Method of Forming a Narrow Self-Aligned, Annular Opening in a Masking Layer

This invention relates to semiconductor manufacturing technology and, more particularly, to photolithography masking techniques used to provide feature dimensions which transcend the resolution limits of available photolithography.

Since the late 1960's, a new generation of integrated circuits has been developed approximately every four years. Each generation has been characterized by a halving of device dimensions, resulting in a four-fold density increase over the preceding generation. Increases in circuit density have been consistently limited by the resolution of the available photolithographic equipment. The minimum size of features and spaces that a given piece of photolithographic equipment can produce is directly related to its resolution capability

It has long been recognized, by those skilled in the fabrication of integrated circuits, that vertical film layers as thin as 0.01 .mu. can be grown with a high degree of accuracy Also, layers as thin as 0.1 .mu. can be deposited by low pressure chemical vapor deposition, hereinafter known as LPCVD. By comparison, the minimum feature size, producible with the present generation of photolithography equipment used to produce 1-megabit SRAMs and 4-megabit DRAMs, is approximately 0.6 .mu.. If deposition layers can be used to define horizontal dimensions within integrated circuits, the result will be increased circuit density.

Many die are typically fabricated on a singular semiconductor wafer. Complex circuitries are created on each die. Because of increasing device density on die, it is necessary to enhance the isolation of the different devices to ensure that no current flows through the substrate from one device to another. In FIG. 1, two active areas are isolated from each other by a field oxide region that has been thermally grown using a standard Locos process. During field oxide growth, patches of silicon nitride protect future active areas from oxidation. Electrical devices (e.g., transistors, resistors, capacitors) will ultimately be fabricated in the active areas. The oxide layer must be of relatively large width to ensure that there is no leakage current from one area to another. This leakage current is the result of what is termed bipolar latch up in the case where the two regions are of opposite types; that is, one is n type and the other is p type. Similar regions are also susceptible to leakage current.

In order to reduce the horizontal width of the oxide layer and maximize die space, trenches have been created via several processes. The trenches, filled with an insulative material such as silicon dioxide, extend into the substrate and act as insulating walls between active areas. Because trenches extend into the substrate, they can prevent bipolar latch up even though they may be narrower than the field oxide region of FIG. 1. In fact, the width can now be as narrow as present technology allows. FIG. 2 shows a trench manufactured with polysilicon deposited over an oxide region. The trench width is difficult to precisely predict when using this method due to variations in the polysilicon deposition. FIG. 3 depicts trench fabrication using an oxide mask created with a pitch doubling process that is the subject of a copending U.S. patent application submitted by Tyler Lowrey and Randal Chance of Micron Technolog,y, Inc. and accorded Ser. No. 519,992, filed 5/7/90, entitled "Method for Reducing, by a Factor of 2.sup.-N, the Minimum Masking Pitch for a Photolithographic Process Used in the Fabrication of an Integrated Circuit." In FIG. 4A an oxide mask is formed by under-etching the photoresist. Next, metal is sputtered onto the wafer. The photoresist shields a portion of the substrate next to the oxide mask from the metal. After the wafer has been sputtered, the metal covering the photoresist is lifted and the photoresist is etched producing a mask of metal and oxide for trench formation, FIG. 4B. There is a percentage of error in the predictability of trench size due to the nature of the sputtering process because of the shielding effect of the photoresist. FIGS. 5A and 5B depict trench fabrication that is the subject of U.S. Pat. No. 4,502,914. This invention provides a structure of polymeric material with vertical sidewalls, the latter serving to make sidewall structures of silicon dioxide or nitride with dimensions in the sub-micrometer range. These sidewall structures can be used as masks directly. For the negative lithography, another layer is alternatively applied over the sidewall structures using a planarization which is partly removed until the peaks of the sidewall structures are exposed. Subsequently the sidewall structures themselves are removed. The resulting opening can then be used as a mask for trench formation. Providing uniformity of the planarization layer over the sidewall structures can be difficult using this method due to the fact that the sidewall structures can disrupt the flow of resist or other material during the spin.

Since the trenches are fabricated after the substrate has been exposed, the key to narrow, self-aligned isolation trenches is exposing a highly predictable narrow substrate region. The etch mask fabrication of the present embodiment facilitates even narrower, self-aligned trenches, with a minimum amount of masking steps using a deposition layer to precisely define the narrow spacing.

