Abstract:
The present invention provides a method of constructing trenches for use in microelectronic circuit structures. A photolithographic method is used to create trenches with sloped walls shaping the photoresist masks into sloped profiles. These photoresist masks effectively shape the underlying substrate during subsequent etch steps producing sloped wall trenches. These trenches can be used as shallow trench isolation structures to isolate microelectronic circuit structures from each other.

Description:
TECHNICAL FIELD 
     The present invention relates to the processes used in the fabrication of integrated circuits. More particularly, the invention relates to an advanced method for fabricating sloped wall isolation trenches using shaped photoresist profiles. 
     BACKGROUND OF THE INVENTION 
     Microelectronic circuitry uses isolation trenches filled with electrically isolating materials to electrically isolate the individual devices comprising integrated circuits from one another. Abrupt transitions between active and isolation regions are necessary to maximize circuit density. Shallow trench isolation is one means for achieving the necessary abrupt transition. Typical trench fabrication methods feature the etching of vertical walled trenches between active circuit areas, then filling the trenches with electrically isolating material (typically silicon dioxide), thereby electrically isolating independent circuit portions. Existing trench fabrication technologies stress the need for creating trenches with nearly vertical trench walls in order to achieve the requisite abrupt transition between active and isolation regions. In doped wafers, the walls and floors of these isolation trenches are heavily doped increasing their isolating effect and improving the reliability of circuit interconnections. Standard vertical wall trenches present numerous difficulties to the doping process. In order to dope the vertical trench walls the entire surface is angled allowing implantation of the trench walls. Unfortunately, this process does not create a uniformly doped trench with any useful degree of reliability. Moreover, in deep narrow trenches it becomes very difficult to dope the bottom segments of trench walls. Thus, the need to facilitate doping and the need for deep narrow trenches are at cross purposes. Additionally, the nearly vertical walls create sharp angles (nearly 90°) at the trench corners (e.g. angle θ of FIG. 15) which lead to enhanced edge conduction. The sharp edges also cause difficulties in controlling, for example, gate oxide uniformity at the corners. These sharp angles are known to cause electrical failures in integrated circuits and are a serious cause of circuit unreliability. These problems are known as edge effects. Additional problems arise when trenches are doped and used as current paths. For example, in addition to edge effects, vertical walled trenches are prone to contact misalignment which leads to contacts shorting against the substrate and self-aligned gate shorting. This leads to catastrophic circuit failure. 
     Other existing isolation processes exhibit similar problems, as well as unique problems of their own. For example, the so-called LOCOS method, which selectively oxidizes patterns on a silicon substrate to create isolation regions, suffers from oxide encroachment of silicon dioxide under the silicon nitride layer. This oxide encroachment is known in the industry as a bird&#39;s beak. This bird&#39;s beak problem reduces the active circuit area between the isolation regions by encroaching into the active regions. As feature size decreases this becomes a more significant problem. Other known processes include a variation of the LOCOS process which is known in the industry as SWAMI, as developed by Hewlett-Packard. The SWAMI process includes outwardly etching the silicon nitride and the pad oxides at 60 degrees such that the subsequent oxidation and beak formation length is limited. Still other methods include using high-pressure oxidation techniques where the oxide is grown faster and at a lower temperature. This results in minimizing the growth of the bird&#39;s beak. But, in the end, all of these methods suffer from the bird&#39;s beak problem which reduces the active surface area, thereby limiting circuit density. 
     The primary object of the present invention is to reliably fabricate sloped wall isolation trenches which avoid self-aligned source shorting and contact misalignment shorting. Another object is to provide isolation trench profiles which are more easily doped, thereby creating a more reliable isolation trenches. Another important object is the fabrication of sloped wall isolation trenches which reduce edge effects and thereby increase circuit reliability. A related object of the present invention is to provide an isolation trench which does not suffer from the bird&#39;s beak problem and thereby conserves the maximum amount of microchip surface area. 
