APPARATUS AND PROCEDURE FOR X-RAY LITHOGRAPHY

Description

BBSCBIZI of the invention entitled: "APPARATUS S FROCEDIMBfTO FOR LITO <ffiAPIA A RAGGI I*a dom di: THE FEEKnr-EUIBt COHPCHATIOH. The present invention relates to Z-ray lithography in general and more particularly to an apparatus for generating X-rays for lithography and to the method of use of such apparatus. The need for more sophisticated lithography techniques used to create cross-stitch prints has led to the development of new types of procedures. Z-ray lithography has been proposed as a solution to the resolution problem. The use of Z-rays allows the use of shorter wavelengths. In fact, the wavelength of X-rays is about 10 Angstroms while that of ultraviolet rays is around 4000 Angstroms. The most frequently used target in the production of Z-rays by lithography was aluminum. The E line of all-unique light is at about 8 Angstroms, which has been found to be a very suitable wavelength for X-ray lithography. Practically, wavelengths between 8 and 14 can be used. Angstrom. However, using an aluminum target has several disadvantages. Aluminum is prone to cracking under stress and has a very low melting point. For these reasons, the amount of power that can be delivered to the system is limited if the aluminum is not to break or melt. Because of these various factors, the exposure time when using an aluminum target is relatively long. For these reasons, the lifetime of an aluminum Z-ray tube operated at high power to allow short exposure times is drastically limited. Conventional X-ray targets used in medical, dental, and similar applications use a high-realignment target such as tungsten. However, tungsten has not previously been used for X-ray lithography because of the short wavelength of radiation it exhibits in the X-ray region; nanometers of 0.5 Angstroms. This wavelength is not absorbed in the emulsion (reel) used on the microcircuit and therefore cannot be used for X-ray lithography. The possibility of using a tungsten target exploiting continuous radiation has been suggested. However, the results were not particularly promising. When it was tested, exposure times were in the order of 5 ♦ 10 hours. With the Mask structure used the broadened spectral distribution gave rise to an exposure contrast between the opaque and transparent regions of nano of 3 : 1. It must be taken into account that in X-ray lithography, in addition to the need for good resolution, the ability to process a large number of circuits in a short time is of primary importance. This requires short exposure times. To be able to use short exposure times, high powers are generally required. To achieve high powers, a source with a high cloud point is required. Another attempt has been made with the use of a rhodium or palladium source with the doping of the resist ion to make it able to absorb the radiation better, which is still relatively short at about 4 Angstroms. While this improves the process trope, I do require the use of a drugged emulsion. In this atheist odo it is not a power factor that gives the shortest exposure time but the response of the doped ion to the radiation. As the need for an improved Z-ray lithography apparatus and a set of tools to use such apparatus rapidly, efficiently, and accurately in order to perform a photolithography process, particularly for use in the fabrication of microcircuits, becomes evident. In its most general aspect, the present invention relates to the design of a target which will generate Z-ray radiation in the V-line in a Z-ray lithography apparatus and to the operation of the apparatus for generating radiation in the K-line. To this end, it has been recognized that although previous investigators have considered the 4-line to be rather inefficient for this purpose, this line can be effective and is so at a wavelength which is readily absorbed by the emulsion, permitting a shorter exposure time than previously possible primarily because of the ability of the target to function at high power levels. Preferably an element with high power density is used, in the illustrated embodiment the element is tungsten, \ In order to efficiently generate the tungsten line it is necessary to drive the X-ray source at a voltage of more than 5Xv, preferably in the region of 20 Kv, THE Furthermore, the window through which the X-rays pass from the target must be kept as thin as possible. The apparatus of the present invention includes a source region having an electron gun directed at the tungsten target. In