This invention utilizes a primary mask of photoresist, created using conventional photolithography, to create a secondary mask, having vertical film layer segments. The primary mask patterns a silicon dioxide layer segment that protects a substrate region where integrated circuit structures will later be fabricated. The silicon dioxide layer segment is part of the secondary mask and its perimeter defines the inner perimeter of the region that will be trenched in order to isolate the circuit structures. An expendable spacer layer is deposited on top of the silicon dioxide layer segment and the exposed substrate. The thickness of the expendable layer determines the width of a perimetric annular opening in the secondary mask. The perimetric annular opening may be created with a width as narrow as 0.1 .mu.. The spacer layer may be polysilicon deposited by LPCVD. A thick protective layer is blanket deposited on top of the spacer layer, the primary consideration being that the spacer layer must be etchable with a high degree of selectivity over the protection layer. A blanket deposition of photoresist follows which results in the planarization of the in-process wafer. The protective layer and photoresist are then plasma etched at the same rate in order to expose the spacer layer adjacent to and capping the silicon dioxide segment, the balance of the spacer layer remaining covered by the protection layer. At this point, the vertical portions of the spacer layer are etched away to expose an annular opening of the substrate that is perimetric to the silicon dioxide layer segment. The foregoing process creates an etch mask, one application of which is etching the exposed substrate to form a narrow trench self-aligned to the outer perimeter of the silicon dioxide layer segment and the remaining spacer layer. The trench may be filled with oxide using either thermal growth or LPCVD, in order to provide isolation for future circuit structures fabricated on the substrate defined by the silicon dioxide segment. Once the silicon dioxide layer segment has been isolated by a trench, it resembles an island surrounded by a moat and will therefore be referred to as a mask island.

Because the width of the annular opening within the secondary mask is dependent on the thickness of the expendable spacer layer, the very narrow trench widths are possible. In addition, the resulting trench is self-aligned around its mask island. The process is easily adapted to current manufacturing techniques and has minimum manufacturing problems.

This masking technique may be used to create a variety of semiconductor structures. Although the technique was originally developed to isolate circuit structures within ultra high density DRAM arrays, the technique may also be used to create structural features of SRAM arrays where an increase in device density is desired.

FIG. 1 depicts the prior art isolation of integrated circuit regions; and

FIGS. 2, 3, 4, and 5 are prior art trench isolation of integrated circuit regions.

FIGS. 6 through 25 depict a portion of an inprocess wafer during different process stages. Figures representing contemporaneous process stages have identical ordinal number designations (e.g., FIGS. 5A and 5B are contemporaneous views). Each "A" view is a cross section through the contemporaneous top planer "B" view.

FIGS. 6A and 6B are unprocessed silicon substrate;

FIGS. 7A and 7B represent the substrate of FIGS. 6A and 6B, respectively, following the thermal growth of a thin-oxide layer;

FIGS. 8A and 8B represent the in-process wafer portion of FIGS. 7A and 8A, respectively, following the blanket deposition of nitride;

FIGS. 9A and 9B represent the in-process wafer portion of FIGS. 8A and 8B, respectively, following the deposition of a base layer of silicon dioxide.

FIGS. 10A and 10B represent the in-process wafer portion of FIGS. 8A and 8B, respectively, masking the base layer of photoresist oxide.

FIGS. 11A and 11B represent the in-process wafer portions of FIGS. 9A and 9B, respectively, following an etch of the base layer of photoresist oxide.

FIGS. 12A and 12B represent the in-process wafer portion of FIGS. 11A and 11B, respectively, following an etch of the nitride layer.

FIGS. 13A and 13B represent the in-process wafer portions of FIGS. 12A and 12B, respectively, following a reoxidation of the substrate.

FIGS. 14A and 14B represent the in-process wafer portion of FIGS. 13A and 13B, respectively, following a polysilicon deposition.

FIGS. 15A and 15B represent the in-process wafer portion of FIGS. 13A and 13B, respectively, following an oxide deposition.

FIGS. 16A and 16B represent the in-process wafer portion of FIGS. 15A and 15B, respectively, following a photoresist application to planarize the wafer.

