PROCESS FOR MAKING A PLANAR TRENCH SEMICONDUCTOR STRUCTURE

Abstract: The process includes the steps of forming a trench or groove in a semiconductor substrate (10, 12) through a surface (14) thereof, depositing an organic or other filling material (20, 22) on the surface of the substrate and into the trench, forming a block (26) of material over the trench on the filling material so as to extend a given distance over the surface of the substrate, etching the filling material with a given etchant so as to remove the filling material disposed over the surface of the substrate until only a segment (28) of the filling material remains over the trench and above the surface of the substrate, forming a layer of material (30) over the surface (14) of the semiconductor substrate so as to cover the segment (28) of the filling material, with the layer of material (30) and the filling material (28) having similar etch rates, and etching simultaneously the layer of material and the segment of the filling material until all of the layer of material is removed from the surface of the semiconductor substrate.

Description: This invention relates to a process for making planar semiconductor structures having filled grooves or trenches. Grooves or trenches filled with insulating material, such as silicon dioxide or polyimide, for isolating devices, e.g., transistors, from one another, in inte­ grated semiconductor circuits have been known to provide significant device or transistor density improvements over other known isolation techniques such as those employing P-N junctions. In IBM Technical Disclosure Bulletins, Vol. 17, No. 10, March, 1975, pp. 2893-2894, there is disclosed a V-groove trench filled with polyimide; in Vol. 24, No. 11A, April, 1982, pp. 5458-5459, there is disclosed a transistor butted against the sidewall of a trench filled with insulating material; in Vol. 23, No. 11, April, 1981, pp. 4917-4919, there is disclosed a trench filled with silicon dioxide and silicon nitride with portions of the silicon nitride being removed by a reactive ion etching (RIE) technique and in Vol. 25, No. 11B, April, 1983, pp. 6129-6130, there is disclosed a method for filling a trench with silicon dioxide and planarizing the silicon dioxide with respect to the surface of the substrate in which the trench was formed. U.S. Patent No. 4,396,460, filed May 21, 1982, and issued August 2, 1983, discloses a method for obtaining surface flatness at a groove in a semiconductor substrate by employing chemical etching and a double layer film which is made of two different substances. In U.S. Patent No. 4,519,128, filed on October 5, 1983, by D. G. Chesebro and F. J. Soychak and issued on May 28, 1985, there is disclosed a trench formed in a semiconductor substrate having thin layers of silicon dioxide and silicon nitride formed on the sidewalls and bottom of the trench, with the remainder of the trench filled with polyimide. Any excess polyimide is removed after being thermally cured by known etch back techni­ ques. It has been found that when the filling material in a trench is etched back an undesired step is formed between the surface of the substrate and the top of the filling material due to either over etching or under etching the filling material and due to the contour of the filling material at the trench. Such a step causes discontinuities in conductive lines or wiring subse­ quently formed over the substrate which reduces the reliability of these integrated circuits. It is an object of this invention to provide an improved simple planarization method for semiconductor substrates having narrow grooves or trenches, e.g., less than 2 microns wide, formed therein and filled with, particularly organic material, such as polyimide. In accordance with the teachings of this invention a method is provided for making a planar surface on a semiconductor substrate having a groove or trench formed therein and filled with a material such as an organic material which may be used to electrically isolate one region of the substrate from an adjacent region. The method includes the steps of forming a groove or trench in a semiconductor substrate through a surface thereof, depositing a filling material, such as an organic ma­ terial, within the trench and on the surface of the substrate, forming a block of material over the trench on the organic material so as to extend a given distance over the surface of the substrate, etching the organic material and the block with a given etchant so as to remove the block and the organic material disposed over the surface of the substrate, forming a layer of material over the surface of the semiconductor substrate so as to cover the remaining portion of the organic material, with the layer of material and the organic material having similar etch rates with a given etchant and etching the layer of material and the organic material until all of the layer of material is removed. In a