GB2171530A - Method for image enhancement in positive photoresist by vapor diffusion image reversal - Google Patents

Abstract

An image reversal method for forming a negative image on a surface using positive photoresist material includes the first step of coating the positive photoresist material on the surface, and soft baking the photoresist or the like to form a uniform layer on the surface. The photoresist layer is then exposed by actinic radiation through a mask or the like in image forming fashion, the exposed portions reacting to form a carboxylic acid or other acid which is soluble in aqueous base solutions. The photoresist is then exposed to a treatment with a vapour of chemical base (e.g. of an amine; tetramethylammonium hydroxide, triethanolamine or ammonia) which renders the acid insoluble to alkali solutions and relatively insensitive to further light exposure. The vapor treatment is followed by a flood exposure of the entire surface to render soluble the photoresist portions which were originally unexposed by the mask. The photoresist is then developed by immersion in an alkali developer to remove the flood exposed portions. The remaining photoresist areas comprise the mask exposed portions in relief from the surface and defining the negative image of the original mask exposure.

Description

SPECIFICATION Method for image enhancement in positive photoresist by vapor image reversal In the semiconductor industry the fabrication of integrated circuits involves extensive use of photoresist processes to define preselected circuit images of predetermined dimensions on a surface of a substrate. This is accomplished by photolithography, in which a pattern corresponding to the circuit design is imaged on the surface of a photosensitive layer by means of an appropriately designed mask. A subsequent developing step then defines the desired openings in the photosensitive resist, so that the uncovered areas of the surface of the substrate may undergo processing, while the remaining areas are shielded from processing.

Initially, negative image masks and negative photoresist materials were favored in chip fabrication. However, the inherent limitations of the negative photoresist materials proved to be insurmountable. These drawbacks include limited resolution, the frequent use of negative image, bright field masks, and solvents that create environmental and regulatory difficulties.

Thus, positive photoresist processes replaced the negative processes, using positive image masks having generally darker fields, and positive photoresist materials which become soluble in aqueous base solutions when exposed to light. The positive photoresist materials generally are formulated from alkali-soluble novolak resins or the like which have been combined with sensitizers such as o-quinone diazide and suspended in commercially available solvents, such as ethyl cellusolve acetate. Absorption of light energy between 300nm and 450nm results in a loss of nitrogen, forming a highly reactive intermediate which rearranges into a ketene compound.

After a short time (in the picosecond range), the ketene reacts with any available water to form indene carboxylic acid. This desired end product is soluble in an aqueous base while the unexposed and unreacted quinone diazide is relatively insoluble. Immersion in an alkaline developer removes the exposed areas and creates the positive image of the mask.

In many of the commercially available positive photoresist materials, the only significant difference is the nature of the monomeric or polymeric structures with which the o-quinone diazide is. combined. However, it is significant to note that each of the commercially available products has been thoroughly researched and documented, so that important properties, such as shelf life, reaction rate vs. light exposure, density, viscosity, etc., are well known.

The definition of these properties permits the accurate and faithful reiteration of the photoresist processes, a primary requirement of integrated circuit manufacturing.

In recent years there has been a renewed interest in negative image processes, to overcome limitations in positive photoresist imaging and processing techniques and increase adhesion, reduce standing wave effects, improve resist contrast and the resulting resolution, increase implant protection properties, and the like. For example, United States patent No. 4,104,070 describes a process for making a negative photoresist image, based on a positive photoresist material. In this process the positive photoresist material described above is modified with the addition of an imidazole compound, such as 1-hydroxyethyl-2-alkylinidazoline. The imidazole compound acts to decarboxylate the reaction product, the remaining indene comprising a dissolution inhibitor which is more hydrophobic than the original resist.The exposed areas thus become insoluble, while subsequent flood exposure and development steps removes the originally unexposed areas.

This process has a significant drawback in that the imidazole compound is highly unstable. The imidazole forms a suspension, rather than a solution, with the positive photoresist, resulting in difficulties in maintaining a uniform dispersal of the material throughout the resist.

