Reactive ion etching method

Abstract: A dry etching method for performing anisotropic etching of a layer of a silicon based material without using a chlorofluorocarbon gas, is proposed. Sulfur halides yielding free sulfur (S) into a plasma under conditions of dissociation by electrical discharge, such as S₂F₂ or S₂Cl₂, are used as main components of the etching gas. This S is used for sidewall protection and for improving selectivity during etching, and is removed by sublimation by heating the wafer after etching. Although etching may be achieved by S₂F₂ alone, suitable measures may preferably be used to increase the S/X ratio of an etching reaction system, which is a ratio of the number of atoms of S to that of X or a halogen, because the layer of the silicon based material is highly susceptible to halogen radicals. Specifically, optimum results may be obtained by (a) adding H₂, H₂S or SiH₄ to the etching gas, (b) introducing hydrogen in advance into an area for etching by ion implantation, (c) using a silicon-containing resist mask, or (d) by coating the surface of a wafer cover with an amorphous silicon layer.

Description: This invention relates to a reactive ion etching method employed for the preparation of semiconductor devices. More particularly, it relates to a method for anisotropic etching of a trench in a substrate of silicon to high accuracy by low temperature etching without employing a chlorofluorocarbon (CFC) gas. In keeping up with the recent tendency towards high integration and high performance of semiconductor devices, such as VLSIs or ULSIs, a strong demand has been raised for a technology of etching of a layer of silicon based material, such as single crystal silicon, polysilicon, refractory metal silicide or polycide, in such a manner as to meet the requirements, such as high anisotropy, high etchrate or high selectivity, with a high level. Typical of the etching processes for single crystal silicon is trench etching for isolation of minor-sized elements or procuring a cell capacity space. Although anisotropic processing with a high aspect ratio pattern is required in this process, the cross-sectional shape of the trench tends to be changed intricately due to fluctuations in mask patterns or etching conditions, so that unusual shapes, such as undercuts or bowing, are encountered frequently to produce difficulties in trench filling or capacity control performed in subsequent steps. Typical of the etching processes for polysilicon, refractory metal silicide or polycide, on the other hand, is gate processing. Since the pattern width in the gate electrode directly affects sidewall dimensional accuracy in an LDD structure, or channel length of a transistor in which the source-drain region is formed in a self-aligned fashion, the gate processing is similarly in need of extremely high processing accuracy. Conventionally, for etching the layer of the silicon based material, a chlorofluorocarbon (CFC) gas, as exemplified by CFC113 (C 2 Cℓ 3 F 3 ), has been used extensively as an etching gas. Since both F and Cl are contained in the molecule of the CFC gas, high anisotropy may be achieved because etching may proceed both by the radical reaction by radicals such as F * or Cl * and by the ion assist reaction by Cl + , CF X + or CCl + ions, whilst sidewall protection may be achieved by a carbonaceous polymer deposited from the gaseous phase. However, several problems have been pointed out in connection with the CFC gas. First, carbon contained in the CFC gas deteriorates selectivity with respect to the layer of the silicon oxide (SiO 2 ). The C-O bond exhibits a higher interatomic bond energy than that of the Si-O bond, so that, if carbon is adsorbed to the surface of a layer of the SiO 2 based material, the Si-O bond may be weakened, or SiO 2 may be reduced to Si, and hence resistivity against halogen-based etchant is lost. This raises a serious problem if gate electrode processing is performed with a thin gate oxide film as an underlying layer. Secondly, there is a risk of contamination by particles by a carbonaceous polymer. If the design rule for semiconductor devices is refined further in future, the carbonaceous polymer deposited from the gaseous phase is likely to turn out to be a serious source of pollution by particles. The most serious problem, however, is that the CFC gas is among the factors responsible for destruction of an ozone layer of the earth, and a ban will be placed in near future on the production and use of the gas. Thus it is incumbent in the domain of dry etching to find out a suitable substitute material for the CFC gas and to establish an effective method of using the substitute material. Among the techniques thought to be promising for not using the CFC gas, there is a low temperature etching, which is a technique of maintaining the temperature of a substrate to be etched (wafer) at a temperature not higher than 0°C to freeze or suppress the radical reaction on the pattern sidewall to prevent occurrence of shape unusualties, such as undercuts, while maintaing the etchrate along the depth at a practical level by the ion assist effect . In Extended Abstracts of the 35th Spring Meeting (1988) of