A dry etching method of a silicon thin film

Abstract: In a method for selectively etching a second silicon layer of a multilayer structure which includes a first silicon layer and the second silicon layer formed on the first silicon layer and doped with impurities according to the present invention, the second silicon layer is selectively etched by using an etching gas including freon-14 gas and a gas selected from a group composed of hydrogen chloride gas and chlorine gas.

Description: The present invention relates to a drv etching method for use on a polysilicon thin film of a thin film transistor which is formed on an insulating film or on a semiconductor substrates constituting a semiconductor integrated circuit and a liquid crystal display device. Thin film transistors (hereinafter, referred to as a "TFT") are widely used as switching elements in liquid crystal display devices and semiconductor integrated circuits. An intrinsic polysilicon thin film (hereinafter, referred to as an "i layer") is used as a semiconductor laver constituting the TFT. Metal films are often used as source electrodes and drain electrodes which constitute the TFT. However, the interface resistance (contact resistance) between metal films and i layers is large. Accordingly, in the TFT an n + polysilicon thin film (hereinafter, referred to as an "n + layer") having a small resistance is formed as a contact layer between the i layer and the source electrode and between the i layer and the drain electrode, In forming such a TFT, an i layer and an n + layer are successively formed and then patterned. For patterning the n + layer to form a channel section, it is required to selectively etch the n + layer. Conventionally methods for selectively etching an n + layer include wet etching and dry etching. The wet etching method uses a water or acetic acid-diluted hydrofluoric acid nitric acid mixture. The dry etching method uses an etching gas of oxygen, carbon tetrachloride and freon-14. However, the wet etching method has the disadvantage that low dilution of the mixed etchant increases the etch rate and stains (oxide skins) are generated on the polysilicon thin film to be etched, degrading the controllability of etching and the uniformity in the etch rate. In contrast, the dry etching method is superior in both controllability and uniformity. However, the dry etching method has the disadvantage that the use of carbon tetrachloride gas will be prohibited in 1996 by the freon regulations. Two dry etching proposals have sought to improve upon the conventional procedure. In that described by Baldi & Beardo in J. Appl. Phys. 57 (6), 15 March 1985 at page 2221 selective etching of n-doped polysilicon over undoped polysilicon is achieved with CF 4 /CF 3 Cl/O 2 plasma. The etch rate was found to increase with CF 3 Cl/CF 4 but there was no selectivity improvement up to 20% CF 3 Cl. This suggests that selectivity is mainly determined by CF 4 , although the reported selectivity ratio of 1.2 is very low for practical purposes. In the method described by Mogab and Levinstein in J. Vac. Scl. Technol. 17(3) May/June 1980 at page 721, selective etching is achieved using C 2 F 6 and Cl 2 . The article investigated the influence of chlorine concentration on etch rate and selectivity and concluded that at up to 20% concentration, selectivity was unaffected, generally in line with Baldi and Bear do . Above 20% chlorine, however, etch rate and selectivity increased rapidly with chlorine concentration to a ratio of 15 at 90% chlorine. The present invention seeks to provide a dry etching method for selectively etching an n + layer using a gas free from restrictions under the freon regulations, in which the etch rate is uniform and etching can be easily regulated. In accordance with the invention, there is provided a method of reactive ion etching n-type polysilicon formed on an intrinsic polysilicon layer which comprises dry etching the n-type polysilicon using an etching gas constituted by CF 4 and a gas selected from hydrogen chloride and chlorine, possibly in admixture with oxygen, the concentration of CF 4 in the etching gas being 67 mole % or more based on CF 4 and hydrogen chloride or chlorine when the etching gas is constituted by CF 4 and one of hydrogen chloride or chlorine, or 58 mole % or more based on CF 4 and hydrogen chloride or chlorine when the etching gas is constituted by CF 4 , one of hydrogen chloride or chlorine, and oxygen, thereby selectively etching the n-type polysilicon with respect to the intrinsic polysilicon. In accordance with the invention, there is provided also a method for fabricating a thin film transistor including a gate electrode, a channel-section