Moreover, imidazoline reacts chemically with the quinone diazide, affecting the photosensitive properties. Thus the positive photoresist becomes difficult to handle and unpredictable in its reactivity. These limitations render the process entirely unsuitable for a manufacturing process.

The present invention generally comprises a unique process for forming a negative image using a negative image mask and positive photoresist material. A salient feature of the process is that it employs a novel post-exposure image reversal step to achieve the negative image, without resorting to any modification of the positive photoresist material. The process also provides enhancement of the image through control of the geometry of the image defining portions of the photoresist.

The image reversal method for forming a negative image on a surface using positive photoresist material includes the first step of coating a positive photoresist material on the surface, the photoresist containing a sensitizer such as o-quinone diazide which forms an acid upon exposure to actinic radiation. The resist is processed to form a uniform layer by drying, soft baking, or other techniques known in the art. The photoresist layer is then exposed by actinic radiation through a mask or the like in image forming fashion, the exposed portions reacting to form the acid, such as indene carboxylic acid, which is soluble in aqueous base solutions. A significant process step is the exposure of the photoresist to a heated amine vapor treatment which neutralizes the acid in the reaction product, such as by decarboxylating the indene carboxylic acid.The remaining indene or the like comprises a strong dissolution inhibitor. The amine vapor treatment also renders the light-exposed areas relatively insensitive to further light exposure.

The vapor treatment is followed by a flood exposure of the entire surface to render soluble the photoresist portions which were originally unexposed by the mask. The photoresist is then developed in an alkali developer to remove the flood exposed portions. The remaining photoresist areas comprise the maskexposed portions in relief from the surface and defining the negative image of the original mask exposure. Thus the present invention takes advantage of the benefits of darkfield mask imaging while also making use of the generally superior qualities of positive photoresist materials.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a functional block diagram of the process for forming photolithographic negative images using positive photoresist compounds.

Figures 2-6 are a sequential series of elevations of a substrate, showing various stages in the process delineated in Figure 1.

Figure 7 is a simplified representation of a high intensity imagewise exposure through a narrow mask opening.

Figure 8 is a simplified representation of a low intensity imagewise exposure through a narrow mask opening.

Figure 9 is a block diagram representation of the ability of the process of the present invention to control image and non-image area contrast and slope geometry through variation of the process parameters.

The present invention generally comprises an image reversal process for creating negative images on the surface of a substrate using positive-acting photoresist materials in a photolithography process. With reference to the block diagram of Figure 1 and the sequential sectional views of Figures 2-6, the preparatory step is to select a suitable substrate 11, such as silicon dioxide, aluminum, or the like, and prepare thereon a clean, generally planar surface 12.

Step 1 A commercially available positive photoresist material containing a sensitizer which forms an acid upon actinic radiation exposure, such as o-quinone diazide or the equivalent, is applied to the surface 12 by any coating process known in the prior art. Such materials may include, but are not limited to commercial brands such as Shipley 1300 or 1400 series, Kodak 800 series, Hunt 200 series, etc., photoresists.

Step 2 The photoresist material is formed into a layer 13 by solvent evolution from the photoresist. This may be accomplished by air drying at ambient temperature, or by baking in a soft bake oven at 900C for approximately 30 minutes, or the like. The thickness of the layer 13 may be approximately 5,000A to 50,000A thick, or greater. (Figure 2) Step 3 The photoresist layer 13 is exposed through a mask 14 in standard fashion, using actinic radiation such as, but not limited to, UV light in the 200-500nm range. (Figure 3) Step 4 The photoresist layer 13 is exposed to a vapor which is a chemical base. The vapor may comprise an amine substance, and may derive from tetramethyl ammonium hydroxide, triethanolamine, anhydrous ammonium hydroxide, or the like.The treatment is carried out for a sufficient time and at a sufficient temperature to cause the base vapor to react with the acid reaction product in the exposed areas 16 of the photoresist and render them insoluble and relatively insensitive to further light exposure. This process requires approximately 0.6-200 minutes at a temperature of 10 - 200 C. (Figure 4) The vapor exposure reaction is driven by increasing temperature and increasing pressure. The pressure may range from near vacuum to greater than atmospheric pressure.