the conventional technique, electrons fired from the gun and hitting the target generate X-rays, In order to maintain good resolution, the beam diameter is kept thin. Preferably, the target is both rotated and water-cooled to allow for better power dissipation. The source operates under vacuum. The source is spaced from the coated substrate with the oval ion that must be exposed in order to distribute the rediasian on it. A mask with absorbing material is placed between the X-ray source and the substrate. The X-rays are emitted through a window, preferably made of beryllium*, which is transparent to X-ray radiation. The window is necessary because of the vacuum operation of the source. Things already noted the window should be as thin as possible. Although a 100 µm window can often operate with reasonable efficiency, it is preferred that the window be 25 µm thick or less. The remainder of the apparatus can be operated in a vacuum, or in an oil atmosphere, or perhaps in air. The apparatus according to the present invention can be used with conventional CGP apparatus. However, a special development process is necessary to obtain good results. These emulsions, which were initially proposed for use in electron beam lithography, have been found to have some development defects if the process of the present invention is not followed. Therefore, in accordance with the present invention, "after a one minute exposure through a mask with preferably gold absorbers" and having generated Z-rays with a tungsten source, the emulsion is developed first in a strong developing solution and then in a weaker one having a strength not exceeding that required for complete development with overlap between the two developing phases. A further feature of the present invention consists in an improved beryllium window seal. Since beryllium is rather rough, indium gaskets are placed on each side of the window, which in turn contact copper coolers. With this arrangement, in which the window is clamped between two indium gaskets, a good vacuum seal is achieved. The use of tungsten as a target offers numerous advantages. As mentioned above, it allows for operation at higher power densities while reducing the exposure time. Furthermore, contamination of the target by the tungsten filament of the electronic cane is no longer a problem. In general, the arrangement according to the present invention allows operation at high power densities, thus requiring shorter exposure times and allowing the X-ray tube, i.e. the source comprising the electron gun and target, to have a longer life than was possible using an aluminum target. Oh further advantage and feature of the present invention is the availability of the L line of tungsten radiation for use in mask alignment. Since the soft K-line radiation can be easily eliminated by a filter that transmits the L-ray which will not be absorbed by the ion nucleus, this becomes considerably advantageous. In the drawings, FIG. 1 is a schematic elevation view of an X-ray lithography apparatus; FIG. 2 is a similar view of one type of apparatus used to test the principle of the present invention; FIG. 3 illustrates a simpler type of gun design which may be used with the X-ray lithography apparatus of the present invention; FIG. 4 illustrates a cross-sectional view of the beryllium window seals used in the apparatus of FIGS. 1 or 2 according to the present invention; and Fig. 5 is a diagram illustrating the development process of the present invention. Fig. 1 is helpful for a general understanding of the X-ray lithography process. As in any conventional X-ray system, X-rays are generated by striking a target 11 with an electron beam 13. In order to obtain good resolution, it is desirable that the X-ray source be as close as possible to a point source. This imposes a small size for the target collision area. The electron beam and the X-ray target are contained in a source chamber 15 which is under vacuum. This requires that there be a sealed window through which the X-rays generated by the target pass. Therefore, it has been represented as «but window 17·window 17 can be made of any material that is transparent to the wavelength of 10 - generated X-rays· Typically, beryllium is used as the window material. The generated X-rays 19* pass through this window, some of which are absorbed. The X-rays are directed towards a substrate 21 containing the emulsion 22 on it. Interposed between the source and the emulsion is a j The mask generically indicated by 23. The mask will comprise