United States Patent (19) 11) 4,387,145 Lehrer Et Al

United States Patent (19) 11) 4,387,145 Lehrer et al. 45) Jun. 7, 1983

54 LIFT-OFF SHADOW MASK Primary Examiner-Richard L. Schilling (75) Inventors: William I. Lehrer, Los Altos; John Attorney, Agent, or Firm-Kenneth Olsen; Michael J. H. Vincak, Mountain View, both of Pollock; Theodore S. Park Calif. 57 ABSTRACT 73) Assignee: Fairchild Camera & Instrument A method for forming a predetermined configuration of Corp., Mountain View, Calif. a film material comprises the steps of forming a layer of 21 Appl. No.: 306,116 a first material on a surface, forming a layer of a second material on the first material wherein the first material 22) Filed: Sep. 28, 1981 has an etch rate greater than that of the second material (51) Int. Cl...... G03C5/00; G03F 5/00; when the first material and the second material are B44C 1/22 exposed to a common etchant, etching portions of the 52) U.S. Cl...... 430/5; 430/314; second material and underlying portions of the first 430/323; 430/324; 156/628; 156/643; 156/652; material to expose portions of the surface, forming a 156/653; 156/655; 156/657 layer of film material on the exposed portions of the (58) Field of Search ...... 430/314, 323, 324, 5; surface, forming a layer of film material on the exposed 156/653, 652, 628, 643, 655, 657 portions of the surface and on the remaining portions of (56) References Cited the second material, and removing the remaining por U.S. PATENT DOCUMENTS tions of the first material such that the overlying second material and the film material thereon is also removed. 3,669,661 6/1972 Page et al...... 430/314 4,108,717 8/1978 Widmann ...... 430/314 4,307,179 12/1981 Chang et al...... 430/314 8 Claims, 7 Drawing Figures