     Finally, the present invention provides fabrication processes for manufacturing microelectronic integrated circuit structures featuring isolation trenches with reliably sloped walls in accordance with the foregoing objects. The key objective of the invention is to overcome the difficulties presented in reliably producing trenches with controlled slope walls. 
     SUMMARY OF THE INVENTION 
     The present invention is a method of fabricating trenches. In particular, the invention produces sloped wall isolation trenches which are easier to dope and exhibit a lower circuit failure rate due to edge effects. The present invention eliminates issues of self-alignment source shorting and contact misalignment shorting. The isolation trenches of the present invention do not suffer from the bird&#39;s beak problem. Therefore the active circuit area size is not reduced by bird&#39;s beak intrusion. This allows higher circuit density. 
     Standard wafer fabrication techniques are used to create the basic semiconductor substrate (typically, crystalline silicon or crystalline gallium arsenide). Later, this basic semiconductor substrate (which may be doped or not) is oxidized. (See FIG. 1). The substrate is then coated with photoresist by methods known to practitioners with ordinary skill in the art (see FIG. 2). 
     Typically, the photoresist layer is exposed through openings in a mask pattern. As a result the mask pattern is transferred to the photoresist. Typically this is done by focusing the light on the surface of the wafer (i.e. photoresist) through the mask pattern exposing the photoresist coated surface to light. The photoresist surface is then developed and rinsed, leaving a pattern. When the exposing light is sharply focused, sharp vertical walls are patterned into the photoresist. This technique is used for both positive photoresists and negative photoresists. 
     A positive photoresist is insoluble when coated, but becomes soluble when exposed to light. The result is that when the soluble portions are washed away during the development step, a pattern remains. A negative photoresist is soluble when coated. In certain types of negative photoresists, exposure to light causes insolublization. Therefore the exposed regions are no longer soluble. As with positive photoresists, the soluble regions are removed during development leaving a pattern which is used for later surface etching processes. A key element in the exposure process involves sharply focusing the exposing light onto the semiconductor surface. Exposing the wafer surface either above or below the focal plane leads to aberrations in the exposure profile (e.g. see, FIGS. 6a and 6c). These aberrations are normally avoided because they do not lead to the vertical exposure profile depicted in FIG. 3b. 
     The present invention is an improved method for fabricating shallow isolation trenches with sloped wall profiles. The cornerstone of the invention is the creation of shaped photoresist mask profiles which lead to sloped wall isolation trench profiles. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a fuller understanding of the present invention, reference is made to the accompanying drawings in the following Detailed Description of the Invention. 
     FIG. 1 shows a typical semiconductor substrate with a barrier layer. 
     FIG. 2 shows the same substrate as depicted in FIG. 2 after being coated with a photoresist layer. 
     FIG. 3A shows the exposure of a positive photoresist through a pattern mask. 
     FIG. 3B shows the surface of FIG. 3A after the exposed regions have been removed. 
     FIG. 3C shows the surface of FIG. 3B after etching creates vertically walled trenches in the substrate. 
     FIG. 4 shows the photoresist profile used in the present invention. 
     FIG. 5 shows the typical starting wafer with photoresist coating in place. 
     FIG. 6A shows an above focal plane exposure of a wafer of the present invention. 
     FIG. 6a shows the resulting approximate developed pattern of a positive photoresist after exposure as shown in FIG. 6A. 
     FIG. 6B shows the exposure of a wafer of the present invention with the wafer exposed while in the focal plane. 
     FIG. 6b shows the resulting exposure pattern after exposure as shown in FIG. 6B. 
     FIG. 6C shows a below focal plane exposure of a wafer of the present invention. 
     FIG. 6c shows a resulting approximate developed pattern of a positive photoresist after exposure as shown in FIG. 6C. 
     FIG. 7 depicts the effects of exposure by the methods of the present invention showing the sloped photoresist profile. 
     FIG. 8 shows the photoresist profile after soluble photoresist is removed leaving a sloped photoresist profile. 
