United States Patent (19) 11 Patent Number: 4,536,252 Mcdonald Et Al

United States Patent (19) 11 Patent Number: 4,536,252 McDonald et al. 45) Date of Patent: Aug. 20, 1985

(54) LASER-INDUCED PRODUCTION OF Assistant Examiner-Jackson Leeds NITROSYL FLUORIDE FOR ETCHING OF Attorney, Agent, or Firm-Anthony T. Lane; Robert P. SEMCONDUCTOR SURFACES Gibson; Jack W. Voigt 75) Inventors: Joseph K. McDonald, Athens, Ala.; 57 ABSTRACT James A. Merritt, Pulaski, Tenn. Nitrosyl fluoride is prepared by laser-induced method 73) Assignee: The United States of America as wherein the nitrosyl fluoride is produced in situ or in represented by the Secretary of the close proximity to where it is used to etch semiconduc Army, Washington, D.C. tor surfaces. 21 Appl. No.: 699,283 A reaction mixture of a catalyst compound and a fluoro compound, wherein the catalyst compound is an oxide 22 Filed: Feb. 7, 1985 of nitrogen selected from the group of nitrogen oxide 51) Int. Cl...... H01 L 21/265 compounds consisting of NO, N2O, and NO2 and 52 U.S.C...... 156/662; 156/646; wherein the fluoro compound is selected from the 423/386; 204/157.1 R; 204/157.1 H group of fluoro compounds consisting of NF3 and N2F4, 58) Field of Search ...... 204/157.1 R, 157.1 H; is irradiated with CO2 laser radiation to produce FNO. 423/386; 156/646, 662 FNO reacts with an exposed silicon material to produce SiF4 and nitrogen oxide. Since the oxide is regenerated, 56) References Cited it can be regarded as a catalyst which can be recovered U.S. PATENT DOCUMENTS and recycled. 3,306,834 2/1967 Fox et al...... 204/157.1 R A production scheme for producing nitrosyl fluoride 3,408,276 10/1968 Rey ...... 204/157.1 R for etching is disclosed wherein the nitrosyl fluoride is 3,554,699 1/1971 Gross ...... 423/386 produced directly in the laser reaction chamber by 4,431,477 2/1984 Zajac ...... passing a CO2 laser beam through the reaction mixture. 4,447,304 5/1984 Malatesta et al...... 204/157.1 R Alternately, the nitrosyl fluoride can also be produced OTHER PUBLICATIONS in a side chamber and the gas flowed into an etching Merck Index, Tenth Edition, p. 953, listing No. 6494. chamber. Primary Examiner-Gary P. Straub 5 Claims, 2 Drawing Figures

2 WASTE GASES =S(N2, FLUOROCARBONS, CO2 LASER NF)

DLUENT 8

UNREACTED FLUORDE COMPOUNDS 36

U.S. Patent Aug. 20, 1985 4,536,252

DLUENT

FUORDE COMPOUNDS 22 FIG.