semiconductor layer electrically separated from the gate electrode by a gate insulating film, a pair of contact layers having contact with the channel-section semiconductor layer, a source electrode having contact with one of the contact layers and a drain electrode having contact with the other contact layer, comprising the steps of: forming an intrinsic polysilicon layer on the gate insulating film, forming an n-type polysilicon layer on the intrinsic silicon layer, and forming the pair of contact layers by selectively reactive ion etching a portion of the n-type polysilicon layer by the above method. In the method of the invention, the pressure of the etching gas is preferably 53Pa (400 mtorr) or more. Further, the etching gas preferably includes oxygen, suitably at a concentration of 12 mole % or less. Suitably, the activation energy of the n-type polysilicon layer is 0.2 eV or less, preferably 0.1 eV. (In the description, 1 electron-Volt (eV) is equivalent to 1.6. 10 -19 Joule (J)). The method of the invention has the advantage of using an etching gas including hydrogen chloride/chlorine and CF 4 (freon-14). The hydrogen chloride/chlorine and the freon-14 are free from the freon regulations so that an n-type polysilicon layer can be selectively etched using such a gas. At the preferred pressure of the etching gas of 53Pa (400 mtorr) or more and with the concentration of the freon-14 in the etching gas being 67 mole % or more, etching can be carried out with satisfactory controllability and at a uniform rate. The method of the invention has the further advantage that the n + /i selectivity can be maintained by using an etching gas containing oxygen in a concentration (mol%) of 12% or less, even though the concentrations of the hydrogen chloride/chlorine and the freon-14 are thereby reduced. The present invention will now be described in greater detail by way of example only with reference to the accompanying drawings, in which Figure 1 is a plan view of a TFT formed by a method of the present invention; Figure 2 is a cross-sectional view of the TFT taken along a line A-A' of Figure 1; Figure 3 is a cross-sectional view of an etching apparatus used in an example of the invention; Figure 4 is a diagram showing dependence of the n + /i selectivity, the uniformity of the etch rate of an n + layer, and the etch rates of the n + layer and the i layer, upon the gas pressure, in the case where the i layer and the n + layer having an activation energy of 0.2 eV are etched by a method of the invention; Figure 5 is a diagram showing dependence of the n + /i selectivity, the uniformity of the etch rate of an n + layer, and the etch rates of the n + layer and the i layer, upon the concentration (mol%) of freon gas in an etching gas, in the case where the i layer and the n + layer having an activation energy of 0.2 eV are etched by a method of the invention; Figure 6 is a diagram showing dependence of the n + /i selectivity, the uniformity of the etch rate of an n + layer, and the etch rates of the n + layer and the i layer, upon the gas pressure, in the case where the i layer and the n + layer having an activation energy of 0.1 eV are etched by a method of the invention; and Figure 7 is a diagram showing dependence of the n + /i selectivity, the uniformity of the etch rate of an n + layer, and the etch rates of the n + layer and the i layer, upon the concentration (mol%) of freon gas in an etching gas, in the case where the i layer and the n + layer having an activation energy of 0.1 eV are etched by a method of the invention. Figures 1 and 2 show an example of a TFT fabricated by a method of the present invention. Figure 1 is a plan view of the TFT, and Figure 2 is a cross-sectional view of the TFT taken along a line A-A' of Figure 1. In the TFT, a gate electrode 9 branched from a gate bus line 10 and an intrinsic polysilicon thin film layer 7 (hereinafter, referred to as an "i layer") are formed on a base substrate 12 made of a glass plate or the like, sandwiching a gate insulating film 8 therebetween. A source region n + layer 5 and a drain region n + layer 6, which are electrically separated from each other, are formed as a contact layer on the i layer 7. A source electrode 2 and a drain electrode 3 are formed an the n + layers 5 and 6, respectively. A source auxiliary electrode la is formed on the source electrode 2 and connected to a source bus line 4. Further, the drain electrode 3 is connected to a pixel electrode 1. The TFT as mentioned above can be fabricated as follows: First, a metal thin film or the like is deposited on the base substrate 12 and etched into a desired configuration and size to form the gate bus line 10 and the gate