GB2331624A - Plasma etching of a nitride layer over an oxide layer - Google Patents

Abstract

A process for plasma etching a silicon nitride film from an underlying silicon oxide film overlying a silicon substrate 43 comprises simultaneously applying a first RF power, from first RF source 42A, having a frequency of 13.56 to 60MHz and a second RF power, from second source 42B, having a frequency of between 0.8-13.56 MHz. The power density at the top surface of the substrate 43 being between 0.2-0.35 W/cm<SP>2</SP> and the power density at the bottom surface of the top electrode 38A being 1.13 W/cm<SP>2</SP>. The etching may be performed in a chlorine or chlorine/oxygen atmosphere, the pressures and flow rates being disclosed. The process may be used to form an oxide element isolation region (Figures 7A-C) or a nitride side wall film (Figures 8A-C).

Description

2331624 METHOD OF FORMING A SEMICONDUCTOR DEVICE The present invention

relates to a method of forming a semiconductor device, and to a method for etching a silicon nitride film from a silicon oxide film and, more io particularly, to a method for etching a silicon nitride film with a high selectivity with respect to a silicon oxide film.

In a conventional fabrication process of a semiconductor device, dry etching (referred to as simply "etching" hereinafter) is performed to the silicon nitride film formed on a substrate with an intervention of 4 silicon oxide film for various purposes such as formation of an element isolation region made of silicon oxide or an etch stop layer on a side wall film of a self-aligned contact hole.

A first conventional process for the etching of a silicon nitride film is directed to formation of an element isolation region made of a silicon oxide film by the etching. Figs. 1 A to IC are sectional views of a semiconductor device for consecutively showing the first conventional process. After consecutively forming a silicon oxide film 14 and a silicon 2 nitride film- 16 on a silicon substrate 12, a photoresist mask 18 is formed by application of photoresist and patterning thereof using a photolithographic technique, as shown in Fig. 1 A.

Subsequently, selective etching using a parallel plates RIE (Reactive Ion Etching) technique, for example, is performed to the silicon nitride film 16. At this stage, the silicon oxide film 14 is often etched together with the silicon nitride film 16, as shown in Fig. 1B. Then, a photoresist mask 18 is removed, followed by thermal oxidation of the silicon substrate 12 and removal of the silicon nitride film 16 and the silicon oxide film 14, to form an element isolation region 20 made of silicon oxide on the silicon substrate, as shown in Fig. I C.

A second conventional process for the etching of a silicon nitride film is directed to formation of an etch stop layer on a side wall film of a self-aligned contact hole. Figs. 2A to 2C are sectional views of a semiconductor device for showing consecutive steps of the second conventional process. After consecutively forming a first silicon oxide film 22, a polycrystalline silicon (polysilicon) film 24, and a first silicon nitride film 26 on a silicon substrate 12, a photoresist mask is formed thereon by application of photoresist and patterning thereof using a photolithographic technique, as shown in Fig. 2A. A selective etching by a parallel-plates RIE using a photoresist mask 28, for example, is then performed to the first silicon nitride film 26 and the polysilicon film 24 in a gas ambient 3 which does 'not etch the first silicon oxide film 22, to obtain the structure in Fig. 2B.

Subsequently, the photoresist mask 28 is removed, followed by consecutive formation of a second silicon oxide film and a silicon nitride film. The second silicon nitride film is etched to form a side wall film 32, which fanctions as an etch stop layer. At this stage, the first and the second silicon oxide films 22 and 30 are also often etched together with the silicon nitride film 32, as shown in Fig. 2C. The polysilicon film 24 is used as a gate electrode. Thereafter, an insulator film (not shown in the figure) having contact holes is formed thereon.

For example, the etching gas for etching the silicon nitride film 16 is fluorocarbon-based gas such as CF4 gas, CF4/CBF3 mixed gas, CHF3/02mixed gas, CF4/02mixed gas, SF6/CHF3 mixed gas described in JP-A-05(1993)-251339, CF4/H2 mixed gas described in JP-A-56(1981)-122129, or CF4/N2rnixed gas described in JP-A-01(1989)-214025.