REACTANT

GASES

REACTION CHAMBER

ETCHING CHAMBER

PRODUCT GASES FG. 2 4,536,252 1. 2 etching of semiconductor surfaces in a combination LASER-NDUCED PRODUCTION OF NITROSYL reaction and etching chamber, and FLUORIDE FOR ETCHING OF FIG. 2 depicts a laser production method and facili SEMECONDUCTOR SURFACES ties for producing nitrosyl fluoride in a side reaction chamber and subsequently flowing same into an etching DEDICATORY CLAUSE chamber to achieve..etching of semiconductor surfaces. The invention described herein may be manufac DESCRIPTION OF THE PREFERRED tured, used, and licensed by or for the Government for EMBODIMENTS governmental purposes without the payment to us of Laser-induced in situ production of nitrosyl fluoride any royalties thereon. provides a continuous method for etching semiconduc BACKGROUND OF THE INVENTION tor materials. Nitrosyl fluoride is produced when a fluoro com Many fluorine-containing compounds are used to pound absorbs CO2 laser and subsequently reacts with a etch semiconductor materials. Some gaseous mixtures 15 nitrogen oxide. Thus, when a mixture of an oxide of will react spontaneously while others require the use of nitrogen (NO, N2O, NO2) and a fluoro compound se plasmas, electric discharges or ion beam. lected from nitrogen trifluoride (NF3) or tetrafluorohy Nitrosyl fluoride (FNO) is an example of a highly drazine (N2F4) is irradiated by a CO2 laser, nitrosyl reactive fluoronating agent. In the presence of masked fluoride is produced. Other fluoro compounds that can silicon wafers it has been shown to react spontaneously 20 absorb the CO2 laser could also react with a nitrogen to produce silicon tetrafluoride and nitric oxide. How oxide to produce nitrosyl fluoride. ever, because of the spontaneously, highly reactive Nitrosyl fluoride reacts spontaneously with masked nature of nitrosyl fluoride, storing and handling of the silicon material to produce silicon tetrafluoride and pure compound is extremely hazardous, particularly, in nitric oxide. Therefore, when nitrosyl fluoride is pro a manufacturing or production environment. 25 duced in situ, i.e., within a laser chamber containing the Advantageous would be a procedure for producing masked silicon material or in an adjoining reaction nitrosyl fluoride at the time it is used, particularly, if the chamber from which the nitrosyl fluoride is transferred nitrosyl fluoride is produced from precursors which do to an etching chamber containing the masked silicon not create a storage and handling problem as does the material, etching of semiconductor surfaces is achieved product of reaction. 30 by a cost-effective method, particularly for etching Even of more significance would be a procedure for large batches of semiconductor materials as further making nitrosyl fluoride in situ or in close proximity to disclosed hereinbelow. where it is used. Using as an example the reaction mixture nitric oxide Therefore, an object of this invention is to provide a (NO) and nitrogen trifluoride (NF3), the etching in procedure to produce nitrosyl fluoride in situ or in close 35 volves the following reaction: proximity to where it is used to etch semiconductor 1. Laser irradiation of the NO and NF3 mixture pro surfaces. duces FNO. Another object of this invention is to provide a laser 2. FNO reacts with the silicon wafer to produce SiF4 induced procedure to produce nitrosyl fluoride in situ and NO. or in close proximity to where it is used to etch semi 40 With further reference to the drawings, FIG. 1 de conductor surfaces. picts a laser-induced production facility of nitrosyl fluo ride for etching of semiconductor surfaces. This laser SUMMARY OF THE INVENTION induced production facility 10 which is outlined within dashed lines A comprises a CO2 laser 12 for inducing a Irradiation of oxides of nitrogen (NO, N2O, NO2 etc.) 45 reaction within a combination reaction and etching or nitrosyl chloride (NOCl) by a cw CO2 tunable laser, chamber 14 which contains silicon materials to be in the presence of nitrogen trifluoride (NF3) or tetra etched 16. The associated fixtures comprise a container fluorohydrazine (N2F4) produces nitrosyl fluoride in 18 for containing a fluorine-containing reactant com situ. pound (1) (NF3, N2F4 etc.), a container 20 for contain The in situ production of nitrosyl fluoride in the gase 50 ing reactant compound (2) oxides of nitrogen (NO, ous state makes it highly desirable for etching silicon N2O, NO2, etc.), a container 22 for containing a diluent chips in accordance with the preferred embodiment gas (N2, He, Ar, etc.), and conduit and valve means 24 disclosed hereinbelow and in the Drawing. for introducing gases into reactant chamber 14. Reac An example of an etching process is comprised of: tant gases in a predetermined ratio with a diluent gas are (1) Laser irradiation of an NO and NF3 mixture to