electrode 9. The gate insulating film 8 is formed on the substrate by a CVD method or the like. Then, an intrinsic polysilicon is formed on the gate insulating film 8 the i layer 7. Further, polysilicon doped with n type impurities is formed on the i layer 7 as an n + layer which will be divided into the n + layers 5 and 6. Subsequently, a step for dividing the n + layer on the i layer 7 into the source region n + layer 5 and the drain region n + layer 6 so as to form the channel section will be described with reference to Figure 3. In the TFT mentioned above an etching prevention film is not provided on the i layer 7 for simplifying the production steps, so that it is necessary to completely control the selective etching of the n + layer and the i layer. First, in a dry etching apparatus 20 of a parallel flat plate type in a RIE (Reactive Ion Etching) mode, substrates each having the i layer and the n + layer deposited thereon in the above-mentioned manner are provided on a susceptor 21. An etching gas including hydrogen chloride gas (HCl), freon-14 gas (CF 4 ), and oxygen gas (O 2 ) with the flow ratio (Viz. the ratio of the concentration (mol%) in the apparatus 20) of 5:15:2, is injected into the dry etching apparatus so as to have a gas pressure of 53Pa (400 m Torr). Then, an electric power having an RF electricity concentration of 0.25 w/cm 2 is applied to the susceptor 21 via an RF power supply 22. Polysilicons of the i layer and the n + layer react with a reaction species generated in this way to form volatile substances, and then the layers are patterned. The i layer has a higher activation energy than the n + layer, so that the i layer is less likely to react with the reaction species. Accordingly, it is possible to selectively etch the n + layer. The i layer can be patterned as a different step before or after the selective etching. In the i layer 7, phosphorus may be diffused and an active layer may be formed depending on the temperature at the formation of the n + layer on the i layer 7 or the thermal hysteresis in other steps. In order to remove the active layer together with the n + layer by etching, overetching can be used. The source electrode 2 and the drain electrode 3 are formed on the n + layers 5 and 6, respectively. Then, a source auxiliary electrode la is formed on the source electrode 2 and connected to the source bus 4, and the pixel electrode 1 is connected to the drain electrode 3. Further, a liquid crystal orientation film is entirely formed on the substrate and liquid crystal is sealed between the substrate and a counter substrate, thereby obtaining a liquid crystal display device. Hereinafter, the results obtained by measuring etching characteristics will be shown, in the case where the etching condition for selectively etching the n + layer and the i layer is varied. Figure 4 shows the results obtained by measuring the dependence of the n + /i selectivity, the uniformity of the etch rate of the n + layer (n + uniformity), the etch rates of the n + layer (n + etch rate) and the i layer (i etch rate), upon the gas pressure, in the case where the i layer and the n + layer having the activation energy of 0.2 eV are etched by using an etching gas including hydrogen chloride gas, freon-14 gas and oxygen gas with the flow ratio of 5:15:2 at a stable RF electrical density of 0.25 W/cm 2 . The n + /i selectivity of a process is a measure of the etch rate of the layer to be etched (n + layer) relative to other material (i layer) on the wafer surface. As is understood from this diagram, the uniformity of the etch rate of the n + layer is decreased (improved) as the gas pressure raises. Further, when the gas pressure is at 53Pa (400 m Torr) or more, the etch rate of the n + layer sharply increases compared with that of the i layer. When the gas pressure is at 53Pa (400 m Torr), the n + /i selectivity becomes minimum. Accordingly, in the case where an etching gas including hydrogen chloride gas, freon-14 gas and oxygen gas is used, the n + /i selectivity, and the uniformity of the etch rate of the n + layer can be simultaneously improved when the gas pressure is at 53Pa (400 m Torr)or more. Figure 5 shows the results obtained by measuring the dependence of the n + /i selectivity, the uniformity of etch rate of the n + layer, and the etch rates of the n + layer and the i layer, upon the gas concentration of freon-14, in the case where the i layer and the n + layer having an activation energy of 0.2 eV are etched at a stable gas pressure of the etching gas of 40Pa (300 m Torr) and a stable RF electrical density of 0.25 W/cm 2 . Herein, the dependence are measured by varying the flow ratio X of freon-14 in an etching gas