WO1997024750A1

WO1997024750A1 - Method for etching silicon dioxide using unsaturated fluorocarbons

Info

Publication number: WO1997024750A1
Application number: PCT/US1996/020141
Authority: WO
Inventors: Robert Charatan
Original assignee: Trikon Technologies, Inc.
Priority date: 1995-12-29
Filing date: 1996-12-27
Publication date: 1997-07-10

Abstract

An oxide such as silicon dioxide is dry etched by use of unsaturated fluorocarbons to achieve a sharp trench profile, minimum microloading and sub-micron features.

Description

METHOD FOR ETCHING SILICON DIOXIDE USING UNSATURATED FLUOROCARBONS

This invention relates to the dry etching of materials and in particular to the dry etching of an oxide such as silicon dioxide by use of unsaturated fluorocarbons and specifically fluorocarbons from the family C_nF_2n.

Background of the Invention

Dry etching of materials is well known. Dry etching employs a number of different methods ranging from plasma etching where the reaction is essentially chemical with the workpiece being etched isotropically by radicals from the plasma to reactive ion etching where the reaction is anisotropic and the etch process utilizes both ion bombardment and the reaction of radicals with the workpiece to be etched.

U.S. Patent 5,429,070 issued July 4, 1995 discloses a number of etch gases suitable for etching oxides and particularly silicon dioxide. Among the etch gases are CF₄, CHF₃, C₂F₆, 0₇ , SF_5; SF₆+ C₂F₆, CHF₃+C₂F₆, SF₆+C₃F„, and CHF₃+0₂ plus He.

In etching an oxide of a material such as silicon dioxide using what is called "reactive ion etching", typically a plasma is generated in any one of several well known manners. Reactive ions from the plasma are then accelerated against the workpiece to bombard the workpiece. Molecules or atoms, particularly of carbon, from a photoresist material on the workpiece used to define the area to be etched, form polymers along the sides of the materials being etched to prevent lateral etching. The ions bombarding the surface being etched (which surface, in reactive ion etching, is substantially perpendicular to the direction of ion bombardment) dislodge materials that would otherwise form polymers as well as compounds or atoms of the

-l- material being etched. The result is a highly directional etch (called an "anisotropic" etch). U.S. Patent 4,208,241 issued June 17, 1980 on an application filed July 31, 1978 by Harshbarger et al. discloses the use of etchant gases which provide both recombinant species and etchant species where the recombinant species either inactivate or reduce the activity of the etchant species at or in the vicinity of the etch walls, thereby resulting in substantially anisotropic etching. O. Joubert et al., in an article entitled "Fluorocarbon High Density Plasma. V. Influence of aspect ratio on the etch rate of silicon dioxide in an electron cyclotron resonance plasma", J. Vac. Sci. Technol.A 12(3), May/Jun 1994, p. 658, disclose using "high C/F ratio gases such as C₂F₄, C₃F₆, and their mixtures with hydrogen," to obtain an etch selectivity of Si0₂ over Si greater than fifty. Joubert et al. determined that the fluorocarbon deposition rate measured on a blanket Si0₂ sample when the sample was operated without RF sample bias, was an interesting parameter to characterize the fluorocarbon discharge. The fluorocarbon deposition rate was shown to be dependent on the flux of polymerizing species produced in the gas phase which controls the etch rate ratio between the oxide and the silicon. Joubert et al. showed that the highest fluorocarbon deposition rates are obtained for C₂F₄ and C₃F₆. Another paper, entitled "Reactive Ion Etching of Silicon and Silicon Dioxide in CF₄ Plasmas Containing H₂ or C₂F₄ Additives" by Simko and Oehrlein, J. Electrochem. Soc, Vol. 138, No. 9, Sept. 1991, discusses the etching of polycrystalline silicon and silicon dioxide using CF₄ with an additive gas of either hydrogen or C₂F₄.

Morgan, in his book entitled "Plasma Etching in Semiconductor Fabrication" mentions, in Chapter 3 relating to silicon and silicon dioxide etching in plasma, the use of C₂F₄. d'Agostino et al. in a paper entitled "Mechanisms of deposition and etching of thin films of plasma-polymerized fluorinated monomers in radiofrequency discharges fed with C₂F₆-H₂ and C₂F₆-0₂ mixtures", J. Appl. Phys. 61(8), 15 April 1987, mentions on page 2754 the use of C₂F₄ or C₃F₆ as feed for deposition of plasma-polymerized fluorinated monomers.