In the conventional techniques, at the step of etching the silicon nitride film, if the size difference between the mask pattern and the resultant pattern formed by the etching is to be suppressed to an allowable level, it arises a problem in that the selective ratio of the silicon nitride film with respect to the silicon oxide film cannot be maintained at a sufficient level, because the selective ratio remains at a low level of -about 1 to 3.

The low selective ratio in the first conventional technique 4 results in etching of the silicon oxide film to thereby expose a portion of the silicon surface, if the thickness of the silicon oxide film underlying the silicon nitride film is 10 nm or less. The exposed surface of the silicon substrate is generally damaged by the etching, resulting in a lower withstand voltage and a resultant current leakage between adjacent element regions.

In the second conventional technique, as shown in Fig. 3 illustrating a detail of a portion in Fig. 2C, an exposed portion of the silicon surface 12 between the element isolation region 20 and the side wall film 32 is damaged. This is because the etching of the second silicon nitride film to form the side wall film 32 also etches the first and the second silicon oxide films 22 and 30 to expose the portion of the silicon surface. In Fig. 3, the damaged region is shown by the marks " X ". In addition, the etching of the second silicon nitride film 32 also etches a region 35A of the silicon substrate underling the element isolation region 20 and in the vicinity thereof, which causes a low withstand voltage between the adjacent element regions 20 if a contact hole is formed thereon.

JP-A-2(1994)-66943 and JP-A-6(1994)-181190 propose improved etching techniques wherein a higher selective ratiDcan be obtained between the silicon nitride film and the silicon oxide film. The proposed techniques use CIF3 gas and NF3/Cl2mixed gas, respectively. However, these techniques involve radical fluorine generated in the etching, which causes a parallel etching wherein the etching proceeds in the direction parallel to the silicon surface. This results in a difficulty to accurately control the size obtained by the etching, which impedes the etching technique from being used to obtain a finer patterned device. JP-A-5-198538 also proposes etching using the NF3/Cl. mixed gas; however it is not so effective because the selective ratio of the silicon nitride film with respect to the silicon oxide film obtained therein remains at a moderate improvement at around a maximum selective ratio of four.

It is therefore an object of the present invention to provide a method for etching a silicon nitride film formed on a substrate with an intervention of a silicon oxide film with a high selective ratio and excellent dimensional accuracy.

The present invention provides a method of forming a is semiconductor device comprising the steps of consecutively forming a film of first material and a film of second material on a semiconductor substrate, and etching said second film from said first film while applying a first RF power having a first frequency and a second RF power having a second frequency to the semiconductor substrate.

By virtue of the above, selective etching of the second film from the first film can be achieved.

Preferably, the first film is a silicon oxide film and the second film is a silicon nitride film.

In accordance with the method according to the present invention, a high selective ratio of the silicon nitride film with respect to the silicon oxide film can be obtained.

6 Preferred features of the present invention will now be described, purely by way of example only, with reference to the accompanying drawings, in which:- Figs. IA to 1C are sectional views of a semiconductor device for consecutively showing the process of the first conventional technique; Figs. 2A to 2C are sectional views of a semiconductor io device for consecutively showing the process of the second conventional technique; Fig. 3 is a detailed sectional view of a portion shown in Fig.

2C; Fig. 4 is a block diagram of an etching system implementing a method according to the present invention; Fig. 5 is a graph for showing etch rates of a silicon nitride film and a silicon oxide film plotted against the power density (W/CM2) at the top surface of the sub strate; Fig. 6 is a graph for showing etch rates of a silicon nitride film and a silicon oxide film plotted against the power density (W/cm2) at the bottom surface of the substrate; Figs. 7A to 7C are sectional views of a semiconductor device for consecutively showing a process according to a first embodiment of the present invention; Figs. 8A to 8C are sectional views of a semiconductor 7 device for:consecutively showing a process according to a second embodiment of the present invention,, and Figs. 9A to 9C are sectional views of a semiconductor device for consecutively showing a process according to a first embodiment of the present invention.

io Embodiment 1 Referring to Fig. 4, an etching system, generally designated by 36, for implementing a method according to the present invention includes a chamber 37, a pair of electrodes including a top electrode 38A and a bottom electrode 38B opposed to each other in the chamber 37, and first and second high (radio) frequency power sources (referred to as "RF power sources", hereinafter) 42A and 42B. One of the electrodes of the first RF power source 42A is connected to the top electrode 38A through a first matching circuit 40A, whereas one of the electrodes of the second RF power source 42B is connected to the bottom electrode 38B through a second matching circuit 40B. The other terminal of each of the RF power sources 42A and 42B is grounded.