including hydrogen chloride gas, freon-14 gas, and oxygen gas with the flow ratio of 5:X:2. CF 4 /CF 4 +HCl+O 2 in this diagram represents the flow ratio (Viz. concentration (mol%)) of freon-14 gas against the gross volume of hydrogen chloride gas, freon-14 gas, and oxygen gas, CF 4 /CF 4 +HCl represents the flow ratio of freon-14 gas against the gross volume of hydrogen chloride gas and freon-14 gas, and O 2 /CF 4 +HCl+O 2 represents the flow ratio of oxygen gas against the gross volume of hydrogen chloride gas, freon-14 gas, and oxygen gas, respectively. As is understood from Figure 5, the uniformity of the etch rate of the n + layer is decreased (improved) as the concentration (mol%) of freon-14 gas is increased and the concentration of oxygen gas is decreased. Further, the n + /i selectivity is small when the flow ratio X of freon-14 gas is 5; and the n + /i selectivity is increased when X is more than 5 and less than 10; and the n + /i selectivity is almost stable when X is 10 or more. Accordingly, when the concentration (mol%) of freon-14 gas against the gross volume of hydrogen chloride gas and freon-14 gas is 67% or more, the n + /i selectivity and the uniformity of the etch rate of the n + layer can be simultaneously improved. Further, when the concentration (mol%) of freon-14 gas against the gross volume of hydrogen chloride, freon-14 gas, and oxygen gas is 58% or more, the n + /i selectivity and the uniformity of the etch rate of the n + layer can be simultaneously improved. Further, when the concentration (mol%) of oxygen gas in the etching gas is 12% or less, the n + /i selectivity and the uniformity of the etch rate of the n + layer can be simultaneously improved. Figure 6 shows the dependence of n + /i selectivity, the n + uniformity, and the etch rates of the n + layer and i layer, upon the gas pressure under the same measurement conditions as those of Figure 4, except that an n + layer having an activation energy of 0.1 eV is used. As is understood from this diagram, the dependence represent the same tendencies as those of a case where the n + layer having an activation energy of 0.2 eV is used. However, the etch rate of the n + layer has a value almost twice as large as that of such a case. Further, the n + /i selectivity is almost doubly improved. As the reason for these results, the following is considered: Etching is promoted in the n + layer having an activation energy of 0.1 eV because electrons of reacting molecules (radical) and those of silicon are exchanged. Accordingly, the etching is smoothly conducted and the etch rate is increased, thereby obtaining the excellent selectivity. Figure 7 shows the dependence of n + /i selectivity, the n + uniformity, and the etch rates of the n + layer and i layer, on the concentration (mol%) of freon-14 gas under the same measurement conditions as those of Figure 5, excepting that an n + layer having an activation energy of 0.1 eV is used. As is understood from this diagram, the dependence represent the same tendencies as those of the case where the n + layer having an activation energy of 0.2 eV is used. However, the n + /i selectivity has further improved compared with that case. Accordingly, excellent etching characteristics can be obtained by using a polysilicon thin film having an activation energy of 0.2 eV or less for the n + layer. The TFT is fabricated by using the above-mentioned respective etching characteristics in the following manner: The i layer 7 is deposited on the substrate with a thickness a of 50nm (500 angstroms) and the n + layer having an activation energy of 0.2 eV is deposited thereon with a thickness of 50nm (500 angstroms). An etching gas including hydrogen chloride gas, freon-14 gas and oxygen gas having the flow ratio of 5:15:2 and a pressure of 53Pa (400 m Torr) is made to flow into the dry etching apparatus. The RF electrical density is set at 0.25 W/cm 2 . As is apparent from Figures 4 and 5, the etch rate of the n + layer is 25nm/min (250 angstroms/min) under the above-mentioned conditions. Thus, in order to divide the n + layer into the drain region 5 and the source region 6 by etching, the n + layer should be etched for about 2 minutes. For the fabrication of the TFT, etching is conducted for about 3 minutes in order to remove the active layer formed in the i layer by the diffusion of phosphorus included in the n + layer, resulting in a 150% etching of an appropriate etching. Under the above-mentioned conditions, the n + /i selectivity becomes 2, and the layers can be selectively etched. Further, the i layer 7 having a thickness of 30nm (300 angstroms) is formed without remaining partially undivided on the n + layer nor being excessively