METHOD FOR FORMING A PHOTORESIST PATTERN AND APPARATUS APPLICABLE WITH SAID METHOD

Abstract: With the method pattern elements with predetermined wall profiles and/or lateral shapes differing from the shapes of the respective pattern elements in the used irradiation mask are formed in a photoresist layer. The method comprises a modification of the conventional photo- lithographic process where a substrate supporting the photoresist layer is shifted laterally relative to the mask or the mask image respectively in a continuous mode and/or in steps during exposure. The apparatus comprises means for shifting a substrate relative to a mask into the x- and/or the y-direction or means between the mask and the substrate to shift the path of the beam relative to the mask and controls for said means. The method - especially in connection with the apparatus - allows to form reproducibly photoresist patterns with a great variety of differently formed wall profiles and/or lateral shapes. Using the method photoresist patterns can be flexibly adapted to many applications.

Description: The invention relates to a method for forming a pattern in a photoresist layer having openings with predetermined shapes and to an apparatus for practising said method. Owing to the small structures (order of magnitude 1 µm) currently required in semiconductor technology, photoresist structures are generated usually by high-resolution projection exposure systems. The positive resists which are used for their favourable characteristics are highly transparent to the radiation (λ = 435 nm) generally employed for projection irradiation, i.e., the photoresist in the irradiated regions is disintegrated relatively uniformly across its entire thickness. After the irradiated photoresist layers have been developed, which - to effectively control this step - is done by diluted basic developer solutions, the holes generated in the photoresist have vertical walls. When the photoresist patterns thus produced are used as etch masks in a subsequent dry etch step, the shape of the holes in the photoresist pattern is accurately transferred to the etched material, i.e., the holes in the etched material have vertical walls, too. When in still further process steps a layer is vapor-deposited on the etched structure, problems arise because the thickness of the deposited layer is non-uniform and too thin, particularly in the region of the hole walls and the hole edges, which leads to poor step coverage and tearing. A typical example of such phenomena are metal lines with weak spots along the edges of contact holes in isolating layers. The described defects are responsible for an increased failure rate of components containing such structures. Measures are known that are taken to prevent holes with vertical walls in etched structures. It is known, for example, to use multi-step etching, wherein by stepwise expansion of the etch mask, steps are etched into the hole walls (contour etching). In practice, this leads to inclined hole walls preventing the above-described difficulties during the deposition of a layer. However, this known process is time-consuming and complicated and thus expensive and not readily reproducible. In another known process, the walls of the holes in the photoresist are inclined by heating the photoresist structure after development, which softens the photoresist, causing it to flow. As a result, the photoresist edges are rounded and the hole walls are inclined, meaning that the dimensions of the hole at the bottom, where it opens towards the material to be etched, are smaller than at the top. However, this process has the disadvantage that the edge angles of the walls depend on the hole diameter and the packing density in the vicinity of the hole. The inclination of the hole walls in the photoresist pattern is transferred to the holes resulting in the etched material. Although being much simpler than the previously described known process, the latter process it too inaccurate for forming holes with very small dimensions that must be reproducible within narrow tolerances. DE-OS 2 645 081 describes a further process for producing photoresist patterns with inclined hole walls, wherein 1. the UV-light used for irradiation defocusses or decollimates and 2. a small spacing has to be kept between mask and photoresist layer and/or 3. a thick (> 2 µm) photoresist layer is used and 4. a disperse (non-collimated) light source is employed. This process however has the disadvantage that the transferred pattern is poorly defined and not readily reproducible. In the EP application 0 227 851 another process for forming a photoresist pattern having holes with inclined walls is described, wherein regions of a layer of a positive photoresist are image-irradiated in a projection exposure system and additionally at least the photoresist regions adjoining the image-irradiated layer regions are irradiated, using a radiation at which the photoresist is highly transparent, and wherein the irradiated regions are developed by means of a basic developer. The additional irradiation may cover the entire photoresist layer or be limited to the regions adjoining the image-irradiated regions. This process avoids