the Japan Society of Applied Physics and Related Societies, page 495, lecture number 28a-G-2, for example, a report has been made of an example in which the wafer was cooled to -130°C and silicon trench etching and etching of a n + -type polysilicon layer was performed using the SF 6 gas. However, if only freezing or suppression of a radical reaction is resorted to for realization of high anisotropy, a lower temperature which may be realized by using liquid nitrogen is necessitated, as discussed above. This raises a problem in hardware aspects because a specific large-sized equipment is necessitated and vacuum sealing is deteriorated in reliability. In addition, the throughput may be lowered and hence economic profitability or productivity may be affected because a time-consuming operation is involved in cooling the wafer and in subsequent heating for resetting the temperature to ambient temperature. The publication of J. Pelletier et al. in Japanese Journal of Applied Physics, Vol. 29, no. 9, September 1990, pages 1846-1853, relates to low-pressure plasma etching processes of Si and includes a comparison of several sulfur monohalides, among them S 2 Cl 2 and S 2 F 2 . The US-A-4 956 314 describes a method for differentially etching silicon nitride comprising the steps of forming a silicon nitride layer in an essentially hydrogen-free environment, implanting hydrogen into various regions of said silicon nitride layer, and differentially etching said silicon nitride layer. In the EP-A-0 258 698 a dry etching method is disclosed, wherein a cooling medium is supplied to the wafer holder to thereby affect the temperatures and the vapor pressures of the etching gas and the neutral radicals. From the EP-A-0 047 663 it is known to add hydrogen gas to the etching gas in a microwave plasma etching system in order to consume excessive halogen radicals to improve the anisotropy of the etching process. The EP-A-0 065 085 relates to a cathode for reactive ion etching which contains silicon or other dummy components for improving the etching rate and the uniformity of etching. In view of the above depicted status of the art, it is an object of the present invention to provide a reactive etching method whereby satisfactory anisotropic etching of a trench in a substrate of silicon may be achieved without using the CFC gas while the wafer cooling temperature is set within a practical temperature range even although the etching is carried out at a lower temperature. It has occurred to the present inventors that, by combining wafer cooling to a lower temperature with sidewall protection, anisotropic processing may be carried out at a temperature higher than heretofore to realize a practical low temperature etching process. If such an etching gas is selected that is capable of producing a carbon-free deposition material in a plasma under discharge dissociation conditions, selectivity of the SiO 2 based material is not lowered, while pollution by the carbonaceous polymer is not produced. The deposition material should be capable of being removed easily after completion of etching. As a result of our researches in line with the above, the present inventors attention has been directed to sulfur as an example of the deposition material. It has now been found that sulfur may be produced in a gaseous phase by discharge dissociation of sulfur halides, having a larger S/X ratio, which ratio is that of the number of sulfur atoms to that of halogen atoms in the molecule. Sulfur is deposited on the wafer surface, depending on the particular etching conditions, if the wafer temperature is controlled to be lower than ambient temperature. Adsorbed sulfur contributes to mask selectivity and underlying layer selectivity on a surface on which ions are incident in a perpendicular direction, while being deposited continuously on the pattern sidewall for exhibiting sidewall protection effects on the pattern sidewall where ions are not incident in a perpendicular direction as a principle. Besides, sulfur remaining on the wafer may easily be removed by sublimation by heating the wafer to 90 to 150°C after completion of etching without the least risk of pollution by particles. According to the present invention, the compounds first enumerated as sulfur halides are four sulfur fluorides, namely S 2 F 2 , SF 2 , SF 4 and S 2 F 10 . These compounds have been proposed in a preceding proposal as main components of an etching gas for a layer of an SiO 2 based material. Our preceding proposal in connection with etching of the SiO 2 based material has been directed to an ion mode etching based on contribution of S + or SF x + . On the other hand, our present proposal is directed to an etching by assisting a F * radical reaction by the incident energy of the above ions by optimizing the operating conditions. The present invention relates to a reactive ion etching method for etching a trench in a silicon substrate according to the features of the appended claim 1. According to the invention hydrogen is introduced into an etching area previously by ion implantation. By introducing hydrogen at a predetermined range by controlling the energy of implantation, hydrogen may be yielded