To date, however, Applicant knows of no commercially viable process which has been developed to use unsaturated fluorocarbon gases to etch silicon dioxide features, particularly to submicron dimensions.

Summary of the Invention

In accordance with thiε invention, unsaturated fluorocarbon gases are used to etch silicon dioxide features at a relatively high etch rate compared to the prior art and with a selectivity relative to polycrystalline silicon (i.e., etch rate in silicon dioxide relative to polycrystalline silicon) greater than 10. The use of unsaturated fluorocarbon gases to provide etching of openings as small as 0.35 micron has been accomplished with the resulting contacts having sharp side walls, good selectivity to the material underlying the Si0₂ and minimal microloading (i.e., deviation in etch rate as a function of critical dimension across the material being etched) of less than about 10%.

In one embodiment, the unsaturated fluorocarbon etch gas is diluted with selected gases such as a small amount of hydrogen, oxygen, methyl ercaptan CH₃SH, SF₆ or additional fluorocarbon gases. The variation of the gases assists in selective polymerization and/or changes the density of fluorine containing radicals present within the plasma and thus enhances the selectivity of this etch when used to etch silicon dioxide. This invention will be more fully understood in conjunction with the following detailed description taken together with the drawings.

Description of the Drawings

Fig. 1 illustrates the chemical formula of the unsaturated fluorocarbon gases C₂F₄ and C₃F₀ used in accordance with this invention.

Fig. 2 illustrates the general structure of the unsaturated fluorocarbon gases used in accordance with this invention.

Detailed Description

In accordance with this invention, unsaturated fluorocarbon gases are used to etch silicon dioxide features of submicron dimensions to achieve sharp side walls, good selectivity to the film underlying the Si0₂ and minimal icroloading for the purpose of microelectronic device fabrication, among others. The silicon dioxide to be etched can be either thermally or plasma deposited with or without doping. The etch is accomplished with one of the unsaturated fluorocarbons, specifically of the family C_nF_2n, a high density plasma source (lθ'°-10¹⁴ cm'³) , a moderate C_nF_2n flow rate <75 SCCM and possibly <40 SCCM and a low pressure (<5 mTorr) . Using a C₃F₆ plasma, it is possible to etch open pads of Si0₂ at rates > 1 μ min^'1 with a selectivity relative to poly-silicon > 15. The etching of contacts as small as 0.35 μ, accomplished with the aforementioned gas, reveals sharp sidewalls of the etched contact (>88°), good selectivity to the film underlying the Si0₂ and minimal microloading (<10%) . In addition, in some cases, the unsaturated fluorocarbon etch gas is diluted with a small amount of hydrogen, oxygen, methyl mercaptan CH₃SH, SF₆ or additional fluorocabon gases, to assist in the degree of selective polymerization and/or changes the density of fluorine containing radical present within the plasma and hence, an enhancement in the selectivity of this etch relative to that of other films.

In accordance with the invention, the combination of a hydrogen - unsaturated fluorocarbon gas plasma and a low pressure is believed essential to provide a process which meets current industry requirements for the etching of silicon dioxide. In one embodiment, a typical process utilizing MORI technology (M = 0 Resonant Induction) is as follows:

MORI Source Power 3 kilowatts

Chuck Power 1.4 kilowatts

Chamber Pressure approximately 1 mT

C3F6 flow 30 seem

Coil currents (using 25 amps inner - 25 a dual coil structure) amps outer (the coils generate a magnetic field used to shape plasma and to launch the helicon wave, as disclosed, for example, in U.S. Patent 5,429,070) .

Chamber temperature 50 degrees C Chuck temperature 10 degrees C Chuck clamping voltage 700 volts He backside pressure 10 Torr

Use of the above operating conditions led to an oxide etch rate of approximately 7,000 Angstrom per minute for 0.425 micron features. The selectivity of the etchant in etching silicon dioxide to etching photoresist used to pattern the silicon dioxide was greater than 2 to 1. The selectivity of the etching of silicon dioxide to the etching of polycrystalline silicon was greater than 10 to 1 on a blanket wafer. No etching of the polycrystalline silicon at the bottom of a trench was observed after a 20% over etch. The profile was greater than 88 degrees thereby approaching the ideal of 90 degrees quite closely. Microloading was 15% comparing the 0.425 and the 0.725 micron features.

The fact that the microloading is 15% is within the expected range of performance of the process in accordance with the invention.