pro 55 admitted through conduit means 24 into the reaction duce FNO; and and etching chamber 14. Product gases (SiF4 and NO) 2. FNO reaction with silicon material (Si3N4, SiO2 or Si are removed through conduit means 26 for separation surfaces) to produce SiF4 and NO. through gas separator 28. Nitric oxide (NO) and unre Since NO is regenerated, it can be regarded as a cata acted fluoride compounds are recovered and stored in lyst and most of it should be recoverable for recycling. container 30 for recycling through conduit and valve Other nitrogen oxides could be used in place of NO means 32. which can also be recycled. The SiF4 can be used for Diluent gases are recovered and stored in container other products or decomposed. 34 for recycling through conduit and value means 36. BRIEF DESCRIPTION OF THE DRAWINGS The diluent gas storage containers can also furnish gases 65 for purging the reactant chamber, gas separator, and In the drawings, conduit systems. FIG. 1 depicts a laser production method and facili Another embodiment is depicted in FIG. 2 wherein a ties for producing nitrosyl fluoride in situ to achieve laser-induced production facility 40 is outlined by 4,536,252 3 4. dashed lines B. The nitrosyl fluoride for etching semi fluoro compound selected from the group of fluoro conductor surfaces is produced in a side reaction cham compounds consisting of NF3 and N2F4, said cata ber 42 and flowed into etching chamber 44. The associ lyst compound introduced into said laser reaction ated fixtures as depicted in FIG. 1 having like numbers cell in a ratio of about 60 torr to about 45 torr of as shown in FIG. 1 perform the same functions in em- 5 said fluoro compound, bodiment of FIG. 2 as FIG. 1 except as required to form (ii) irradiating said gaseous mixture with a power nitrosyl fluoride in a side reaction chamber. level from about 25 to about 150 watts of a cw Additional description of the CW CO2 laser reaction CO2 tunable laser, tuned to a radiation line that is cell, CO2 Spectrum Analyzer, and other details of the resonant with an absorption band of said fluoro experiments of this invention are set forth below. 10 compound, said irradiating being carried out for a The laser induced chemical reactions are initiated by predetermined time period based on a predeter an infrared laser beam transmitted through a ZnSewin mined power level of said laser to thereby achieve dow (5 cm diameter) secured at the end of a stainless a continuous production of gaseous nitrosyl fluo steel cell (e.g. 5x10 cm.). The laser beam entering ride, through the ZnSe window traversed a 10 cm path. 15 The fluoro compounds, NF3 and N2F4, exhibit a se (iii) exposing a semiconductor silicon material to said ries of very strong absorption bonds between 900-1050 gaseous nitrosyl fluoride to react said gaseous ni cm. Any resonant laser line with sufficient energy trosyl fluoride with the exposed semiconductor could be used to initiate the reaction. silicon material to achieve etching while producing Infrared laser excitation in the range of 10.4 or 9.4 20 SiF4 and NO from said reacting; and, pum is provided by a Coherent Radiation Laboratories (iv) recovering said etched semiconductor silicon model 41 continuous-wave (cw) CO2 laser. The exact material, said SiF4, and said NO. laser frequencies are verified using an Optical Engineer 2. The method as defined in claim 1 wherein said ing CO2 Spectrum Analyzer. catalyst compound of an oxide of nitrogen is NO and Experiments were performed on a masked silicon 25 wherein said fluoro compound is NF3. wafer. A mixture of NO and NF3 in a ratio of 60 torr to 3. The method as defined in claim 2 wherein said 45 torr respectively was irradiated by 50 watts/cm2 of nitrosyl fluoride is produced in situ is a combination the P(36) (929.02 cm) CO2 laser line. The irradiation reaction and etching chamber containing a plurality of time was 3 sec. The etching was repeated 3 times. A semiconductor silicon material and wherein said recove smooth etching of the exposed silicon was observed. 30 ered NO is recycled. We claim: 4. The method as defined in claim 1 wherein said 1. A method for laser-induced production of gaseous catalyst compound of an oxide of nitrogen is NO2 and nitrosyl fluoride for the continuous etching of a semi wherein said fluoro compound is NF3. conductor silicon wafer by gaseous nitrosyl fluoride, 5. The method as defined in claim 4 wherein said said method comprising: 35 nitrosyl fluoride is produced in a separate reaction (i) filling a laser reaction cell with a gaseous mixture chamber and allowed to flow into an etching chamber of a catalyst compound of an oxide of nitrogen containing a plurality of semiconductor silicon material selected from the group of nitrogen oxide com and wherein said recovered NO is recycled. pounds consisting of NO, N2O, and NO2 and a : k 32 s 40