preferred embodiment of this invention, the organic material is polyimide, each of the etching steps is carried out with a dry etching process, the etchant being 100% oxygen gas, the block of material is photo­ resist and the layer of material is also photoresist. The main advantage offered by the invention is that the process provides for improved surface continuity in that the surface of the trench filling material is flat and at virtually the same height as that at adjacent semiconductor surfaces. This results in higher fabri­ cation yield because the likelihood of discontinuities in subsequently deposited layers, such as metallic lines crossing the trench, is substantially reduced. The foregoing and other objects, features and advan­ tages of the invention will be apparent from the following and more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings, in which Figs. 1, 2, 3 and 4 are sectional views of a struc­ ture having a trench formed in a semiconductor substrate taken at succeeding stages during the fabrication of the structure made in accordance with the teachings of the present invention. Referring to the drawings in more detail there is illustrated in the sectional view in Fig. 1 a structure which includes a semiconductor substrate 10, preferably made of silicon, having a P type conductivity and an epitaxial layer 12 having a surface 14 and of N type conductivity grown on the substrate 10 by known techni­ ques with an N+ subcollector diffusion region 16 formed at the junction of the substrate 10 and the epitaxial layer 12. By using an appropriately patterned photoresist mask, with, for example, nitrogen triflouride, not shown, and known reactive ion etching (RIE) techniques, a trench 18, e.g., 2 microns wide, is formed through the epitaxial layer 12 and the N+ subcollector region 16 into the substrate 10 to a depth of approximately 5.0 microns below the surface 14 of the epitaxial layer 12 to form isolated sections 12A and 12B in the epitaxial layer 12. The trench 18 is then filled with polyimide, preferably by depositing a first layer of about 3.5 microns of DuPont 2560 polyimide on the surface of the epitaxial layer 12 and allowing the polyimide to flow into the trench 18 during a spinning operation, curing the first polyimide layer at 80°C for 13 minutes and then deposi­ ting a second layer of about 3.5 microns of DuPont 2560 polyimide over the first polyimide layer and curing the second polyimide layer at 400°C for 25 minutes. Using reactive ion etching (RIE) techniques with oxygen as the etchant, the cured polyimide, not shown, on the surface 14 of the epitaxial layer 12, is removed with polyimide 20 remaining in the trench 18 to a level somewhat below the surface 14 of the epitaxial layer 12. The trench 18 is then completely filled with DuPont 2560 polyimide until a layer 22 of cured polyimide, about 1.3 microns thick, is formed on the surface 14 of the epitaxial layer 12, as indicated in Fig. 1 of the drawings. After curing the polyimide layer 22, which may have been performed at 400°C for 25 minutes, a depression or groove 24 is known to form in the cured polyimide layer 22 over the trench 18. It has also been noted that after curing the polyi­ mide, there are variations in the thickness of the polyimide layer 22 over the surface 14 of the epitaxial layer 12. A layer of photoresist, about 2.4 microns thick, is now deposited on the polyimide layer 22 and, with an appropriately patterned mask, a photoresist segment or block 26 is formed over the trench 18 with extensions of the block 26 overlapping the surface 14 of the epitaxial layer 12 adjacent to the trench 18. The photoresist block 26 is made preferably with substantially vertical sidewalls. By again using reactive ion etching (RIE) techniques with oxygen as the etchant, the polyimide layer 22 and the block 26 are removed from the surface 14 of the epitaxial layer 12 except for the polyimide in the vicinity of the trench 18 that was initially covered by the photoresist block 26 where a polyimide segment 28, about 1.3 microns thick, is formed over the trench 18 and on the surface 14 of the epitaxial layer 12 adjacent to the trench 18, as indicated in Fig. 2 of the drawings. The polyimide segment 28 is then covered with a layer of photoresist 30, as shown in Fig. 3, which may be of the same type as photoresist block 26, having a thickness of about 1.4 microns and having an etch rate similar to that of the polyimide block 28. The layer of photoresist 30 and the polyimide segment 28 are now removed by reactive ion etching (RIE) techniques with oxygen as the etchant to provide a planarized surface wherein it has been found that the upper surface 32 of the polyimide 20, as shown in Fig. 4, in the trench 18 is always less than 0.5 microns above the surface 14 of the epitaxial layer 12 or less than 0.3 microns below the surface 14 of the epitaxial layer 12. Thus, when a conductive line or electrical wiring is extended