the disadvantages described above however in its preferred embodiment the thickness of the photoresist layer is substantially reduced which can only be avoided by using an additional mask which makes the process more complicated and more expensive. A further method for influencing the wall profile of openings in photoresist layers is described in the article "A novel method for submicron structurization using optical projection lithography" by K. Ismail, published in Microelectronic Engineering 1 (1983), page 295. In this method called the "double exposure technique" the photoresist is exposed through a mask where the exposure time is relatively short such that the incident energy would not be high enough for developing the gate area. The mask is then shifted for example by using a micrometer screw and exposed again for the same period where the shift is such that the two exposures overlap. The overlapping region is doubly exposed so that by developing the resist only this region is completely developed. The exposed opening has inclined walls. However the angle range achievable of the wall inclination is restricted especially when the ratio of the thickness of the resist layer to the dimension of the mask opening is large. All the methods described above have in common that it is rather difficult if not impossible to shape the topography of the inclined walls in a predetermined way. This option is very desirable. For example, if the photoresist pattern is used as a mask for doping a substrate by ion implantation the doping profile in the substrate can be formed by shaping the wall profile in the photoresist pattern accordingly. Sometimes it is of interest to tailor the openings in the photoresist layer with respect to their wall profile and/or with respect to their lateral shape, independently of the shape of the respective openings in the used irradiation mask. (In this context lateral shape of an opening means the shape of the projection of the opening onto a surface aligned in parallel to the surface of the photoresist layer.) This applies especially to methods with which the reduction of the size of an opening in a mask can be achieved to a size below the resolution limit of the used light. The above cited article published in Microelectronics describes the formation of an opening in a photoresist layer being narrower than the respective opening in the mask. However the width of the opening is coupled to the inclination of its wall in the sense that with decreasing width also the inclination angle decreases. Another method for achieving narrow openings in photoresist layers uses a bake cycle causing the unexposed photoresist to flow into the developed opening whereby its size is reduced. However this method is not reproducible enough for the application in the VLSI technology. It is therefore one object of the invention to provide a method for forming a photoresist pattern with openings having inclined walls where the inclination angle can be varied in a wide range independently of the photoresist thickness. It is another object of the invention to provide a method for shaping the topography of the inclined walls in a predetermined way. It is a third object of the invention to provide a method to form reproducibly predetermined wall angles and/or predetermined wall topographies. It is still a further object of the invention to provide a method for forming in the same exposure step openings with predetermined wall profiles in the photoresist pattern and pattern elements in the photoresist having a lateral shape being changed with regard to the respective mask pattern elements. It is still another object of the invention to provide a method for forming a photoresist pattern with elements having lateral shapes differing from the shapes of the respective elements in the mask pattern as projected on the photoresist surface. It is still a further object of the invention to provide a method being applicable in a manufacturing environment. It is also an object of the invention to provide an apparatus applicable for executing the above methods. These objects are achieved by methods as defined in claim 1 and by an apparatus as claimed in claim 8. To shift the substrate relative to the mask or the mask projection includes the cases of shifting the mask with regard to the substrate and vice versa and shifting the mask projection with regard to the substrate. "Shifting in steps" in this context means a fast shift lasting short in relation to the exposure time. It is also possible to switch off the light during the step. Inspite of its simplicity the inventive method as defined in claim 1 is able to produce wall profiles of a great variety and - if necessary - complexity without being dependent from or being influential to the other parameters of the photolithographic process. Also the application of the inventive method is not dependent from a special mask pattern or from the thickness or the characteristics of the photoresist, i.e., the method is applicable to positive as well as negative photoresists and also in connection with the reversal process using a positive resist to form a negative pattern. The inventive method allows to form non-symmetric wall