The frequency of the first RF power supplied from the first RF power source 42A is between 13.56 MHz and 60 MHz, 8 whereas the: frequency of the second RF power supplied from the second RF power source 42B is between 0.8 MHz and 13.56 MHz.

The etching system 36 further includes a monitor system 46 for monitoring the end of the etching operation, which includes a photodetector 46A for detecting light having a 580-- : 10 nm wavelength and emitted during the etching and a controller 46B for receiving an output from the photodetector 46A, to deliver an end signal for indicating the end of the etching when the intensity of the light having the specified wavelength reduces to a specified level. Etching of the silicon nitride film generally emits light having a wavelength of 580 rim, which is monitored by the monitor system during the etching.

The etching system 36 further includes an electrostatic support member 44 for holding a silicon substrate 43 on the upper surface of the bottomelectrode 38B, a gas supply system for supplying an etching gas'to the chamber 37 from the top thereof and a gas exhaust system connected to the vent 60 of the chamber 37 provided on the bottom side of the chamber 3 7 for 2o exhausting the etching gas.

Fig. 5 is a graph for showing etch rates of a silicon nitride film and a silicon oxide film when the power density on the top surface of the substrate 43 is changed. More specifically, the graph was obtained by changing the output from the second RF power source 42B connected to the bottom electrode 38B while 9 the substrate 43 having the silicon nitride film and the silicon oxide film on the etching side thereof is placed on the bottom electrode 38B so that the etching side of the substrate 43 is directed upward. In this example, the silicon oxide film was made Of Si02, and the etching gas was chlorine. When the power density on the top surface of the substrate 43 is changed stepwise from 0 to 0.56W/cM2, the etch rate of the silicon nitride film rises in proportion to the rise of the power density, whereas the etch rate of the silicon oxide film is substantially unchanged within the range of 0 to 0.35W/cm. As a result, it is understood that by setting the power density at the top surface of the substrate in the higher range, for instance, from 0.20 to 0.35W/cM2, the silicon nitride film can be etched at a selective ratio as high as 10 or more with respect to the silicon oxide film.

Fig. 6 shows similar graphs when the power density at top surface of the substrate 43 is fixed to 0.28W/cm' whereas the power density at the bottom surface of the top electrode is changed stepwise from 0.5 to 3.39W/CM2. The etch rate of the silicon oxide film is substantially unchanged, whereas the etch rate of the silicon nitride film increased rapidly, thereby achieving a high selective ratio of 10 or more, when the power density at the bottom surface of the top electrode rises above 1.13W/CM2.

Thus, it is confirmed based on Figs. 5 and 6 that a high 25 selective ratio of the silicon nitride film with respect to the silicon oxide film, as high as 10 or more, can be obtained in the chlorine gas ambient by controlling the power densities at the bottom surface of the top electrode and at the bottom surface of the substrate 43 within the specified ranges.

Based on the results of the above founding, the method of the first embodiment employs the process shown in Figs. 7A to 7C. The structure of Fig. 7A is similar to that of Fig. IA. In the present embodiment, after forming a 10-nm-thick silicon oxide film 14 and a 120-nm-thick silicon nitride film 16 consecutively on a silicon substrate 12, a photoresist film 18 is formed thereon by application of photoresist and a subsequent photolithographic step, to obtain a structure shown in Fig. 7A.