a masking membrane 29 with a figure cut into it in an absorbent material 27 such as gold. The absorbent material absorbs X-rays in the areas where it is present, preventing the corresponding emulsion areas from being exposed to the radiation. After exposure for the required period of time the emulsion is developed and further processes are performed on the substrate. As shown in the figure, absorption into the emulsion causes a welding of cross-links (negative emulsion) or a breaking of chains (positive emulsion) in the emulsion to allow a selective development of the emulsion itself. \ normally 1* is isolated from a net negative λ which means that 1· areas eh· ​​are not exposed to the sun and will not be removed during the BTilding process. Of course, a positive emulsion can also be used. The part of the apparatus in which the ma—a eh era and the substrate are arranged is referred to as the exposure chamber 29* Basa can operate in vacuum» in a helium atmosphere or even in atmospheric air. This last application is available when the distances are short. As previously indicated it was typical in the previous art to make the X-ray target 11 out of aluminum. However, due to the low melting point of aluminum, keeping in mind that the hit area must be small even with a cool target ~ «The water data · rotating poet to expose different areas of the target at successive times» large* ·,power Ò limited · exposure times are long· As an alternative, the use of palladium as a source with a doped emulsion has been tried. This increases the absorption of the emulsion allowing you I - 12 - shorter exposure times and lower power However, this means that a special sanitation must be used in this area, reducing the flexibility of the system. With the wavelengths produced by an aluminum target the thickness of the necessary absorbers is about 0V4 jm. Obviously an absorber that must be too thick cannot be used due to the problem of making the Mask, which makes it difficult to finish small geometries in large thicknesses. Using the commonly used palladium the required thickness is 0.7 jw. Typically the width of an absorber is 1 jm or nano so both of these thicknesses are suitable although the thicker mask required for palladium creates some difficulties. In accordance with the present invention, therefore, the X-ray target is made of tungsten. <<,The electron beam 13 is generated at a sufficiently high voltage, for example 20 V, so as to generate a tungsten line having a wavelength of approximately 7 angstroms. It was previously thought that this line could not generate sufficient radiation. Λ However, tests have shown that this is the case and very good results have been obtained. The thickness of the absorber 27 can be as low as 0.5 mm using tungsten. Figure 2 illustrates the experimental apparatus used to test the present invention. The parts that are the same as in fig. 1 have the same numerical reference. The electron beam 13 is generated by an electron gun 31. In the case of the experimental model this electron gun was an electron gun for welding. The beam 13 hits the target 11 made of tungstan. The target is water cooled with a water inlet line 33 and an outlet line* . 35. Directly below the target is an aperture plate 37 at ground potential. Below aperture plate 37 is an electron deflector 36 consisting of a plate 38a at ground potential and a plate 36b at high voltage to collect any electrons that are reflected from source 11. This plate 38b has high potential so as to deflect these electrons to the plate 3θα· 14 - ! With the above mentioned provision, a conventionally cooled sealed window 17 is provided. The aperture plate 37 and the deflector assembly 38 protect the window 17 from electron beams which could cause overheating. The portion of the stretched enclosure containing the electronic circuit 31 and the area of ​​the apparatus above the window 17 is maintained under vacuum with a vacuum outlet 39 coupled to a pump. The lower door of the apparatus is kept in a helium atmosphere with a helium inlet line 41 and an outlet line 43 provided for this purpose. The sample to be exposed is held in a conventional manner on a vacuum support 47. Window 17 was about 25 mm thick. As shown in fig. 1 there will be a mask (not a trout) directed above the sample. Table 1 provides some performance data calculated from experimental work in the published literature and provides a comparison of the performance of the tungsten target of the present invention, the conventional aluminum target, and the