profiles. For example, when inclined walls are needed only on one side of the opening by using the inventive method it is possible to form substantially vertical walls on the other sides of the opening. By this remarkable savings of space are achieved. The method as defined in claim 1 allows also to modify the shape of the pattern elements in the photoresist with regard to the respective mask pattern elements in a predetermined way. These changes can include an enlargement or a shrinkage of the pattern element in all or in one or several privileged directions but also a complete new shaping of its contours. The walls of these openings in the photoresist pattern are essentially vertical provided that the exposure dose reaching the exposed area is sufficient at least for full exposure everywhere in said whole area. With the method defined in claim 1 it is also possible to combine the tailoring of the wall inclination and/or the wall topography of the openings in the photoresist layer with the modification of their lateral shape with regard to the shape of the respective mask openings. In most applications the modifications of the shape of the pattern elements is in no way coupled to the change of the wall profile or the wall topography. These modifications can include an enlargement or a shrinkage of the pattern element but also a complete new shaping of its contours. The different results are achieved by controlling the shifts of the substrate relative to the mask in an appropriate and predetermined way and in adaptation to the thickness of the photoresist, its sensitivity at the wavelength of the applied light and the intensity of the exposure. It is advantageous to use the inventive methods not only in connection with projection printing but also in connection with the application of x-rays and e-beams. These applications are especially therefore valuable because so far no other method is known to form inclined walls by using x-ray or e-beam lithography. Known means being able to shift the projection of the mask relative to the substrate, like a plain parallel glass plate being suspended between mask and substrate in such a way that it can be tilted in a controlled manner can be used to perform the inventive method. However more preferred is the concept to achieve the relative shift in the inventive apparatus by shifting the substrate and having the mask in a fixed position. Such a shift can be performed by applying the means used in conventional exposure tools or wafer steppers respectively to align a substrate relative to a mask. Practically conventional exposure tools are used for the exposure during the execution of the inventive method. By additionally using said means for shifting the substrate the inventive method becomes even more efficient. Beyond that it is no problem to achieve accuracies of ± .1 µm. The shifting of the substrate into the x- and/or the y-direction can be accomplished by piezo translators or linear motors. Other advantageous embodiments of the inventive methods are disclosed in the sub-claims. The invention will be described in detail below with reference to examples illustrated in the accompanying drawings, in which Fig. 1 is a schematic cross-sectional detail representation of a projection mask and an opening with inclined walls in a photoresist layer formed using the inventive method on a substrate, where - plotted against the exposure time - the respective lateral shifts of the substrate relative to the mask are indicated, Fig. 2A is a representation as in Fig. 1, where the opening formed has a smaller dimension at the lower surface of this photoresist than the corresponding opening in the mask, Fig. 2B is a representation as in Fig. 1, where the opening formed differs from the one shown in Fig. 2A by an additional enlargement of the dimension of the opening at the upper surface of the photoresist, Fig. 3 is a representation as in Fig. 1, where the walls are shaped in a special way in order to use the pattern in the photoresist layer as ion implantation mask, Fig. 4 is a representation as in Fig. 1 with the difference that also part of a dielectric layer underneath the photoresist layer are shown, where the opening formed is used to form a via hole in the dielectric layer to a conductor being very close to adjacent conductors, Fig. 5 is a schematic cross-sectional detail representation of a mask having two openings and a structured photoresist layer including a spacer having unsymmetrical sides with the dimension at the lower surface of the photoresist spacer being smaller than the respective dimension of the mask, Fig. 6 illustrates in a schematic representation the shifting of an x-y-table relative to a mask and optical means into the x- and/ or y-direction where the table supports a substrate coated with a photoresist layer to be exposed according to the invention and Fig. 7 is a schematic representation of one embodiment of the inventive apparatus where the shifting into the x- and/or y-direction is program controlled. The inventive method is enclosed in the conventional photolithographic process. In this process a substrate is coated with a photoresist layer in a predetermined thickness usually by spin