Step 5 The photoresist layer 13 is then flood exposed over the entire surface, using uniform actinic radiation in a wide range of exposure intensity. All previously masked areas 17 are exposed, and react with the exposure to become soluble in developer solvents appropriate for standard development of the photoresist. (Figure 5) Step 6 The photoresist layer is developed in standard positive photoresist fashion, using standard developer solutions, followed by rinsing and drying. The resulting structure comprises the portions 16 of the photoresist layer 13 adhering to the surface 12 in those areas where mask exposure was first obtained, with the remaining portions 17 completely dissolved and the surface 12 exposed in these areas. The portions 16 thus comprise the negative of the image provided by the mask 14, formed in relief on the surface of the substrate.

It should be noted that the heated amine vapor exposure step may be carried out in an evacuated, heated oven, such as the Star 2001/IR model produced by Imtec Products, Inc., Sunnyvale, California. However, other processing techniques may also be used, operating at standard pressure or higher in combination with appropriate temperatures, as is known to those individuals having ordinary skill in the art.

A significant advantage of the process described above is that a negative image is formed using an unmodified positive photoresist. Thus, in the frequent situations in which a negative image mask provides a darker field than a positive image mask, the former may be used to help minimize reflections and diffractions which degrade the geometry and dimensions of the final image.

Moreover, the process of the present invention provides heretofore unrealizable control of the contrast, geometry, and dimensional accuracy and resolution of the image features. This advancement of the photomicrolithography art is due to the fact that the intensity and total energy of the projected actinic radiation in the initial image exposure step is related directly to the definition of the upper portion of the image features, while the intensity and total energy of the actinic radiation used in the flood exposure step is related directly to the slope of the sidewalls and the base width of the image features. For example, with regard to Figure 7, a representative image exposure is made through a one micron slit in an opaque mask 14 to expose the positive photoresist material 17.Due to the proximate nature of the wavelength of the exposing light to the width of the slit, there occur significant diffraction effects and standing wave patterns in the photoresist portion beneath the opening 21. During exposure to image projecting light at standard resist sensitivity wavelengths, the primary diffraction effect spreads the incoming light and irrdiates a swath which widens with increasing depth. Indeed, the side lobes 22 which diverge laterally from the boresight of the opening 21 are significantly exposed, although not as fully exposed as the photoresist within the boresight. The boresight portion is generally fully exposed throughout the resist thickness.

Increasing or decreasing the image projecting exposure from the nominal exposure, which faithfully replicates the mask feature, creates dimensional increases or decreases, respectively, In the upper portion of the image 23. Decreasing the normal exposure does not irradiate fully the entire width of the boresight.

Image projecting exposure energies may fully irradiate an image to the full depth of the resist film, while the side lobe portions 24 are only partially exposed, as shown in Figure 8.

Also, attenuation and depth of focus effects act to cause a negative exposure gradient with increasing depth to the resistsurface interface. Thus the intensity and total energy of the initial exposure can define volumes 22 or 24 in which partial exposure has taken place.

Indeed, a continuum of exposure variation beneath the mask opening may be defined and controlled.

It is significant to note that in the partially exposed portions of the photoresist some of the quinone diazide or similar inhibitor has been reacted to form indene carboxylic acid or other acid, while a significant amount of the inhibitor remains unreacted. The vapor reversal treatment will fix the reaction product, but the subsequent flood exposure will then render soluble at least some of the remaining inhibitor. As a result, the marginally exposed portions 22 and 24 may be either substantially dissolved during development, or remain substantially intact, depending upon the degree of flood exposure energy delivered to the photoresist and the amount of inhibitor that remains thereafter.The efficacy of the flood exposure process, which is easily controlled, interacting with the continuum of exposure variation in the resist, permits the selective and accurate removal or retention of virtually any portion of the exposed areas. Therefore the flood exposure step determines the image wall slope angle, profile, and image base dimension.