palladium target using a A < "A ' V doped emulsion, with chlorine. The other two are! They use a traditional emulsion. In Table 1, for example, the data for the palladium-chlorine system are based on published values. The published data are for 5 Kilowatts and in the table are extrapolated to 10 Kilowatts. With this power the system with the all-wicker or using 1* CCP exile, for a resolution of 0.24us· requires an exposure time of 60 seconds» The tungsten example uses a narrower collision area to gain an advantage in power performance, that is, the area itself is more concentrated. In this case the exposure time is 280 seconds. In example 2 the resolution is held constant at 0.24 pMm. The power for the systems with aluminum and palladium is still limited to 10 Kilowatts, the power in the case of tungsten is now brought to 57 Kilowatts, tungsten being the only material in the group capable of withstanding this power. The results, as far as exposure time is concerned, are that the palladium-chlorine system still has an exposure time of 60 seconds, 1 * allied to 1280 seconds, while the tungsten system now requires a time of only 49 seconds. Example 3 is again one in which the resolution is kept constant. In addition, the distance between the target and the substrate is optimized based on calculations and experiments. Therefore, instead of the conventional distance of 50 cs, a reduced distance of 15 cs is seen. Because of this reduction in distance, the size of the area must be correspondingly smaller. With a small area size, aluminum and palladium systems must operate at reduced power. Thus the aluminum system is shown operating at 1.7 Kilowatts and the palladium system at 1.6 Kilowatts. The tungsten system also operates at a reduced power of 9.2 Kilowatts. With these parameters the exposure time for aluminum is 205 seconds, for palladium-chlorine 35 seconds and for tungsten 37 seconds. Finally, example 4 is one where the resolution is set to 0.5. This value is chosen because it is thought that in the current state of the art the above resolutions are not necessary. Furthermore, the optimal distance Dfc15 approx. is used. Since the size of the area is now larger, each of the systems can be operated at higher power. With these parameters the exposure times for the tungsten and palladium-chlorine systems are 17 seconds for both and 39 seconds for aluminum. The palladium and tungsten systems are at least an order of magnitude lower. However, as noted above, the palladium system is much less versatile and requires the use of a doped emulsion. As regards the universal use of the various systems, the last column of the table is instructive. This shows, under the same conditions as in the example, exposure times when using a traditional emulsion such as Kodak 747. As illustrated, the exposure time for tungsten ether increases to 60 seconds while for aluminum the increase is to 300 seconds and, using a dopant-free palladium source the time increases to 600 seconds. TABLE I COMPARISON OF THEA I SYSTEM OF LITHOGRAPHY AND ESSAYS Σ. ssapio w Al Pd PARAMETERS CdP OOP CL CONSTANTS 1.POWER (Kw) 10 10 10 POWER, D, d SYS0ELUZI0NE(^m)O.O6 0.24 0.24 T3CP0 OF EXPOSURE SIZICNB (sec.) 280 280 60 D * 50 ci 2.POTHfZÀ (KW) 57 10 10 P, d, D, S HISOLUTION^ua ) 0.24 0.24 0.24 EXPOSURE TOPO (sec.) 49 280 60 D * 50 cs 3.P0TBIZA (KW) 9.2 1.7 1.6 P, d, D, s RESOLUTION^!·) 0.24 0.24 .r C\l . or EXPOSURE TIME (sec.) 37 205 35 D « 15 cm 4. POWER (KW) 20.0 3.75 3*4 P, d, D, S RESOLUTION (^m)0.5 0.5 0.5 EXPOSURE TIME (sec.) 17 93 17 D = 15 cm 747 EMULSION 60 300 600 p - RESOLUTION D b SUPPORT - DISTANCE FROM SAMPLE d - MASK - DISTANCE FROM SAMPLE b DIAMETER OF COLLISION AREA. The use of tungsten also allows for a simpler gun design since tungsten contamination is not a problem with the tungsten target. Such a design is illustrated by Figure 3j. As shown, the target 11 is then mounted on a shaft 51 which permits rotation. The shaft contains appropriate passages 54 and 56 for cooling water so it can be cooled at the same time. The electron gun 53 is particularly simple and allows the positioning of the window 17 closer than previously possible* As shown the gun comprises a ring cathode 57 from which electrons are released and strike the target 11. The electron deflector 59 is quite simple since it acts close to the cathode* Since window 17 is closer it can be made thinner and still withstand a pressure of about 1 atmosphere* Of course, by making the beryllium window thinner it will absorb less radians making the