coating. This photoresist layer is - optionally after and/or prior to a baking cycle - patternwise irradiated where when a negative photoresist or a positive resist in reverse mode is used the irradiated resist becomes insoluble and when a positive photoresist is used the irradiated resist becomes soluble. In the context of this application only the irradiation through a mask having opaque and transparent regions is considered. The following description only deals with irradiation using uv-light where when small structures (order of magnitude 1 µm) are to be generated high-resolution projection exposure systems are used. (During projection exposure the pattern in the projection mask is usually transferred to the photoresist layer reduced by a factor of 5 which is not considered in the Figures 1 to 5.) It is however emphasized that the inventive method is equally suitable for the irradiation with x-rays and electrons, where the mask pattern is transferred to the photoresist layer in the ratio 1:1. If uv-light is used the natural resolution limit is in the order of 0,5 to 1 µm. It is also important to mention that the radiation is diffracted along the edges of the mask pattern which influences to a certain degree the region exposed to the uv-light. The latent image produced in the photoresist layer by the irradiation is developed using the developer appropriate for the photoresist where the photoresist layer is structured in dependency to the transferred pattern. In the following the invention is described with reference to the Figures 1 to 7. The generated photoresist pattern can serve very different purposes. Among these are the application as etching mask for the selective etching of the underlying material, the application in a lift-off process which is an additive process for forming conductors, and the application as an ion implantation mask for the selective doping of the underlying material. It is the irradiation part of the photolithographic process with which the invention is concerned. Fig. 1 shows in a schematic cross-sectional view a photoresist layer 1 overlying substrate 8 and having been irradiated through mask 2 according to the inventive method. For ease of appreciation mask 2 has only one opening 4 but it should be understood that in practice the mask has multiple openings of different shapes. Also mask 2 does not have - as usual in projection photolithography - five times the size but the same size as the pattern to be transferred into photoresist layer 1. Photoresist layer 1 is shown after development of an opening 3 having inclined walls 5 and 6. To achieve the shown wall profiles of opening 3 the photoresist layer was shifted relatively to the mask in the x-direction according to the diagram shown in which the amplitude of the shift is plotted against exposure time. The exposure cycle goes like this: In the first section the exposure takes place without shifting the substrate relative to the mask. This exposure time is long enough for the full development of the region of the photoresist layer underneath the mask opening. In the second section the substrate is shifted into the +x-direction with the amplitude being larger than the dimension of the mask opening. Subsequently the substrate is shifted back with the light switched off (switching off light is not necessary, if the time for shift back << exposure time) towards the original position, however not completely since in order to achieve the steady slope of wall 6 it is necessary to irradiate additionally the lower part of wall 6 a little bit. This is because the amplitude of the shift in the second section was larger than the dimension of mask opening 4 with the consequence that at the end of the shift into the +x-direction the lower part of wall 6 was shaded. Therefore it is necessary to make in the next section of the irradiation a small shift (indicated in the diagram) into the -x-direction. When the substrate is again in its original position the bottom part of opening 3 and wall 6 are irradiated. It remains now only to repeat the indicated shifts in a mirror-symmetrical way in order to irradiate the region of wall 5. The inclined walls are achieved because the areas of the photoresist being closer to the center of the opening are exposed for longer times than the areas at its periphery. By varying the shift amplitude the inclination angles of walls 5 and 6 can be made steeper or shallower. Naturally it is also possible to generate differently inclined walls 5 and 6 including the case where wall 5 is inclined and wall 6 vertical. While for ease of appreciation in Fig. 1 only shifts in the x-direction are contemplated it is also possible to make the shift in the y-direction or simultaneously into the x- and the y-direction including shifts causing the walls all around to be inclined. It must be emphasized that the function x(t) can be different from the one indicated in Fig. 1 as long as the local dose remains the same to which the different areas of the photoresist surface are exposed. These statements apply accordingly to the description of the embodiments illustrated by Fig. 2A to 5. To produce the opening 3 shown in Fig. 2A in principle the same irradiation procedure