Indeed, careful control of the process parameters, including vapor reversal process temperature, vapor concentration, pressure and time, as well as flood exposure energy, can determine whether the portions 22 or 24 will remain in the developed image, either fully or partially retained. These portions generally define not only the sloping sidewalls and lower outer portions (the base) of the image features, but also the edge portions of the upper surface of the features. These physical characteristics are directly related to image contrast and controlled by bake and initial exposure conditions, vapor exposure conditions, flood exposure, and development.Due to the fact that these process parameters can be controlled accurately and repeatably, the process of the present invention yields enhanced images in which the geometry of the image feature may be tailored to the specific use in integrated circuit production. For example, a one micron image opening in a mask may be replicated with a slightly shrunken geometry, yielding a image line width of 0.5 micron or less. This effect comprises a significant image enhancement.

The determination of these geometrical properties through variations in the process of the present invention are depicted in greatly simplified form in Figure 9. When the more intense image exposure (Figure 7) is followed by a relatively intense flood exposure, the side portions 22 are substantially eliminated, and the fully exposed boresight yields an image feature profile 27 having a planar top surface with well-defined edges and generally vertical sidewalls. Indeed, the sidewalls may taper inwardly with increasing flood exposure. If the intense image exposure is followed by a relatively low flood exposure, the resulting image feature profile 29 retains the planar top surface and good edge definition, with sidewalls sloping outwardly to a substantially widened base.

When a low image exposure (Figure 8) is followed by a strong flood exposure, the resulting image feature profile 26 is defined by substantial removal of the portions 24. The structure 26 thus comprises generally a flat, narrow upper portion and base portion. A low image exposure followed by a low flood exposure yields a final structure profile 28 in which the portions 24 are substantially intact. The feature 28 includes a narrow upper extent and sloping base portion.

It should be noted that the simplified representations of Figure 9 comprise extreme cases, and that there is a continuum of variation available in the exposure processes. Furthermore, this fine control of the final image is available without bias throughout the range of dimensions common in integrated circuit fabrication, from less than one micron to more than ten microns. The extreme cases should be compared with a nominal imagewise exposure and a nominal flood exposure which yields an image feature profile 30 having vertical side walls, a generally horizontal upper surface, and good definition of the upper edges.

The dimension of this feature replicates its corresponding mask feature size.

Example A thermal oxide layer 5000 A thick was formed on a silicon substrate by techniques known in the prior art. A positive photoresist material (Shipley 1470) was coated onto the oxide surface using an MTI spin coater to form a layer 10,000 A thick. The substrate was then soft-baked in a convection aven at 90 C for 30 minutes.

The substrate then underwent image-wise exposure in a TRE-800 exposure device, using a test pattern mask having 0.75100. micron features. The exposed resist coated substrate then underwent exposure to a heated amine vapor, produced by one of the substances named in Step 4 above, in a heated vacuum oven (Star 2001/IR). The vapor exposure was carried out at 900C for 105 minutes. The sub strate was then exposed to a UV flood expo sure of approximately 500mj/cm2, using a source of broadband, UV radiation. The sub strate was then developed in an aqueous alkali solution (Shipley MF-314) and rinsed in a MTI spray/stream device. A standard hard bake then completed the process.

It may be noted that after the substrate is placed in the Star 2001/IR oven and before the vapor exposure begins, the oven may be programmed to vacuum bake the resist film and gain the known advantages of post-expo sure baking.

After this processing, all 2 micron lines and spaces were perfectly resolved. In addition, all image lines to 0.8 microns and spaces to 0.7 microns were clearly resolved. Thus it may be appreciated that the process of the present invention provides these benefits: 1) Improved resolution capabilities of existing equipment, which prior to this invention were limited to 1.25 microns at the focus setting in the example; 2) The ability to produce a bias-free image throughout the photomicrolithographic range of 10 microns down through submicron features; 3) Reduction of standing wave effect on images; ; 4) The ability to realize the benefits of reverse polarity masks, such as but not limited to greater critical dimension control due to reduction in uncontrolled reflections from underlying surfaces, suitablility for liftoff processing of a chemically vapor deposited, physically vapor deposited, plated, or in the like manner deposited film, greater ability to detect underlying alignment marks, etc.; 5) The ability to control feature definition through control of the size, shape, and slope of the image features, 6) the ability to produce an enhanced image through contrast enhancement.