The parameter ranges within which the process of this invention operates to achieve superior etch results compared to the prior art are as follows:

Parameter Ranges

Parameter Range MORI power 500 - 5000 W (13.5 MHz)

Coil Currents variable (inner/outer)

Chuck Power 800 - 2000 W (for 13.5 MHz)

Chuck Bias Frequency variable Process Pressure 0.1 - 5 mT Gas Flow C₃F₆ 10 - 75 SCCM Chemistry Pure C₃F₆ or addition of H₂, 0₂, SF₆, gases from the mercaptan family or additional fluorocarbon gas (whether or not from the family C_nF_2n) . Also noble gases-helium- reduced etch rate but gave substantial increases in selectivity. Also tried argon - which lowers etch rate of everything.

Chamber Wall Temperature 50 - 100°C

Chuck Temperature < 20°C

Chuck Clamping Voltage 500 - 800 V

He backside pressure on 10 - 20 Torr wafer

Within the above parameter ranges the following parameter values represent the preferred embodiment.

Parameter i?angre

MORI Power 5000 W ( 13 . 5 MHz )

Coil Currents 25 A/ 25 A (inner/outer)

Chuck Power 1400 W (for 13.5 MHZ) Chuck Bias Frequency 13.5 MHZ

Process Pressure 0.5 mT

Gas Flow C₃F₆ 20 SCCM Chemistry Pure C₃F₆ or < 20% addtn .

H₂

Chamber Wall Temperature 50°C

Chuck Temperature 5°C

Chuck Clamping Voltage 700 V He backside pressure on 10 Torr wafer

While the Joubert et al. and Simko et al. references cited above in the Background of the Invention disclose the use of certain unsaturated fluorocarbon gases either alone or with other gases to etch silicon dioxide features, a number of issues are important to obtain an oxide etch process which is capable of being used to manufacture usable semiconductor devices. Among the issues that are important (in no particular order) are: (1) etch rate uniformity, (2) etch rate, (3) etch selectivity to resist, (4) etch selectivity to Si substrate, (5) etch selectivity to etch stop material (for example, poly- Si, a suicide, TiN or silicon) , (6) etch profile (the shape of the contact after etching and resist/polymer removal) , (7) critical dimension (CD) control (control of the width of the feature, ideally the same before and after etching) , (8) CD uniformity, (9) wafer throughput, (10) process cleanliness, and (11) process reproducibility.

Joubert et al., in their above cited article, state on page 659 (and later throughout their paper) that experiments are performed with the wafer self bias of 100 volts. In accordance with this invention, a wafer self bias is on the order of 500 volts or greater in the preferred embodiment. Indeed, at less than approximately 300 volts measured self bias (which in one embodiment corresponds to 800 Watts applied chuck power), small round contact features (i.e., contacts less than about 0.5 micron diameter) were not able to be etched in silicon dioxide. The wafer bias is a function of applied chuck power as well as the chamber pressure and plasma source power. Joubert et al. discuss on page 662 of their above cited article an inability to etch material with an aspect ratio (AR) greater than 1.6 using C₃F₆ gas under conditions similar to those disclosed herein but with a self bias voltage of about 100 volts. For a 0.4 micron contact, a size contact of increasing importance as semiconductor device features decrease in size, neglecting the increase in aspect ratio which is attributable to the presence of photoresist prior to etch initiation, an AR of 1.6 corresponds to a maximum attainable etch depth of only 0.72 microns. While the maximum attainable etch depth will change by a factor dependent on the oxide to photoresist selectivity, this etch depth will certainly decrease for oxide selectivities to photoresist greater than one (as the etch proceeds, walls of the contact get steeper) , a fact explicitly stated by Joubert (page 661, photoresist selectivity to oxide at least 1.8 to 1) . Thus, Joubert et al. cannot etch features of the size which are etchable using the preferred embodiments of this invention.

The experiments discussed by Si ko et al. in their above cited paper are performed in a reactive ion etching (RIE) mode and are clearly different from those disclosed herein. On page 2749 of Simko et al. the experimental section details a 25 T pressure and a 200 W RF power. To the contrary, the maximum pressure used in this invention is 5 T and the power used is between 1 and 5 kilowatts. The RIE regime, which depends on gas phase nucleation for polymer deposition is either not present or severely attenuated in the low pressure regimes utilized by this invention. Indeed, one would expect to see a severe taper (i.e., contact profile "V" shape) in the etch profile should the process be run at 25 T. Simko et al. report low etch rates for Si0₂ (maximum etch rate of approximately