     FIG. 9 shows the effects of the first etch of the surface of FIG. 8. 
     FIG. 10 shows the fabrication of a barrier layer over the surface of FIG. 9. 
     FIG. 11 shows FIG. 10 after masking for a second etch. 
     FIG. 12 shows the second etch which removes the barrier layer from the surface of FIG. 11. 
     FIG. 13 shows FIG. 12 after the photoresist is stripped. 
     FIG. 14 shows the surface of FIG. 13 after a third etch. This view also shows the final shallow isolation trench profile, including sloped trench walls. 
     FIG. 15A shows a vertical walled trench with an idealized dopant implantation profile. 
     FIG. 15B is an exemplar of the problems encountered when trying to implant a vertical walled trench with dopant ions. Also shown are the sharp corner angles. 
     FIG. 15C shows a typical doping profile in a vertical walled trench illustrating the difficulties in doping the bottom segments of the vertical trench wall. 
     FIG. 16 shows the doping profile achieved by a typical sloped wall trench. FIG. 16 also displays the less radical trench edge profiles. 
     FIG. 17A shows a typical dopant profile in vertical walled trench structures. 
     FIG. 17B shows a well aligned contact in place on the structure of FIG. 17A. 
     FIG. 17C shows a typical misaligned contact problem common with vertical wall trenches. 
     FIG. 18A shows a typical doping profile in a sloped wall trench, FIG. 18B shows how the doping profile achieved in a sloped trench avoids the misaligned contact problem. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The methods of the present invention are used to fabricate sloped wall trenches. These trenches can serve as electrical isolation between adjacent microelectronic circuit components. The following embodiment is the best mode for creating these structures. 
     Ordinarily a semiconductor substrate (typically, crystalline silicon or crystalline gallium arsenide) is provided with a first barrier layer. Using a silicon substrate the first barrier is typically silicon dioxide. FIG. 1 depicts a semiconductor substrate (101) with a first barrier layer (102), which is typically between 100 Å-300 Åthick. It should be noted that the first barrier layer (102) is not necessary to practice the invention. FIG. 2 shows this basic wafer coated with a layer of photoresist (103). Either positive or negative photoresists may be used. The invention is illustrated using a positive photoresist. However, all the techniques and methods disclosed are applicable to negative photoresist embodiments. As shown in FIG. 3A, the photoresist layer (103) is exposed to light (301) as it is passed through openings in a mask pattern (302). When positive photoresists are used, the areas exposed to light (303) become soluble while the unexposed areas (304) remain insoluble. Because the light (301) is sharply focused, the photoresist profile (103A) after exposure and development, is approximately vertical in nature (as shown in FIG. 3B). This leads to approximately vertical walled trenches (305) as shown in FIG. 3C after typical directional plasma etch as is known in the art. As disclosed above, vertical wall trenches have a number of performance limitations. Sloped wall trenches overcome many of these limitations. The key to the present invention is the reliable fabrication of a sloped photoresist profile (R of FIG. 4). The inventor&#39;s method of fabricating sloped wall isolation trenches begins with the same starting materials as the prior art. 
     FIG. 5 shows a semiconductor substrate (101) with a first barrier layer (102). Silicon dioxide is typically used as a barrier layer (102). This first barrier layer is typically between 100 Åto 300 Åthick. Photoresist (103) covers the first barrier layer (102). The pictured embodiment depicts the use of positive photoresist (103). However, the invention may be practiced with equal effectiveness by using either positive or negative photoresists. 