over the trench 18 between isolated sections 12A and 12B of the epitaxial layer 12, there is little or no likelihood that discontinuities will occur in the line or wire at the surface of the trench 18. It should be understood that preferred polyimides for filling the groove or trench 18 are those that have a molecular weight greater than approximately 8000 grams/­ gram-mole. This type of polyimide is desired because the mechanical properties of lower molecular weight polyi­ mides tend to be brittle and result in material defects such as delamination and in-situ fracturing after a trench filling operation. Although these higher molecu­ lar weight polyimides tend to produce non-planar surfaces due to the low relative solids content and viscous nature, by the use of the teachings of this invention a process is provided wherein very planar surfaces are repeatedly produced even with the use of polyimides having a molecular weight greater than approximately 8000 grams/gram-mole. In the preferred embodiment of this invention, polyimide is used as a trench filling material, however, this invention may be practiced by using materials other than polyimide, e. g., glass, given that appropriate reactive ion etching conditions and etchants are used to provide similar etch rates between the photoresist and the glass when reactively ion etching as indicated in Fig. 3 of the drawings. It should also be noted that the process of this invention may be used advantageously to isolate one semiconductor region from another semiconductor region when making either bipolar transistors in regions 12A and 12B or when making field effect transistors. Accordingly, it can be seen that this invention provides a very simple process for producing a highly planarized semiconductor structure when one or more trenches are formed in the structure resulting in impro­ ved reliability and higher yields for highly dense semi­ conductor circuits.

Claims: 1. A process for making a planar trench semicon­ ductor structure which comprises the steps of forming a trench (18) in a semiconductor body (10, 12) through a surface thereof, depositing a first material (20, 22) into said trench and on said surface, forming a block of material (26) over said trench so as to extend a given distance over said surface, etching said first material (22) and said block (26) while leaving a segment (28) of said first material over said trench, forming a layer (30) of a second material over the surface of said body so as to cover the segment (28) of said first material, said layer (30) of a second material and the material of said segment (28) having similar etch rates for a given etchant, and etching simultaneously said layer (30) of a second material and said segment (28) until the surface of said body is exposed. 2. A process as set forth in Claim 1 wherein said first material is a polyimide. 3. A process as set forth in Claim 2 wherein said polyimide has a molecular weight greater than 8000 grams/gram-mole. 4. A process as set forth in Claim 1 wherein each of said etching steps is a dry etching process. 5. A process as set forth in Claim 4 wherein each of said etching steps is a directional dry etching process. 6. A process as set forth in Claim 5 wherein each of said etching steps includes the use of a 100% oxygen gas etchant. 7. A process as set forth in Claim 1 wherein said block (26) is made of a photoresist material. 8. A process as set forth in Claim 1 wherein said layer (30) of a second material is a photoresist mate­ rial. 9. A process as set forth in Claim 1 wherein said first material (22) and said block (26) have similar etch rates for a given etchant. 10. A process for making a planar trench semiconductor structure which comprises the steps of forming a trench (18) in a semiconductor substrate (10, 12) through a surface (14) thereof, flowing a polyimide (20, 22) having a molecular weight greater than about 8000 grams/gram-mole into said trench (18) and on the surface (14) of said substrate, curing said polyimide, forming a block (26) of photoresist on said cured polyimide (22) and over said trench (18) so as to extend a given distance over said surface from said trench, said cured polyimide and said block of photoresist having similar etch rates for a given etchant, etching simultaneously said cured polyimide (22) and said block (26) until said surface (14) is exposed so as to form a segment (28) of said cured polyimide above the surface of said substrate at said trench, flowing a layer of photoresist (30) having a low molecular weight compared to that of said polyimide over the surface of said substrate so as to cover said polyi­ mide segment (26), said layer of photoresist and said polyimide having similar etch rates, and etching said layer of photoresist (30) and said polyimide (28) until the surface (14) of said substrate is exposed.

Inventors: DOUGHERTY, JAMES JOHN
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21