concentratedly when the etching depth reacnes the vicinity of its range to increase the consumption of the halogen radicals. Simultaneously, sulfur is deposited on the etching bottom to lower the etchrate abruptly. This is effective in providing for a uniform etchrate, regardless of the aperture size, for suppressing microloading effects. Figs. 1a to 1c are schematic cross-sectional views showing a process according to an embodiment of the present invention applied to silicon trench etching, step by step, wherein Fig. 1a shows the state in which hydrogen is ion-implanted or a single crystal silicon substrate, Fig. 1b shows trench etching being performed and Fig. 1c shows the state of completion of trench etching. The present invention is applied to trench processing in which hydrogen was previously introduced by ion implantation into a single crystal substrate to a depth not less than the desired trench depth and etching was carried out using S 2 Cl 2 . The present example is explained by referring to Figs. 1a to 1c. A wafer in which silicon oxide etching mask 12 was formed on a single crystal substrate 11 as shown for example in Fig. 1a was prepared. The etching mask 12 had a sidewall formed by etchback by reactive ion etching (RIE). In this manner, a first aperture 13a was approximately 0.2 µm in diameter which is in excess of the limit of resolution of the present status of photolithography. A second aperture 13b was approximately 0.5 µm in diameter. H 2 + ions were then implanted at a dosage of approximately 10 16 atoms/cm 2 into the single crystal substrate 11 through the first aperture 13a and the second aperture 13b. As a result, high hydrogen concentration regions 14a, 14b were formed to a depth of not less than 2 µm from the surface of the single crystal silicon substrate 11. It is noted that the energy of ion implantation may be controlled for setting the range to a value approximately equal to the desired trench depth. This wafer was set in a magnetic microwave plasma etching device and, as an example, the single crystal silicon substrate 11 was etched under the conditions of the S 2 F 2 flow rate of 5 SCCM, a gas pressure of 1.3 Pa (10 mTorr), a microwave power of 850 W (2.45 GHz), an RF bias power of 100 W (2 MHz) and a wafer temperature of -70°C. Since S 2 Cl 2 has a melting point of -80°C and a boiling point of 137°C, and hence is a liquid material at ambient temperature, it was gasified by bubbling using an He gas before being introduced into the system. During this process, an ion assist reaction by S + , Cl X + or SCl + ions and the radical reaction by Cl * radicals proceeded simultaneously to effect etching at a high etchrate, while sulfur yielded from S 2 Cl 2 was deposited on a pattern sidewall to produce a sidewall protection film 16 for forming trenches 15a, 15b with high anisotropy. Since the efficiency of incidence of active etchants is higher at the second aperture 13b than at the aperture 13a narrower than the aperture 13b, the etchrate was higher at the second aperture 13b. As a result, in the course of etching, the bottom of the trench 15b within the second aperture 13b first reached a region of high hydrogen concentration 14b, as shown in Fig.1b. With the conventional etching process, the trenches 15a and 15b tend to differ from each other markedly in depth under the microloading effects. However, according to the present invention, Cl * are consumed at this stage by hydrogen supplied from the region of high hydrogen concentration 14b so that the apparent S/Cl ratio is increased abruptly. The result is that formation of sulfur deposit 17 on the bottom of the trench 15b proceeds competitively with removal thereof by sputtering so that the etchrate is lowered. As a result, the trench 15b was scarcely increased in depth. As etching proceeded further, the bottom of the trench 15a reached a region of high hydrogen concentration 14a and a sulfur deposit 17 was formed by the same effect. Since etching was substantially terminated, the trenches 15a and 15b having different diameters were formed to substantially the same depth without being affected by the microloading effects.

Claims: A reactive ion etching method for etching a trench of a desired depth in a silicon substrate (11) by using an etching gas containing S 2 F 2 , SF 2 , SF 4 , S 2 F 10 , S 3 Cl 2 , S 2 Cl 2 or SCl 2 , capable of releasing a halogen radical into a plasma under conditions of discharge dissociation, said method comprising the steps of forming a region (14a, 14b) of high hydrogen concentration at the desired depth from the surface of the substrate (11), wherein the energy of the ion implantation is adjusted for setting the range to a value approximately equal to the desired trench depth, and the dosage is adjusted to a value, so that in the etching step hydrogen is released in an amount so that the etching rate is lowered abruptly due to sulfur deposition on the bottom of the trench, subseqently etching the substrate (11) while cooling the substrate (11).

Inventors: KADOMURA SHINGO, NAGAYAMA TETSUJI, TATSUMI TETSUYA, KADOMURA, SHINGO, NAGAYAMA, TETSUJI, TATSUMI, TETSUYA
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L21, C23F4