     An important aspect of the invention is the creation of a photoresist pattern having sloped edges (as shown in FIG. 4). This can be accomplished by a number of photolithographic means which are known to persons with ordinary skill in the art. Previously, these effects have been considered aberrations to be avoided. Ordinarily, a stepper is used to focus the exposing light on the substrate surface (e.g. as in FIG. 3A and FIG. 6B) leading to substantially vertical photoresist walls (as in FIG. 3B). However, by exposing the photoresist (103) with unfocused light, the walls of the photoresist profile may be altered. The photoresist profile of the present invention may be formed using the staged incremental photolithographic exposure techniques set forth below. For example if, as in FIG. 6A, the photoresist (103) is exposed while above the focal plane, an exposure profile similar to that of FIG. 6a is created. If exposing again while in focus, as in FIG. 6B, an exposure profile like that of FIG. 6b is achieved. In a final exposure, below the focal plane (FIG. 6C), a photoresist exposure profile similar to that of FIG. 6c is added to the existing profiles. By a novel stage combination of these exposure profiles (6a, 6b and 6c) in conjunction with different amounts of exposure energy at the different exposure focal planes an exposure profile similar to that of FIG. 4 is created. An important aspect of the invention is to use an imaging system with a high enough numerical aperture so that these effects can be created. Typically, these effects can be created by imaging systems with numerical apertures as low as 0.35. However, it is preferable to use systems with a numerical aperture of about 0.45. This exposure can be done in as many focal increments as desired leading to very precisely shaped photoresist profile, although it is more practical to use fewer increments, e.g. 3, to increase throughput. 
     Additionally, the inventors use the intensity of the exposing light to shape the profile. More intense light leading to narrower profiles and less intense light leading to wider profiles. The juxtaposition of a series of varied intensities leads to a developed photoresist profile similar to FIG. 4. Both methods may be used in conjunction to more efficiently shape the photoresist profile. For example, when using a positive photoresist, a first exposure above focus (as in FIG. 6A) may be used with a high intensity light exposure, followed by a second exposure in focus (as in FIG. 6B) using a lower intensity light exposure. Finally, a third exposure below focus (as in FIG. 6C) using a lowest intensity light will lead to a profile like FIG. 4. If a negative photoresist is used, the process is just the opposite (i.e., a low intensity light is used when the exposure is above focus, as in FIG. 6A; a higher intensity light is used when exposure is in focus, as in FIG. 6B; and a highest intensity light is used when the exposure is below focus, as in FIG. 6C.) The invention may be practiced by using any number of subtle combinations of exposure intensities and focal states. All of which will be used to shape the photoresist profile and thereby influence the final shape of the isolation trench. 
     FIG. 7 depicts a positive photoresist after exposure. The exposed regions (7a) are solubilized and will be removed. FIG. 7 also depicts the sloped profile of the unexposed photoresist (7b). The solubilized photoresist (7a) is developed out by any of the standard means known to those with ordinary skill in the art. After this step, a shaped photoresist remains (7b) as depicted in FIG. 8. 
     Once the sloped profile photoresist is in place, the semiconductor substrate is anisotropically etched. Dry etch techniques are used because of their directional etch characteristics. Any anisotropic etch technique (e.g. plasma etching, reaction ion etching (&#34;RIE&#34;), ion milling, etc.) may be used, but the preferred method is plasma etching. The first etch step serves to weaken the surface of the semiconductor substrate (101) in the regions defined by the photoresist creating a trench region (101A) as is shown in FIG. 9. The plasma etch also erodes a portion of the photoresist. The original photoresist profile is shown by the dotted line (901), with eroded profile depicted as (902). As a result of the photoresist erosion, the trench region manifests a sloped profile (101A). After the first etch is complete, the remaining photoresist (902) is stripped. 
     FIG. 10 depicts the surface after coverage with a barrier (1004) material which is chosen for good etch selectivity between the barrier layer (1004) and the substrate (101)(for silicon substrates this is typically silicon nitride (Si 3  N 4 ), but may consist of silicon oxynitride (SiO x  N y , where x and y describe the ratio of oxygen to nitrogen) or any other suitable barrier material). This barrier layer (1004) is about 500 Åand 2000 Åthick, preferably forming a layer approximately 1700 Åthick. Methods of forming the barrier layer are known to any practitioner with ordinary skill in the art. The barrier layer (1004) is masked (as in FIG. 11) with a second photoresist (1105) by any of the prior art methods. The second photoresist (1105) defines the final trench widths (w) (usually between 0.1 μm and 0.7 μm). 