Subsequently, the silicon nitnde film 16 is etched by using the etching system 36 of Fig. 5 under the condition as follows:

(1) Etching gas:C12 (chlorine) gas, (2)Cl2gas flow rate: 50 to 150sccm,, (3)Cl2gas pressure: 5 to 15mTorr, (4) Power density at the bottom surface of the top electrode: within the range of 1. 13 to 3.39W/cM2, and (5) Power density at the top surface of the substrate: within the range of 0.20 to 0.35W/cM2.

The power densities at the bottom surface of the top electrode and at the top surface of the substrate are controlled within the above range for etching the silicon nitride film 16 at a high selective ratio, for instance, 10 or more with respect to the 11 silicon oxide film 14. During the etching, the monitor system 46 monitors the intensity of light at the wavelength of 580 nm. When the intensity of light reduces below the specified level, the end of the etching is indicated to finish the etching. Fig. 7B shows the substrate or wafer thus obtained. The size difference between the pattern of the photoresist mask and the resultant pattern formed on the silicon nitride film is, for example, 0.03,U m or less. That is, an excellent result can be o btained with respect to the dimensional accuracy.

Thereafter, the photoresist film 18 is removed, followed by thermal oxidation and removal of the silicon nitride film 16 and the silicon oxide film 14, thereby obtaining the element isolation region 48, such as shown in Fig. 7C.

In the present embodiment, the silicon nitride film 16 is etched at a high selective ratio with respect to the silicon oxide film 14. Accordingly, since the silicon oxide film. 14 is substantially unetched, as shown in Fig. 7B, the silicon surface is not exposed to damage. Thus, there is substantially no reduction of withstand voltage and no current leakage between the element isolation regions.

Embodiment 2 Referring to Figs. 8A to 8C, a process according to a second embodiment of the present invention includes the step of forming a 8-rim-thick first silicon oxide film 22, a 200-nm-thick polysilicon film 24 and a 100-iun-thick first silicon nitride film 12 26 consecutively on a silicon substrate 12. A photoresist film 28 is formed thereon by application of photoresist and patterning thereof by using a photolithographic technique, to obtain the structure shown in Fig. 8A. Then, the first silicon nitride film 26 and the polysilicon film 24 are etched to obtain the substrate shown in Fig. 8B.

A 20-nm-thick second silicon oxide film 30 and a 60-nmthick second silicon nitride film 32 are then formed consecutively on the entire surface, followed by etching the second silicon nitride film 32 using the etching system 36 under the condition as follows: (1) Etching gas:Cl2gas, (2)Cl2gas flow rate: 50 to 150sccm, (3)Cl2gas pressure: 5 to 15mTorr, (4) Power density on the bottom surface of the top electrode: 9W/CM2 within the range of 1. 13 to 3.3., and (4) Power density on the top surface of the substrate: within the range of 0.20 to 0.35W/cM2.

The power densities at the bottom surface of the top electrode and at the top surface of the substrate are set within the above range, to etch the second silicon nitride film 32 at a high selective ratio, for instance, 10 or more with respect to the second silicon oxide film 30. Similarly to the first embodiment, the end of the etching is monitored by the monitor system 46, during etching the second silicon nitride film 32, which monitors 13 the intensity Of light at the wavelength of 580 nm. When the intensity of the light becomes below a specified value, the end of the etching is indicated to finish the etching. Fig. 8C shows the substrate obtained by the present embodiment. The side wall film 34 on the side wall of the second silicon oxide film 30 is made from the second silicon nitride film 32.

In the present embodiment, the second silicon nitride film 32 is etched at a high selective ratio with respect to the second silicon oxide film 30. Accordingly, as shown in Fig. 8C, since the first and the second silicon oxide films 22 and 30 are substantially unetched, the silicon surface is not exposed to damage, thereby preventing the vicinity of the el ement isolation region from being etched. Thus, there is substantially no current leakage caused by the crystal defect of the silicon surface.

Embodiment 3 Referring to Figs. 9A to 9C, a process according to a third embodiment of the present invention is similar to the second embodiment except for the step of Fig. 9C. Specifically, in the present embodiment, the structure shown in Fig. 9B is formed by etching (first etching), similarly to the second embodiment.