has to be followed as described in connection with Fig. 1. Different are only the variation of the amplitude with the exposure time and the absolute value of the amplitude. The consequence is, that an opening is formed having a dimension at the lower surface of the photoresist - i.e. the surface adjacent substrate 8 - being smaller than the dimension of the opening 4 in mask 2. Using this embodiment of the inventive method, where it is intended to form an opening having a smaller dimension than the respective opening in the mask, one is not so free in the determination of the inclination angle because the inclination angle is coupled with the dimension of the opening at the lower surface and to a lesser degree with the dimension of the opening at the upper surface of the photoresist. The smallest dimension of the opening at the lower surface of the photoresist layer for a given shift is indicated in Fig. 2A by the wall profile drawn with solid lines. By increasing the exposure time the dimension of the opening at the bottom surface of the photoresist layer is increased and with it the inclination angle. This is indicated in Fig. 2A by the wall profile and the curve in the diagram drawn with dotted lines. A cross-section of an opening as shown in Fig. 2B is generated by performing the same exposure cycle as applied for generating the opening shown in Fig. 2A and by subsequently superimposing the exposure cycle as shown in Fig. 1 but with a shorter exposure time. The wall profile shown in Fig. 3 is formed by applying the exposure cycle indicated in the respective diagram. Photoresist patterns of the kind displayed in Fig. 3, i.e. with a stepped topography, can e.g. be used as an ion implantation mask for doping the underlying substrate in a predetermined way. Using the photoresist pattern shown in Fig. 3 as implantation mask the formed doping profile in the substrate will be equally stepped. The possibility to shape doping zones reproducibly not only laterally but also vertically in a predetermined way becomes more and more important with the increasing density of integrated circuits. Fig. 4 shows in a cross-sectional view a structure comprising a photoresist layer 1 on a dielectric layer 7 overlying substrate 8. On substrate 8 three conductive lines are applied being parallel and in close vicinity to each other. In order to connect conductive line 9 to a second level metallization a via hole 12 is to be etched into dielectric layer 7. The pattern formed in photoresist layer 1 by means of the inventive method serves as etching mask for producing the via hole. Opening 3 in photoresist layer 1 was formed by performing the exposure cycle indicated in the diagram. In the etch process the via hole 12 having the same wall profile as the opening 3 will be formed in dielectric layer 7. A via hole with such a wall profile serves two purposes. The inclined wall profile in the upper part of via hole 12 guarantees - as explained in detail at the beginning of this specification - a good step coverage at the edge of the via hole when metal is applied by evaporation to it. If the wall inclination would already start in the lower part of the via hole the danger that a short circuit occurs between conductive lines 10 and 11 and the evaporated metal cannot be excluded. By shaping the via hole, so that it shows the wall topography as represented in Fig. 4, a good step coverage is guaranteed and the danger of a short circuit is minimized even when the density of the conductive lines is high. Fig. 5 illustrates the formation of a spacer 13 of photoresist on substrate 8. In mask 2 two openings 4 are shown having a dimension of e.g. 1 µm and a distance being about equal to said dimension (it is assumed that the mask pattern is not reduced upon exposure). In using the exposure cycles indicated in the respective diagram the resulting two openings 3 have vertical walls 5, inclined walls 6 and a dimension of the opening at the lower surface of the photoresist layer being larger than the dimension of the openings 4 in mask 2. Consequently the spacer 13 has a vertical and an inclined side and the dimension of the spacer at the lower surface of the photoresist layer is - in the present example - clearly smaller than 1 µm i.e. below the resolution limit. A spacer of this kind can be included in an implantation mask to form doped regions having an extremely small distance. In addition to tailoring the wall profile and the dimension of the pattern elements transferred into the photoresist layer their shape can be changed by using the inventive method. For example a mask element having a circular shape may be transformed into a straight or curved line in the photoresist layer by performing the appropriate shifts of the substrate. This variation of the inventive method is valuable when, for example, a special conductive pattern is needed only once or in a few cases so that the generation of a suitable irradiation mask would be very uneconomical. The above description of various embodiments of the inventive method shows its flexibility and effectiveness in forming very differently shaped photoresist patterns and its usefulness and adaptation capability for many applications. Fig. 6 