FIGS. 17A and 17B represent the in-process wafer portion of FIGS. 15A and 15B, respectively, following a uniform rate etch of the photoresist and final oxide deposit.

FIGS. 18A and 18B represent the in-process wafer portion of FIGS. 17A and 17B, respectively, following a polysilicon selective etch.

FIGS. 19A and 19B represent the in-process wafer portion of FIGS. 18A and 18B, respectively, following a final oxide etch.

FIGS. 20A and 20B represent the in-process wafer portion of FIGS. 19A and 19B, respectively, following a substrate etch to form trenches.

FIGS. 21A and 21B represent the in-process wafer portion of FIGS. 20A and 20B, respectively, following etching of the oxide deposits.

FIGS. 22A and 22B represent the in-process wafer portion of FIGS. 20A and 20B, respectively, following oxidation of the trenches.

FIGS. 23A and 23B represent the in-process wafer portion of FIGS. 22A and 22B, respectively, following a final etch.

FIGS. 24A and 24B depict the trenches formed around two integrated circuit regions

FIGS. 25A and 25B represent the in-process wafer portion of FIGS. 24A and 24B, respectively following the oxidation of the trenches.

In the semiconductor industry, electronic circuitry is fabricated on silicon substrate. Since the substrate is not an insulator, unwanted currents can flow from one circuit region to another through the substrate. Although the masking method of the present embodiment fabricates a narrow trench around an individual circuit region to isolate it from other circuit regions, the fabrication is very flexible, and may be used to pattern a multitude of features within semiconductor circuitry. Therefore, the description of the present embodiment emphasizes the fabrication of narrow spacings in the substrate for subsequent circuit patterning. Also, more than one circuit region may be treated in a similar manner; that is, a trench may be fabricated around each circuit region on an individual substrate by duplicating the following method pertinent to the isolation of one circuit region.

In FIGS. 6 through 25, the "A" figures represent a cross sectional view of narrow spacing and trench fabrication processes and the "B" figures represent a top planar view of each corresponding figure "A". FIGS. 6A and 6B depict a portion of an in-process silicon wafer showing substrate 1.

In FIGS. 7A and 7B, a thin oxide layer 3 is thermally grown on the substrate 1 in a diffusion oven. The thin oxide layer 3 offers protection for the substrate 1 during subsequent layerization and etching.

In order to provide a future barrier against oxidation of substrate 1 during subsequent circuit fabrication steps, a nitride layer 5 may be optionally deposited by LPCVD as shown in FIGS. 8A and 8B.

FIGS. 9A and 9B show the initial layerization of the in-process wafer after deposition of a base layer of silicon dioxide 7. The layerization is conventionally masked with a primary photoresist mask 9 shown in FIGS. 10A and 10B.

FIGS. 11A and 11B depict the layerization after an anisotropic etch of the base layer of silicon dioxide 7, the optional nitride layer 5 and thin oxide layer 3 remaining intact.

The optional nitride layer 5 is etched to form the mask island 8 shown in FIGS. 12A and 12B. The layered mask defines the region in the substrate reserved for electronic circuitry and includes the 3 initial layers of the thin oxide 3, nitride 5 and silicon dioxide 7. The trenches of the present embodiment will isolate the substrate defined by the mask island from other areas of the substrate.

In FIGS. 13A and 13B, the substrate is subject to thermal reoxidation in order to ensure that the thin oxide layer 3 remains intact. FIG. 13B clearly illustrates the circuitry region defined by the mask island 8.

Referring now to FIG. 14A, a conformal expendable polysilicon spacer layer 13 is blanket deposited by LPCVD on the mask island 8 and thin oxide layer 3. It is important that the thickness of the polysilicon spacer layer 13 correspond to the desired trench thickness after deposition since the polysilicon thickness defines the subsequent width of the substrate 1 exposed for eventual trench fabrication. FIG. 14B depicts the blanket deposition of polysilicon on the in-process wafer.

Referring to FIGS. 15A and 15B, a protective oxide layer 13 is blanket deposited on the polysilicon spacer layer by either LPCVD or by the tetra ethyl ortho silicate, (TEOS), process. The polysilicon 13 having been selected for its high degree of selectivity with respect to its being etchable over the mask island and oxide layer

FIGS. 16A and 16B depict the final layerization of the wafer with a planarized layer of photoresist resin 17.