Claims (16)

1. A method for making a negative photoresist image on a substrate, consisting essentially of the steps of: applying to the substrate a layer of positive working photoresist material containing a sensitizer which forms an acid reaction product upon exposure to actinic radiation, imagewise exposing the photoresist layer with actinic radiation, exposing the substrate to a base-containing material which neutralizes the acid of the reaction product and renders the exposed photoresist areas relatively insoluble in aqueous alkaline solutions and relatively insensitive to further actinic radiation exposure, flood exposing the photoresist layer with actinic radiation to expose all areas not initially exposed or fully exposed in the image expo sure, and developing the image with an alkali developer to generally retain the image exposed areas and remove the flood exposed areas of the photoresist.

2. The method of claim 1, wherein the step of exposing the substrate to a base-containing material comprises exposing the substrate to a base-containing vapor.

3. The method of claim 2, wherein said base-containing vapor comprisea an amine vapor.

4. The method of claim 3, wherein said ex posure to amine vapor is carried out in a tem perature range of approximately 10 C-200 C.

5. The method of claim 3, wherein the ex posure to amine vapor is carried out for a period of approximately 0.6 minutes to 200 minutes at, above, or below atmospheric pres sure.

6. The method of claim 1, wherein the in tensity of the flood exposure is approximately 10-20,000mj/cm2 at peak sensitivity wavelength.

7. The method of claim 6, wherein increased flood exposure intensity generally provides increased slope control, profile control, and selectively of wall angle for the image.

8. The method of claim 6, wherein increased flood exposure intensity generally provides increased geometrical definition of the lower portions of the image adjacent to the substrate.

9. The method of claim 1, wherein the intensity of the imagewise exposure range is approximately 10-3000mj/cm2 at peak resist sensitivity wavelength.

10. The method of claim 9, wherein increased imagewise exposure intensity generally provides increased geometrical dimension and definition of the upper portion of the photoresist image.

11. The method of claim 1, further including the step of post-exposure baking the photoresist after imagewise exposure and before the base-containing material exposure step.

12. The method of claim 6, wherein decreased flood exposure intensity generally provides a broader base portion and sloping sidewalls for the image features.

13. The method of claim 9, wherein decreased imagewise exposure intensity provides generally smaller upper portions of the features of the image.

14. A method for making a negative photoresist image on a substrate, using an unmodified positive acting photoresist material, consisting essentially of the steps of: applying to the substrate a layer of positive working photoresist material which includes an o-quinone diazide sensitizer or resist sensitizer counterpart, imagewise exposing the photoresist layer with actinic radiation to break down the exposed o-quinone diazide and form indene carboxylic acid or a counterpart, exposing the substrate to an amine vapor to decarboxylate the indene carboxylic acid and render the exposed photoresist areas partially insoluble in aqueous alkaline solutions and partially insensitive to further actinic radiation exposure, flood exposing the photoresist layer with actinic radiation to expose all areas not initially exposed or fully exposed in the image exposure, and developing the image with an alkali developer to retain the image exposed or partially areas and remove the flood exposed areas of the photoresist.

15. A method for enhancing an image formed in a positive photoresist layer having a sensitizer which forms a first reaction product upon exposure to actinic radiation, comprising the steps of; imagewise exposure of the photoresist layer by actinic radiation to form a first reaction product concentration gradient therein corresponding to the continuous exposure gradient of said imagewise exposure, reversal treating said photoresist layer by exposing said photoresist layer to a substance which reacts with said first reaction product to form a second reaction product having substantially differing solubility, said second reaction product having a concentration gradient throughout said layer, flood exposing the photoresist layer with actinic radiation to expose substantially all of the remaining unreacted or partially reacted sensitizer, developing the image by dissolving and removing said first reaction product and retaining said second reaction product, the final image comprising areas in the photoresist layer in which said concentration gradient of said second reaction product is relatively high, enhancing the image by controlling the extent of said imagewise exposure, said reversal treatment, and flood exposure steps to determine the concentration gradient of said second reaction product throughout the photoresist layer.

16. A method of making a negative photoresist image on a substrate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.