3300 A/min) on a blanket deposition (Simko et al., page 2751) compared to etch rates achieved by this invention of approximately 8,000 A/min to make a .4 micron contact. Simko et al. also report low selectivities to poly Si. Indeed Simko et al. primarily report the consequences of adding C_nF_2n gas to CF₄, not using C_nF_2n as a stand-alone etchant. Given the DC wafer self biases reported in Simko et al. Fig. 8, page 2751, problems would have been encountered similar to those reported by Joubert et al. in the etching of contacts. D'Agostino et al. mention the use of unsaturated fluorocarbons but give no discussion of a process suitable for using the unsaturated fluorocarbons to achieve the results described in this specification. In accordance with this invention, it is important to differentiate low pressure etching (of a few mT or less) from RIE (typically 10 's to 100's of mT) . In the RIE mode, one can take advantage of gas phase nucleation to effect polymerization above the photoresist surface. During low pressure operation, the reduced probability for collision occurrence between plasma species attenuates the degree of gas phase nucleation. In accordance with this invention the unsaturated fluorocarbons are thought to best polymerize on the surface of the material not to be etched (i.e., photoresist and the material beneath the Si0₂) . The unsaturated fluorocarbon polymerization is a substitute for the gas phase nucleation present at RIE pressures. Joubert et al. do not teach all of the ingredients to effect a manufacturable etch process using unsaturated fluorocarbons and in particular Joubert et al. do not teach the correct balance of the net deposition and etch rates and the correct wafer bias.

It has been observed that optimization of the process of this invention is dependent on the etch stop material and type of photoresist, among other parameters. For example, for one set of experiments, optimization for etch processes for deep UV and I-line photoresist appear to occur at MORI source powers of 5 k and 2 kW, respectively. These results are applicable over a large parameter space. While certain prior art low pressure etches use a catalytic Si plate as a substitute for the lack of gas phase nucleation, these prior art reactors are extremely temperature sensitive. The invented process replaces gas phase nucleation with an alternative chemistry rather than surface catalysis. As an alternative, the inclusion of methyl mercaptan to C₃F₆ was investigated. Methyl mercaptan is a molecule which contains relatively weak carbon-sulfur bonding. It was conjectured that the release of sulfur could assist in crosslinking photoresist during the dry etch analogous to the crosslink of polymers during vulcanization. The alternative chemistry approach is inherently more stable.

The addition of SF₆ to the etch gas probably minimizes the plasma sheath, reduces microloading and increases oxide etch rate and oxide selectivity to photoresist. The invention effectively etches contact openings having submicron feature size and provides fairly rapid etch rate thus resulting in optimal through-put for commercial etching.

While certain embodiments of this invention have been described, other embodiments of this invention will be obvious in view of this disclosure.

Claims

CLAIMSI claim:

1. The method of dry etching of an oxide of silicon which comprises the use of unsaturated fluorocarbon gas in a low pressure environment at a self-bias voltage sufficient to provide an acceptable etch rate and aspect ratio for the etched feature.

2. The method Claim 1 wherein the unsaturated fluorocarbon gas comprises C₃F₆.

3. The method of Claim 2 wherein said unsaturated fluorocarbon gas is mixed with no more than 20% hydrogen.

4. The method of Claim 1 wherein said process utilizes power in the range of 500 to 5000 watts at a selected frequency.

5. The method of Claim 4 wherein the selected frequency is 13.5 MHz.

6. The method Claim 4 wherein said process pressure is beneath 5 mT.

7. The method of Claim 6 wherein said process pressure is in the range from 0.1 mT to 5 mT.

8. The process of Claim 6 wherein said oxide of silicon is part of a structure which is mounted on a chuck and said chuck power is in the range of 800 to 2000 W at 13.5 MHz.

9. The method of Claim 8 wherein said chuck power is approximately 1400 W at 13.5 MHz.

10. The method of Claim 1 wherein the oxide of silicon being etched is subjected to a self-bias of about three hundred (300) volts or more.

11. The method of Claim 10 wherein the oxide of silicon is being etched to form submicron contacts.

12. The method of Claim 1 wherein the oxide of silicon is silicon dioxide.

13. The method of Claim 1 wherein the unsaturated fluorocarbon etch gas is diluted with a small amount of gas selected from the group consisting of hydrogen, oxygen, SF₆ and fluorocarbon carbon gases other than C_nF_2n.

14. The method of Claim 13 wherein the small amount of gas is less than 20 percent of the gas in the etch process.