     FIG. 12 shows the surface of FIG. 11 after the barrier layer (typically Si 3  N 4 ) has been etched away in a second etch. The second etch is accomplished using anisotropic etch techniques, typically plasma etching. The second etch etches away the barrier layer until the semiconductor substrate is reached in the trench region (101A). The photoresist layer (1105) is then stripped from the remaining barrier layer (1201). FIG. 13 shows the surface in preparation for a third etch. The remaining barrier layer serves as an etch mask (1201). This surface is anisotropically etched for a third time, leading to the sloped trench wall profile (101B) shown in FIG. 14. Further, the trench region (101A) having been etched by all prior etch steps is often weakened and can be more susceptible to subsequent etching. This means that the trench region (101A) can be etched away faster still, resulting in a final etch profile with sloped walls. This slope is shown by (101B) of FIG. 14. 
     The sloped trench structures of FIG. 14 have several advantages over vertical walled prior art trench structures. When used as isolation structures in CMOS systems, vertical walled trenches present certain process difficulties. FIG. 15A depicts, for example, a vertical walled isolation trench (1502) in a lightly doped substrate (1501). In order to enhance the isolation properties of the trench (1502), the trench boundaries (1503) are heavily doped with ions of the same charge as the lightly doped substrate (1501). The dopant profile (1503) is ideal and is not realized in actual trenches. This doping is typically accomplished using an ion implanter. As is shown in FIG. 15B, when implanting deep trenches the substrate surface (1501S) blocks the ion implantation beam (I) from implanting the bottom portions of the isolation trench (1502B). This leads to the non-uniform dopant implantation profile (1503) shown in FIG. 15C. The thin dopant profile in the bottom of trench walls (1502B) leads to isolation failure and circuit unreliability. This is remedied by using isolation trenches featuring sloped walls, as shown in FIG. 16. The sloped walls (1601A) allow dopant implantation (I) at an angle normal to the surface. This is not possible with vertical trench walls. The result is a more uniform dopant profile (1603) in the isolation trench wall and better isolation. Another key advantage of the invention is that the sloped trench walls reduce the edge effect and enhances circuit reliability. A typical vertical walled trench has sharp trench corners (θ=nearly 90° as in FIG. 15B). These angles (θ=90°) cause enhanced edge conduction. This edge conduction leads to circuit failure. The present invention (as shown in FIG. 16) has edge angles (φ) of greater than 90°. This significantly reduces circuit failure. 
     Another advantage of the present invention is the prevention of misaligned contact shorting. Semiconducting substrate surfaces are often doped to create current paths rather than isolation regions. The resulting doped current paths (17A) in prior art vertical trenches are shown in FIG. 17A. When metallic contacts (17B) are put in place, as in FIG. 17B, they supply microcircuit structures with current. Unfortunately, there are frequently incidents of misalignment (as shown in FIG. 17C). In such cases the misaligned metallic contact (17C) touches the substrate (1701) shorting out the circuit against the trench wall causing failure. The sloped wall structure of the present invention (1801) as depicted in FIG. 18A avoids this problem. The walls are doped (1802) with an ion implanter (I) to create current paths. When misalignment occurs (as in FIG. 18B) there is no shorting because the metallic contact (1803) is always in contact with a doped current path (1802). Therefore, shorting is not possible. The present invention has been particularly described with respect to a certain preferred embodiment. However, it is readily apparent to one with ordinary skills in the art that various changes and modifications in form, semiconductor material, dielectric material, isolation material, oxide material, negative or positive photoresist, material conductivity, i.e. N-type, P-type, and detail may be made without departing from the spirit and scope of the invention. The inventions disclosed herein may be practiced without any element which is not specifically disclosed herein.