U.S. Patent Jun. 7, 1983 4,387,145

3 'THERMALOXIDE FG, SEMCONDUCTOR O MATERIAL

6 4. 3 2 SiO2

a1a1a1aaaaa/a1111111111 O GeO2

-P-1111111111ZZ - - -

3 2 FG4

O 6 4. 3 2 FG.5

FIG.6

FG,7 O 4,387,145 1. 2 LIFT-OFF SHADOW MASK BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-section of a semiconductor substrate TECHNICAL FIELD having a layer of thermal oxide formed thereon. This invention relates to the manufacture of devices FIG.2 is a cross-section as in FIG. 1 having a layer of by photofabrication and particularly to a process for germanium dioxide formed on the thermal oxide layer and a layer of silicon dioxide formed on the germanium forming masks employed in such manufacture. dioxide layer. BACKGROUND FIG. 3 is a cross-section as in FIG.2 having a layer of In many arts, such as the semiconductor art, a film or 10 patterned photoresist formed thereon. layer of material is formed into a predetermined config FIG. 4 is a cross-section showing the structure of uration on a selected surface. For example, after form FIG. 3 following etching of the silicon dioxide and ing an integrated circuit on a surface of a semiconductor germanium dioxide layers utilizing a common etchant. chip, it is necessary to make electrical contact with the FIG. 5 is a cross-section as in FIG. 4 following re numerous elements or regions of the circuit. Such 15 moval of the photoresist. contact may be accomplished by forming a predeter FIG. 6 is a cross-section as in FIG. 5 following depo mined configuration of contacting material over the sition of film material. FIG. 7 is a cross section as in FIG. 6 following re surface of the integrated circuit, the contacting material moval of the silicon dioxide and germanium dioxide. being separated from the surface by a layer of dielectric. 20 In other instances, it is desired that a predetermined BEST MODE OF CARRYING OUT THE configuration in the form of a narrow cut in a passivat INVENTION ing layer of material, for example silicon monoxide, may FIG. 1 shows a substrate 10 of semiconductor mate be made incident to forming a device in or on a sub rial. A layer 12 of thermal oxide having an upper sur strate. In other instances, independent masks, that is, 25 face 13 is formed on the substrate 10. masks not on the surface of an actual device, are formed According to the method of the present invention, a for employment in exposing and processing of an actual layer 14 of germanium dioxide is formed on the surface device. Each of these various applications may involve 13 followed by the formation of an overlying layer 16 of the formation of a film or layer of material into a prede silicon dioxide as shown in FIG. 2. Preferably, both termined configuration. 30 germanium dioxide layer 14 and silicon dioxide layer 16 It is well known to form predetermined configura are formed by chemical vapor deposition. tions of a film of material by etching, by lifting or by a The germanium dioxide layer 14 is deposited in a combination of these techniques. Lifting processes in conventional CVD reactor which is maintained at volve depositing the film of material partially upon a about 400° C. Nitrogen gas is introduced to the reactor surface and partially upon a lifting materials which is 35 at a flow rate of about 40 liters/min. Oxygen is intro formed in a pattern upon the surface. The lifting mate duced to the reactor at a flow rate of about 1.5 liters/- rial is then removed along with the film material depos min. Germane is introduced to the reactor at a flow rate ited thereover leaving a desired pattern of the film ma of about 15 cc/min, that is, such that the ratio of oxygen terial on the surface. to germane is about 100:1. Germanium dioxide layer 14 When a film material is applied to the lifting material is deposited on surface 13 at a rate of about 200A/min and the exposed portion of the surface, the film material to a thickness of about 7500 A. takes a continuous form. Thus, when the lifting material Following deposition of germanium dioxide layer 14, is removed, the layer of film material must be broken. silicon dioxide layer 16 is deposited by maintaining the Breaking of the film material creates problems not only reactor at about 400 C. and introducing nitrogen gas at in its precise definition but also in its functional charac 45 a flow rate of about 40 liters/min. Oxygen is introduced teristics. to the reactor at a flow rate of about 1.5 liters/min. SUMMARY OF THE INVENTION Silane is introduced to the reactor at a flow rate of about 15 cc/min, that is, such that the ratio of oxygen The present invention provides a method for forming to silane is about 100:1. Silicon dioxide layer 16 is depos a lift-off shadow mask which permits formation of a film 50 ited on the germanium dioxide layer 14 at a rate of about material of precisely-defined predetermined configura 1000A/min to a thickness of about 2500 A. tion. The method relies on the sequential chemical Utilizing conventional photolithographic techniques, vapor deposition of two different semiconductor insu a patterned layer of photoresist is formed as shown in lating oxides, the lowermost of which exhibits a faster FIG. 3. The photoresist material may be a photoresist etch rate when the two oxides are exposed to a common 55 such as AZ. The photoresist is applied in the form of a etchant. By simultaneously etching the dual oxide struc continuous layer by standard photoresist coating appa ture utilizing a common etchant, preferably CF4-based ratus. The photoresist is then exposed and developed plasma, an undercut shadow-mask geometry is pro according to well-known photoengraving techniques duced. leaving an image 18 of the photoresist material such as The shadow-mask structure formed by the preferred shown in FIG. 3. method of the present invention offers the advantages Next, as shown in FIG. 4, exposed areas of the silicon of being water-removable and serviceable to deposition dioxide layer 16 and the underlying germanium dioxide temperatures of up to 800 C. for short periods. Accord layer 14 are removed. Utilizing a CF4-based plasma, ing to the preferred method, silane (SiH4) and germane germanium dioxide exhibits 5 to 10 times the etch rate of (Geh) are employed as source gases in a commercial 65 undoped silicon dioxide. Therefore, it is possible to chemical vapor deposition reactor to produce a layered deposit the layered structure shown in FIG. 2, mask structure of silicon dioxide (SiO2) and underlying a with photoresist and then dry etch in a CF4-based germanium dioxide (GeO2). plasma reactor to produce the undercut shadow-mask 4,387,145 3 4. structure shown in FIG. 4. According to the preferred (d) forming a layer of film material on said exposed method, the silicon dioxide and germanium dioxide are portions of said surface and on remaining portions etched in an IPC Barrel Etcher utilizing a 4% CFA/O2 of said layer of second material; and plasma at about 0.4 Torr. and about 300-400 watts. (e) removing remaining portions of said germanium The photoresist 18 is then removed, preferably by 5 dioxide such that overlying second material and "ashing' in an O2 plasma, to expose the underlying film material thereon are also removed. silicon dioxide layer 16 as shown in FIG. 5. 2. A method as in claim 1 wherein said second mate The film material 20 to be patterned is then deposited rial is silicon dioxide. upon the exposed surface of silicon dioxide layer 16 and 3. A method as in claim 2 wherein said etching is upon the exposed portions of the surface 13 of the ther 10 accomplished by plasma etching. mal oxide layer 12. As shown in FIG. 6, in the preferred 4. A method as in claim 3 wherein said common embodiment, following the deposition of film material etchant is CF4-based plasma. 20, a gapor pocket exists beneath the undercut portion 5. A method for forming a predetermined configura of silicon dioxide layer 16. The entire structure is then tion of a film material comprising: immersed in water. Since germanium dioxide produced 15 (a) forming a layer of a water-soluble material on a in the abovedescribed manner is completely soluble in surface; water while silicon dioxide is insoluble in water, the (b) forming a layer of a water-insoluble material on germanium dioxide layer is dissolved carrying with it said layer of water-soluble material, said water-sol the overlying silicon dioxide and leaving the patterned uble material having an etch rate greater than that film material 20 as shown in FIG. 7. 20 of said water-insoluble material when said materi There are several salient advantages to the abovede als are exposed to a common etchant; scribed process. First, the lift-off film, that is, silicon (c) covering said layer of water-insoluble material dioxide/germanium dioxide, can be deposited in large with photoresist and patterning said photoresist to volumes by low pressure chemical vapor deposition expose portions of said layer of water-insoluble techniques. Second, the materials used are "clean di 25 material; electrics' and will not cause any contamination even if (d) plasma etching said exposed portions of said layer used at high temperature. Third, the lift off solvent is of water-insoluble material and underlying por water which is a distinct advantage. Fourth, since ger tions of said layer of water-soluble material utiliz manium dioxide and silicon dioxide are compatible ing a common etchant to expose portions of said oxide materials, mixtures can be made to vary the de 30 surface; gree of etch rate and the geometry of the indentation. (e) forming a layer of film material on said exposed I claim: portions of said surface and on remaining portions 1. A method for forming a predetermined configura of said layer of water-insoluble material; tion of a film material comprising: (f) immersing remaining portions of said water-solu (a) forming a layer of germanium dioxide on a sur 35 ble layer in water to dissolve said remaining por face; tions of said water-soluble layer such that said (b) forming a layer of a second material on said layer overlying remaining portions of said water-insolu of germanium dioxide, said germanium dioxide ble layer and film material thereon are removed. having an etch rate greater than that of said second 6. A method as in claim 5 wherein said water-soluble material when said germanium dioxide and said 40 material is germanium dioxide. second material are exposed to a common etchant; 7. A method as in claim 6 wherein said water-insolu (c) etching portions of said second material and un ble material is silicon dioxide. derlying portions of said germanium dioxide utiliz 8. A method as in claim 7 wherein said plasma is a ing a common etchant to expose portions of said CF4-based plasma. surface; 45 k t e s c