Then, a second etching is conducted to obtain the structure shown in Fig. 9C under the following condition:

(1) Etching gas: C12and02mixed gas, (2)Cl2gas flow rate: 50 to 150sccm, 14 (3)02gas flow rate: 10'YO Of Cl2gas flow rate, (4) Pressureof Cl2gas and02gas: 5 to 15mTorr, (5) Power density at the bottom surface of the top electrode: within the range of 1. 13 to 3.39W/cm 2, and (6) Power density at the top surface of the substrate: within the range of 0.20 to 0.35W/CM2.

The power densities at the bottom surface of the top electrode and at the top surface of the substrate are set within the above range to etch the second silicon nitride film 32 at a high selective ratio, for instance, 10 or more with respect to the second silicon oxide film 30. Similarly to the first and the second embodiments, the monitor system 46 monitors the intensity of light at the wavelength of 580 nrn during the etching. When the intensity of light decreases below a specified level, the end point.of the etching is judged to finish the etching. The side wall film 50 on the side wall of the second silicon oxide film 30 is made from.the second silicon nitride film 32.

In the present embodiment, the second silicon nitride film 32 is etched by using the mixed gas ambient containing02gas and C12 gas during the second etching. The etching in the mixed gas ambient exhibits a selective ratio of the silicon nitride film with respect to the silicon oxide film higher than that Of the first etching. Accordingly, the silicon oxide film 30 can be left with more certainty, compared with the second embodiment, and the side wall film 50 can be formed without the residue of the is silicon nitride film 32.

Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

Tle description of the invention with reference to the drawings is by way of example only.

Tlie text of the abstract filed herewith is repeated here as part of the specification.

A process for etching a silicon nitride film (16) from an underlying silicon oxide film (14) overlying a silicon substrate (12) includes the step of applying a first RF power having a frequency of 13.56 to 60 MHz and a second RF power having a frequency of 0.8 to 13.56 MIU. The power densities at the top surface of the substrate (43) and at the bottom surface of the top electrode (38A) opposing the top surface of the substrate (43) are from 0.20 to 0. 35 W/CM2 and above 1.13 W/cm', respectively. The etching is performed in a chlorine gas ambient to form an element isolation region (48) made of silicon oxide or a side wall film (32) made from the silicon nitride film.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

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Claims (10)

1. A method of forming a semiconductor device comprising the steps of consecutively forming a film (14, 30) of first material and a film (16, 32) of second material on a semiconductor substrate (12), and etching said second film (16, 32) from said first film (14, 30) while applying a first RF power having a first frequency and a second RF power having a second frequency to the semiconductor substrate.

2. A method according to Claim 1, wherein the first film is a silicon oxide film and the second film is a silicon nitride film.

3. A method according to Claim 2, wherein said first frequency is within the range of 10 MHz to 80 MHz, and said second frequency is equal to or less than 20 MHz, the first frequency being higher than the second frequency.

is

4. A method according to Claim 3, wherein said first firequency is within the range of 13.56 MHz to 60 MHz, and said second frequency is within the range of 0.8 MHz to 13.56 MHz.

5. A method according to any of Claims 2 to 4, wherein said etching is stopped when the intensity of light having a wavelength of 580t10 rim emitted during said etching step decreases below a specified level.

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6. A method according to any of Claims 2 to 5, wherein said etching step is one of the steps for forming an element isolation region (48).

7. A method according to any of Claims 2 to 5, wherein said etching step is one of the steps for forming a side wall film (32) made from said silicon nitride film.

8. A method according to any of Claims 2 to 7, wherein power densities at a top surface of said substrate (43) and at a bottom surface of n electrode (38A) opposing said top surface during said etching step are within the range of 0.20 W/CM2 to 0.35 W/CM2, and equal to or above 1.13 W/cm, respectively, and said etching step is effected in an atmosphere containing chlorine gas.

9. A method according to Claim 8, further comprising, subsequent to said etching step, a step of etching said silicon nitride film (32) in an atmosphere containing chlorine gas and oxygen gas.

10. A method of forming a semiconductor device substantially as herein described with reference to Figure 4 and any of Figures 7 to 9 of the accompanying drawings.