is basically a schematic representation of a conventional wafer stepper used for repeatedly transferring a chip mask five times reduced on a wafer coated with a photoresist layer. Such a wafer stepper can - with modifications - advantageously be used for practicing the inventive method. The wafer 18 onto which the chip pattern 19 is to be transferred is supported by an x-y-table 17 being movable into the x- and the y- direction with an accuracy of ± .1 µm. The uv-light 25 passes the mask 22 with the five times enlarged chip pattern 19′ and optical means 24 providing the image reduction and impinges on wafer 18. The wafer stepper is operated in a step and repeat mode which means, that after each exposure the table 18 is moved (naturally this movement has to be extremely accurate) for a chip length, then the exposure is repeated and so on until the whole wafer surface is exposed to the irradiation. The means for moving table 18 can be used for shifting the photoresist layer relative to the mask as described in detail above. It is however also possible - if for any reasons individual means for shifting are more appropriate - to install such shifting means additionally in the wafer stepper. In another embodiment that might also be preferred - instead of shifting the table 18 - means are brought into the path of the beam between mask and photoresist layer, said means being able to shift the beam laterally in a controlled way. In practicing the inventive method the shift of the substrate or the beam has to be controlled in a predetermined way. Fig. 7 shows schematically one suitable arrangement to accomplish this for the case where the substrate is shifted. It is preferred - especially in a manufacturing environment - that the shift is computer controlled. In Fig. 7 numeral 30 indicates a computer, numeral 31 a digital analog converter (DAC), numerals 32 and 33 shifting means into the x- or y-direction respectively, numeral 18 the x-y-table and numeral 34 the shutter for switching on and off the light. The software controlled instructions from the computer 30 are transformed in the DAC into machine readable form and then fed into shifting means 32 and 33. These perform the necessary shifts. It depends from the chosen arrangement whether the modification necessary for the adaptation of the pattern generator to practicing the inventive method is only software related or whether the control means (computer and DAC) and the shifting means or only the latter have to be installed additionally. In the following the formation of an opening as shown in Fig. 1 is described in even greater detail. Five samples were processed in the same way. The samples were photoresist coated silicon wafers. The photoresist employed was a positive photoresist marketed under the trade designation S1400-31 by Shipley. (This photoresist consists basically of a novolac resin and a diazonaphtoquinone sensitizer.) The thickness of the resist layer was about 2 µm. The samples were in each case heated prior to and after irradiation - prior to irradiation for 20 minutes to 85°C and after irradiation for 10 minutes to 95°C. For the irradiation the projection exposure system distributed by GCA under the type name DSW6300 was used at a wave length of 436 nm and an energy of 110 mj/cm². The projection exposure system has a numeric aperture of 0.3 and the mask image is transferred to the photoresist layer five times reduced. The mask used for the irradiation had multiple square pattern elements with an edge length of 5 µm, i.e. if the pattern elements are transferred to the photoresist layer without practicing the present invention the transferred pattern elements have an edge length of 1 µm. The resist coated wafers were brought into the projection exposure system and the center of the wafer was positioned under the mask pattern. Subsequently the wafer was exposed where during part of the exposure the wafer was shifted according to the diagram shown in Fig. 1. For the shifts the x- and y-stage drivers of the projection exposure system were used. The provided modification of the exposure system for this experiment was exclusively software related. During the first section of the exposure cycle (this section took 1.3 seconds) the substrate was not shifted. In the second section that had the same duration the substrate was shifted 1.25 µm; then the light was switched off and the substrate was shifted back until the shift compared to the original position was .25 µm. Subsequently said light was switched on again and the substrate was shifted back into the original position within about .7 seconds. Subsequently the shifts performed from the second section on were repeated but in the opposite direction. The development of the irradiated and postbaked samples was effected at 25°C using a 0.1 n KOH solution for 3 minutes in a tank. From 10 openings of each of the five samples the dimensions were measured. It was found that the dimension of the openings at the upper surface of the photoresist layer was 3.5 µm and the dimension of the opening at the lower surface was 1 µm. The reproducibility of these dimensions was ± 60 nm (3 σ) which was mainly caused by the variations of the development process.