Referring to FIGS. 17A and 17B, the photoresist 17 and the oxide 15 are anisotropically etched with a plasma etch at the same rate to a level being at or below the top of the spacer layer 13 in order to expose the polysilicon spacer layer 13 above the mask island 8 and vertically adjacent to the island and extending away from the island a distance equal to the thickness of the polysilicon deposition. The remaining deposition of oxide 15 protects the polysilicon 13 that was not exposed by the etch. FIG. 17B illustrates the fact that there is now an island of polysilicon 13 resulting from the etching of the photoresist 17 and oxide 15. Alternately it is possible to eliminate the planarization of the wafer with photoresist and, instead, mechanically etch the oxide layer 15 to a level being at or below the top of the spacer layer 13 in order to expose the polysilicon spacer layer 13 of FIGS. 17A and 17B. It is also possible to planarize a portion of the polysilicon spacer 13 layer during the previous mechanical and plasma etches to the same level as the protective oxide layer.

FIGS. 18A and 18B depict the result of isotropically etching the exposed polysilicon 13 adjacent to and capping the mask island 8. The polysilicon 13 protected by the remaining oxide 15 is not etched. The result of the polysilicon etch is an annular opening 19, whose sidewalls are the initial masked island 8 and the oxide 15 covering the remaining polysilicon 13. The floor of the annular opening consists of the thin oxide layer 3 exposed during the polysilicon etch. The width of this annular opening is equal to the original thickness of the polysilicon layer 13. The thin oxide layer 3 forming the floor of the depression is now over-etched to expose the substrate 1.

It can be seen from FIGS. 19A and 19B that the exposed substrate 21 surrounds the area defined by the mask island. The width of the exposed substrate 21 is equal to the thickness of the original polysilicon layer 13. Therefore, the width of the exposed substrate 21 is highly predictable and very narrow. At this juncture the etch mask for subsequent trench formation in the exposed substrate has been completed. This etch mask fabrication process also provides for the self-alignment of the exposed substrate 21 to the mask island 8 perimeter and the oxide 15 covered polysilicon 13.

The trench 23 is now etched in the exposed substrate 21 using an anisotropic process that is highly selective for silicon. The mask island 8 and the remaining oxide 15 protecting the remaining polysilicon 13 function as a secondary patterning mask for trenching the substrate. The resultant trench is self-aligned to the mask island 8 and the protected polysilicon 13. The fabricated trench 23 is shown in FIGS. 20A and 20B. Although FIGS. 19B and 20B look identical, the substrate in FIG. 20B has been trenched, whereas the substrate of FIG. 19B is in its original state.

In FIGS. 21A and 21B, the silicon dioxide layer 7 and the protective oxide layer 15 have been isotropically etched leaving the nitride layer 5 surrounded by a narrow trench 23. At this time, oxide is either deposited in the trench or thermally grown in a diffusion oven from each sidewall of the trench. This oxide growth is possible because the trench is so narrow. A 0.2 .mu. trench will consume only 0.1 .mu. of silicon. This is equal to the radius of the trench and will therefore fill it. In either case, minimal oxide is needed because of the narrowness of the trench. FIGS. 22A and 22B depict the trench once it has been filled with oxide 25. The resultant trench has its inner perimeter self-aligned to the perimeter of the island and its outer perimeter self-aligned to the remaining polysilicon. The trench has a narrow width exactly equal to the thickness of the polysilicon deposition spacer layer 13, thus facilitating even denser circuit fabrication. The method of forming trenches in this embodiment results in the exact placement and a predictable width of the isolation trenches. These facts, coupled with the minimum masking steps this method entails, creates a significant advance in isolation trench formation.

In FIGS. 23A and 23B, the nitride has been etched and the die is ready for further fabrication.

FIGS. 24A and 24B are a representation of trench fabrication around a plurality of mask islands 27. The process is essentially the same as fabricating trenches around one island. Trenching each island ensures each circuit region individual protection from latch up. FIGS. 25A and 25B depict the trenches 28 of the islands of FIG. 24 once they have been filled with oxide 29.

Although only several embodiments of the process have been described herein, it will be apparent to one of ordinary skill in the art, that changes may be made thereto without departing from the spirit and the scope of the process, as claimed.