H-line radiation more efficient. Above 100 pM there will be a significant loss of efficiency. Preferably the window will have a thickness no greater than 25 ^ua. In accordance with another feature of the invention as shown in FIG. 4, the beryllium window 171, beryllium being a rather rough substance, is held between two indium seals 65. The indium is relatively soft and will therefore fill the gaps. On each side of the indium seals are water-cooled copper portions 67 to carry heat away from the beryllium window 17. Regarding the design of the gun it should be noted that with an aluminum target it was necessary to ensure that the electron gun was not in sight or had a direct line of sight to the aluminum target. This is to avoid the possibility of tungsten being deposited on the aluminum, rendering it inoperable. With the tungsten target of the present invention, this problem no longer exists, as any tungsten deposited will be the actual element from which the target is made. Although tungsten is the preferred material, any element capable of generating the 1 line can be used as a target. Materials with high power density are preferred. Table II below lists the wavelengths in Angstroms of the R lines for possible alkyne elements. It must be kept in mind that when talking about targets made with these elements, the use of their alloys and compounds must also be evaluated. Using a tungsten source and the K-line, the thickness of the absorbers on the mask should be about 0.5 fm. This is only slightly more than that required when using an aluminum source and just a little less than that required for a palladium source. With the other elements given in table M, the * thickness will obviously depend on the wavelength, the thickness being smaller for longer wavelengths. TABLE II X-RAY SOURCE OF THE MELaCHfTO LINE HORDE LENGTH OF THE M LINE TO BIGA CKDA LENGTH or A Yb 8.14 Lu 7.84 Hf 7.54 Ta 7.25 V 6*98 Be 6.53 Os 4.49 Ir 6.26 Pt 6.05 Au 5.85 Although it is preferable to actually install the beryllium window, other listed ones can also be used* Generally, a low atonic copper is required. Other options include lithium and boron, although both are difficult to work with. Again, the general criterion is that the material passes the wavelength to be used. It should also be noted that doping the emulsion with e.g. bromine is also possible if one wishes to increase the speed of the process. As mentioned above, conventional CCS emulsions made by HEAD CHBDGAL can be used with the apparatus of the present invention. However, it has been found that a more extensive development process is required to obtain good results. This can be better understood by examining the dif· 5. In this figure the developed thickness in percent has been plotted as a function of the developer concentration. The developed thickness is a quantity that is directly proportional to the incident dose of X-rays. The developer concentration shown is that of the HBC developer versus the ethanol solvent. It can be seen that with a concentration of less than 3 : 3 there is incomplete development. In general it has been seen that using a ratio of around 5:3 for full development causes some problems. Thus, in accordance with a further feature of the present invention, after exposure to X-rays the exposed emulsion is first developed with a high concentration of developer, for example 5 : 1.8 after which the emulsion is fully developed in a 5 : 2.7 solution. The development is performed with the spray technique using - 24 - a traditional nozzle spray with an overlap between the two phases. Figure 4 also shows that with a developed thickness greater than 80Jt imperfections and defects may be revealed. Thus it is preferred that 1"80ϋί be the upper limit of the developed thickness although it is possible to go further 1*8036. Using a developed thickness of 50°C the emulsion will be re-sprayed by spraying with the 5 : 1.8 solution first for 5 seconds; then spraying with both the 5 : 1.8 and 5 : 2.7 solutions (overlapping) for 10 seconds; and then with the 5 : 2.7 solution for another 5 seconds. Subsequent washing is optional. As noted above the use of the tungsten source also allows the utilization of the L line radiation which is at 1.476 1 for alignment purposes. Λ TO This L-line radiation is not capable of impressing the emulsion but can be used in an alignment system. To use the alignment system, a filter of, say, 25 ^m aluminum is placed directly under the window 17 shown in figures 1, 2, or 3. The source is then activated Λ