Claims: Method for forming a photoresist pattern having openings with predetermined wall profiles and/or for changing the shape of the pattern elements in the photoresist with regard to the respective mask pattern elements where from a first position in which the mask and the substrate supporting the photoresist are aligned to each other the substrate is shifted laterally in continuous mode or in at least two steps relative to the irradiation mask or the mask projection during exposure with the substrate being displaced at least during part of the exposure. Method according to claim 1 wherein said wall profiles and/or said shapes are controlled by the amplitude of the shift, the shift as a function of time and the exposure intensity. Method according to any one of claims 1 and 2 for forming an opening in a layer of a positive resist having defined sloped walls at least in one direction comprising the steps a) after positioning the photoresist layer with regard to the mask in a first position exposing the photoresist to the light with a dose sufficient for full development, b) exposing the layer with the same or a reduced dose through the mask while performing the shift of the substrate either continuously or in several steps by a predetermined amplitude in a predetermined direction into a second position, c) shifting the mask into the opposite direction until the first position is reached, and d) optionally repeat step b) and optionally c) at least into one other direction, where the dimension of the opening at the upper surface of the photoresist results from the dimension of the respective mask opening and the amplitudes and the directions of the shifts. Method according to any one of claims 1 and 2 for forming a spacer in a layer of a positive resist having in one direction a substantially vertical wall and in the opposite direction a sloped wall comprising the steps a) after positioning the photoresist layer with regard to the mask in a first position with the mask having at least two adjacent openings exposing the layer through the mask with a dose sufficient for full resist development, b) optionally shifting the substrate for a predetermined amplitude opposite to the direction where the sloped wall is to be formed either extremely fast or with the light switched off and subsequently continue the exposure with the same dose, c) continue the exposure with the same dose while shifting the substrate in the direction defined for optional step b), where the dimensions of the openings at the upper surface of the photoresist are equal to the dimensions of the respective mask openings and the amplitude of the shift or optionally the shifts, where the dimensions of the openings at the lower surface of the photoresist are equal to the dimensions of the respective mask openings and optionally the amplitude of the shift in step b) and where the formed spacer of the photoresist between the at least two adjacent openings in the photoresist is optionally narrower than the distance between said two mask openings. Method according to any one of claims 1 and 2 for forming an opening in a layer of a positive photoresist having defined sloped walls at least in two opposing directions comprising the steps a) after positioning the photoresist layer with regard to the mask in a first position exposing said layer through the mask while performing a shift of the substrate with a predetermined amplitude in a predetermined direction into a second position, b) returning the substrate to the first position, and c) repeat step a) but into the opposite direction, where the minimum dimension of the opening at the upper surface of the photoresist equals to the dimension of the respective opening in the mask plus the amount of the two shifts and where the exposure time determines the dimension of the opening at the lower surface of the photoresist and the slope of the wall with the minimum dimension being the dimension of the respective opening in the mask minus the amount of the two shifts provided the exposure dose in the area covering the opening at the lower surface is just sufficient for full resist development and where said dimension is increased by increasing the exposure dose. Method according to any one of claims 1 to 5 wherein the shifts as a function of time satisfy a linear equation or represent oscillations with uniform or variable amplitudes and uniform or variable frequencies with the shifts being directed in all or in privileged directions. Method according to any one of claims 1 to 6 wherein the photoresist is exposed by x-rays or E-beams or with UV-light using projection printing. Apparatus for carrying out the method of any one of claims 1 to 7 comprising the means contained in an exposure tool for aligning a substrate to a mask and programmed controls for shifting said substrate through said means in a continuous mode or in steps into the x- and/or the y-direction.

Inventors: BOETTIGER, ULRICH CHRISTOPH, HAFNER, BERNHARD, Böttiger, Ulrich Christoph
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: G03F7
IPC codes: H01L21, G03F7, G03F9