etched. Further, the n + layers 5 and 6 are excellent in uniformity. A liquid crystal display device having an excellent display characteristics can be fabricated by using the TFT formed in this manner. Further, the TFT having the same structure as mentioned above is fabricated with an n + layer made of crystalline polysilicon having an activation energy of 0.1 eV. As is apparent from Figures 6 and 7, the etch rate becomes 50nm/min (500 angstroms/min) under the above-mentioned conditions, so that only 1 minute of etching is required to divide the n + layer into the drain region 5 and the source region 6. Also, for the fabrication of this TFT, a 150% etching is conducted in order to remove the active layer formed in the i layer by the diffusion of phosphorus included in the n + layer. Under the above-mentioned conditions, the n + /i selectivity becomes 4, and the layers can be selectively etched. Further, the i layer 7 having a thickness of 20nm (200 angstroms) is formed without remaining partially undivided on the n + layer nor being excessively etched. Further, the n + layers 5 and 6 are excellent in uniformity. The liquid crystal display device having an excellent display characteristics can be fabricated by using the TFT formed in this manner. In the above-mentioned examples, a mixed gas of hydrogen chloride gas and freon-14 gas added with oxygen gas is used for the etching. However, the etching gas used for the present invention is not limited to such a mixed gas and the same effect can be obtained with a mixed gas including chlorine gas instead of hydrogen chloride gas. Further, a cylindrical apparatus can be used as a dry etching apparatus for the present invention. Several kinds of modes such as a RIE (Reactive Ion Etching) mode, a PE (Plasma Etching) mode and an ECR mode can be applied for the etching mode. As is apparent from the above-mentioned descriptions, according to the present invention, the n + layer and the i layer having an satisfactory controllability of etching and an uniform etch rate can be selectively etched by the dry etching method using an etching gas including hydrogen chloride gas and freon-14 gas free from the freon regulation. Further, a TFT having an i layer of a thin film thickness can be fabricated by taking an advantage of the above-mentioned characteristics of the present invention, so that the optical pumping current can be reduced when the TFT is turned off. In addition, the distortion of the threshold voltage generated when the optical pumping current is introduced into a gate insulating film can be restrained, so that a liquid crystal display device having excellent display capacities can be obtained.

Claims: A method of reactive ion etching n-type polysilicon formed on an intrinsic polysilicon layer which comprises dry etching the n-type polysilicon using an etching gas constituted by CF 4 and a gas selected from hydrogen chloride and chlorine, possibly in admixture with oxygen, the concentration of CF 4 in the etching gas being 67 mole % or more based on CF 4 and hydrogen chloride or chlorine when the etching gas is constituted by CF 4 and one of hydrogen chloride or chlorine, or 58 mole % or more based on CF 4 and hydrogen chloride or chlorine when the etching gas is constituted by CF 4 , one of hydrogen chloride or chlorine, and oxygen, thereby selectively etching the n-type polysilicon with respect to the intrinsic polysilicon. A method according to claim 1, wherein the pressure of the etching gas is 53 Pa (400 mtorr) or more. A method according to claim 1 or claim 2, wherein the etching gas contains oxygen in a concentration of 12 mole % or less. A method for fabricating a thin film transistor including a gate electrode (9), a channel-section semiconductor layer electrically separated from the gate electrode by a gate insulating film (8), a pair of contact layers (5,6) having contact with the channel-section semiconductor layer, a source electrode (2) having contact with one of the contact layers (5), and a drain electrode (3) having contact with the other contact layer (6), comprising the steps of: forming an intrinsic polysilicon layer (7) on the gate insulating film (8), forming an n-type polysilicon layer (5, 6) on the intrinsic silicon layer (7), and forming the pair of the contact layers (5, 6) by selectively reactive ion etching a portion of the n-type polysilicon layer by a method according to any one of claims 1 to 3.

Inventors: BAN ATSUSHI, KAJITANI MASARU, KATAYAMA MIKIO, KAWAI KATSUHIRO, SAKURAI TAKEHISA, UJIMASA HITOSHI, BAN,ATSUSHI, KAJITANI, MASARU, KATAYAMA, MIKIO, KAWAI, KATSUHIRO, SAKURAI, TAKEHISA, UJIMASA, HITOSHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, H01L29, G02F1