dry-etching. By heating the etching-treated surface, a clean and flat silicon surface was available. Subsequently, this clean surface was etched in a layer-by-layer fashion with BiF 3 : about ten layers of oscillation of reflection high energy electron diffraction (RHEED) representing etching in units of a single atomic layer were observed. Residual Bi on the surface could be removed by heating the surface to about 400°C. Because, without irradiation of BiF 3 under the same conditions, no RHEED pattern is observed, BiF 3 is considered to remove the oxide film on the Si surface. Example 2 A Si(100) plane and a Si(111) plane were placed on a substrate holder to observe the differences. This showed that the Si(100) plane became largely rougher, while the Si(111) plane became smoother, as observed from changes in the RHEED pattern. The etching rate (ER) is therefore considered to be: ER(100) > ER(111) This is attributable to the fact that the supply rate limited range for the decomposition of BiF 3 is not reached on a Si surface even at 730°C, and the decomposition rate limited range has yet been kept. The Si(100) plane has a larger number of dangling bonds. Reactivity of the surface is considered to be simply proportional to this number. Example 3 The difference in reactivity between Si(111) and GaAs(100) was investigated. Fig. 1 shows a diagram illustrating the relation between RHEED specular beam intensity and time for etching Si and GaAs in units of a single atomic layer. The period of this oscillation corresponds to the etching of a single layer [double layer (a coupled single layer at Si(111)), a coupled GaAs single layer at GaAs(100)]. We could confirm the progress of etching in units of a single layer. ER was investigated by the observation of RHEED oscillation on the GaAs(100) plane, where the supply rate limited range was almost satisfied on GaAs at a substrate temperature of 500°C. In the decomposition of BiF 3 , therefore, exchange of electrons with the substrate is very important: the difference in this catalyst-like function (i.e., between the GaAs substrate and the Si substrate) represents a large difference in ER (>50). In the area of device application, growth of GaAs on Si is now a subject of active discussion. The selective etching would therefore become an important technology. According to the prior art for etching GaAs, Si layers (Si as a dopant) remain on a crystal surface. However, as described in Fig. 1, the present method realizes the etching of both GaAs and Si. Additionally, by using the present method, AlAs was etched with a similar etching rate (ER) to the Si case. Then, the present method is also used for etching AlAs on GaAs. According to the present invention, as described above in detail, it is now possible to carry out the removal of an oxide film on a solid surface and the etching of the surface in units of a single atomic layer, both by dry-etching. It permits the achievement of a simple and perfect dry-process giving a high reproducibility.

Claims: A method of dry-etching a solid surface, which comprises the step of etching a solid surface with a gaseous bismuth halide compound in an ultra high vacuum. A method of dry-etching a solid surface as claimed in claim 1, wherein a solid crystal surface of a semiconductor is etched. A method of dry-etching a solid surface as claimed in claim 2, wherein the semiconductor is silicon or a compound semiconductor. A method of dry-etching a solid surface as claimed in claim 3, wherein a silicon surface is etched with a gaseous bismuth halide compound with an accuracy of a single atomic layer. A method of dry-etching a solid surface as claimed in claim 3, wherein a silicon surface with a specific plane orientation is selectively etched. A method of dry-etching a solid surface as claimed in claim 2 or 3, wherein both a silicon surface and a gallium arsenide surface formed on the silicon surface are etched selectively. A method of dry-etching as claimed in claim 1, wherein the solid surface to be etched is an oxide. A method of dry-etching a solid surface as claimed in claim 7, wherein an oxide layer on a surface of a semiconductor crystal is etched. A method of dry-etching a solid surface as claimed in claim 8, wherein the semiconductor is silicon.

Inventors: KANEKO TADAAKI, KAWAMURA TAKAAKI, KANEKO TADAAKI,, KAWAMURA TAKAAKI,
Applicants: JAPAN RES DEV CORP, RESEARCH DEVELOPMENT CORPORATION OF JAPAN
Application year: 1996
Main classification: H01L21
IPC codes: H01L21, C23F1, C23F4