Claims

1. with the filter that excludes the soft radiation of the l1 line but lets the hard radiation of the L line pass. This radiation has no effect on the emulsion but can be detected with suitable fluorescence detectors. Finally, although it is the radiation of the l1 line that mainly affects the emulsion, there will be a further contribution from the radiation of the continuum.

1. X-ray lithography apparatus consisting of: a) a source chamber; b) an electron gun arranged in said chamber; c) a target capable of generating the radiation of the X-ray line arranged in said chamber so that it can be struck by the electrons emerging from said gun; d) a window transparent to the radiation generated by said target through which said radiation can pass;ee) means for causing effective radiation of the II line from said apparatus.

2. The apparatus of claim 1 wherein said target is selected from a group comprising tungsten, tungsten alloys and tungsten compounds so as to generate the tungsten M line.

3. The apparatus of claim 2 wherein said means e) includes means for operating said electron beam at a voltage greater than 5 kV.

4. The apparatus of claim 3 wherein said voltage is in the range of 20 kV.

5. The apparatus of claim 1 wherein said means e) further includes said window being made of beryllium and having a thickness not exceeding 100 µm.

6. The apparatus of claim 5 wherein said thickness is less than 25 µm.

7. The apparatus of claim 6 wherein said means e) further includes said window being made of beryllium and having a thickness not exceeding 100 µm. said voltage is of the magnitude of 25 KV·8· Apparatus according to claim 5 wherein said beryllium window is drawn between two indium seals· *; 9. A method of performing X-ray lithography comprising the steps of: a) coating the substrate on which the lithography is to be performed with an emulsion; b) arranging a Z-ray apparatus having a target capable of generating the M-line of the Z-ray radiation so as to strike said coated substrate; c) operating said Z-ray source so as to generate the M-line radiation; d) arranging a mask between said source and said substrate; e) exposing said substrate to said radiation.

10. A method according to claim 9 wherein said target is selected from a group containing tungsten.

11. The method of claim 10 wherein said step of operating comprises operating said Z-ray beam at a voltage above 5 KV.

12. The method of claim 11 wherein said step of operating comprises operating at a voltage in the vicinity of 20 KV.

13. The method of claim 10 wherein said step of operating comprises operating at a voltage in the vicinity of 20 KV.

12. The method of claim 13 wherein said step of operating comprises using as a window in said Z-ray apparatus a window made of aluminum foil A, S and having a thickness of not more than 100 µm.

14. The method of claim 13 wherein said step of preparing comprises preparing a window having a thickness of not more than 25 µm.

15. The method of claim 9 wherein said emulsion is a CQP emulsion and further comprises the steps of first developing the emulsion in a concentrated high-temperature developer solution for a short period of time and then developing said emulsion in a developer concentration slightly higher than that required to achieve complete development until all of such emulsion is developed.

16. The method of claim 15 wherein said developing steps comprise: a) developing for a first period of time said highly concentrated developer solution; b) develop for a second period of time withboth said solutions being highly concentrated and with a concentration slightly higher than that required to achieve complete development; and c) developing for a third period only with said developer using a concentration slightly higher than that required to achieve complete development.

17. The method of claim 16 wherein all said development steps are carried out by spray.

18. The method of claim 16 wherein said emulsion is exposed so as to obtain approximately 1 / 4 of developed thickness and wherein said first development step is carried out with a concentration of 5:1.8 for 5 seconds; said second step is performed for 10 seconds using a concentration of 5:1,βe 5:2,7 and wherein said third step is conducted for 5 seconds the concentration being the ratio of the NEC developer to the solvent anethanol.

19. The method of claim 9 further comprising: the step of placing a filter between said source and saidsubstrate before exposing said filter being such as to eliminate the radiation of the M line but to transmit the radiation of the L line so that alignment of the mask and the substrate can be performed before exposure.

20. Method according to claim 9 comprising the further step of doping the emulsion with bromine.

21. Improved method of developing an emulsion A - which has been exposed to X-radiation comprising the steps of first developing said emulsion in a highly concentrated solution for a short period of time and then developing said emulsion in a lower concentration until development is complete.

22. Method according to claim 21 wherein said development steps comprise: a) developing for a first period of time with said highly concentrated solution; b) developing for a second period of time both with said highly concentrated developing solution and with said developer at a concentrationslightly higher than that required to obtain complete development; and c) developing for a third period only with said concentration slightly higher than that required to obtain complete development.

23. The method according to claim 22 wherein all said development steps are carried out by spray technique.

24. The method according to claim 22 wherein said emulsion is exposed so as to obtain approximately 1 / 50 of developed thickness and wherein said first development step is carried out with a concentration of 5 : 1.8 for 5 seconds; said second phase is conducted for 10 seconds using concentrations 5:1.8 and 5:2.7 and wherein said third phase is conducted for 5 seconds the concentration being the ratio of the KEC developer to the ethanol solvent % * Ί - 31 - 25 Improved method, in an X-ray lithography apparatus comprising a window made of beryllium through which the radiation passes, for sealing said beryllium consisting of arranging saidberyllium between two indium salts. p. PEHKIK ELUSE COEP.p. ITALPAfBiT