Patent Publication Number: US-2004051053-A1

Title: Universal pattern generator with multiplex addressing

Description:
RELATED APPLICATIONS  
     [0001] This application claims priority of Provisional Application Ser. No. 60/382,672 filed May 22, 2002, which is herein incorporated by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] The invention relates generally to ion beam lithography and more particularly to ion beam lithography systems without stencil masks.  
       [0003] As the dimensions of semiconductor devices are scaled down in order to achieve ever higher levels of integration, optical lithography will no longer be sufficient for the needs of the semiconductor industry, e.g. DRAM and microprocessor manufacture. Alternative “nanolithography” techniques will be required to realize minimum feature sizes of 0.1 μm or less. In addition, the next generation lithography technologies must deliver high production throughput with low cost per wafer. Therefore, efforts have been intensified worldwide in recent years to adapt established techniques such as X-ray lithography, extreme ultraviolet lithography (EUVL), electron-beam (e-beam) lithography, and ion projection lithography (IPL), to the manufacture of 0.1 μm-generation complementary metal-oxide-semiconductor (CMOS) technology. Significant challenges exist today for each of these techniques. In particular, there are issues with complicated mask technology.  
       [0004] Conventional ion projection lithography (IPL) systems require many stencil masks for semiconductor circuit processing. An ion source with low energy spread is needed to reduce chromatic aberration. A small beam extracted from the source is accelerated and expanded to form a parallel beam before impinging onto a large area stencil mask which contains many small apertures. The aperture pattern is then projected onto a resist layer on a wafer after the beam is reduced in size and made parallel by an Einzel lens system. Different masks with particular patterns must be used for each layer to be formed on the wafer.  
       [0005] In the conventional IPL setup, the stencil mask is extremely thin, e.g. about 3 μm, to minimize beam scattering inside the aperture channels. Since the beam energy is high, about 10 keV, when it arrives at the mask, both sputtering and mask heating will occur, causing unwanted mask distortion and instability.  
       [0006] An alternative IPL system, the plasma-formed IPL system, eliminates the acceleration stage between the ion source and stencil mask. Instead a much thicker and more stable mask is used as a beam forming electrode, positioned next to the plasma in the ion source. The extracted beam passes through an acceleration and reduction stage onto the resist coated wafer. Because low energy ions, about 30 eV, pass through the mask, heating, scattering, and sputtering are minimized. However, a separate mask is needed for each new feature pattern to be projected onto the wafer.  
       SUMMARY OF THE INVENTION  
       [0007] Accordingly it is an object of the invention to provide an ion projection lithography (IPL) system which has no stencil mask.  
       [0008] It is also an object of the invention to provide an IPL system which can generate a variety of different beam patterns using a single apparatus.  
       [0009] It is another object of the invention to provide an efficient addressing system for the beamlet generator of such an IPL system.  
       [0010] The invention is an addressing system for a maskless micro-ion-beam reduction lithography (MMRL) system which produces feature sizes down to 0.1 μm or less. The MMRL system operates without a stencil mask. The patterns are generated by switching individual beamlets on or off using a universal pattern generator which is positioned as the extraction electrode of the plasma source. Each aperture of the pattern generator is independently controlled to pass a beamlet. The pattern generator is a two electrode blanking system. A multicusp ion source with magnetic filter produces ion beams with low energy spread, as low as 0.6 eV. The low energy plasma ions are selectively passed through the pattern generator by applying suitable voltages to the electrodes to produce the desired pattern. A beam accelerator and reduction column after the pattern generator produces a demagnified pattern on the resist. The MMRL system is described in U.S. patent application Ser. No. 09/289,332, which is herein incorporated by reference.  
       [0011] The invention provides a multiplex addressing system to the individual apertures of the MMRL system to reduce the number of electrical connections. An additional layer of control electrodes is added. All apertures in each row of a first layer are connected to a single row address line. All apertures in each column of a second layer are connected to a single column address line. By using the combination of row and column lines, each aperture can be controlled.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 shows an MMRL system.  
     [0013]FIGS. 2, 3 are sectional and perspective views of a two electrode pattern generator for the MMRL system.  
     [0014]FIG. 4 is a cross-sectional view of the electrodes associated with an aperture of a universal pattern generator having a multiplexed addressing system of the invention.  
     [0015]FIG. 5 shows the row and column connections to an array of beamlet forming apertures in a multiplexed addressing system of the invention.  
     [0016]FIGS. 6A, B provide a general and a layer view of the X-Y multiplexing system of the invention.  
     [0017]FIG. 7 is a graph of transmission currents due to varying switching voltages.  
     [0018]FIGS. 8A, B show the electron beam current for on and off conditions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0019] A maskless micro-ion-beam reduction lithography (MMRL) system  10 , shown in FIG. 1, has an ion source  12  with a pattern generator  14  formed of a pair of electrodes  16 ,  18  positioned to form a multi-beamlet ion beam  20 . The extracted beam  20  passes through an acceleration and reduction column  22 , of length L, formed of a plurality of electrode lenses  24 . Column  22  reduces the micro-beamlet pattern produced by pattern generator  14  by factors greater than 5 to achieve feature sizes less than 100 nm. The beam from column  22  is incident on a resist layer  26  on a wafer  28  which is mounted on a mechanical stage or support  30 . The wafer  28  with exposed resist layer  26  is processed by conventional techniques.  
     [0020] The MMRL system  10  is made up of the following major components:  
     [0021] A. Ion Source—Multicusp Plasma Generator  
     [0022] As shown in FIG. 1, ions are produced in a plasma generation region  11  of an ion source  12  which may be of conventional design. Plasma is generated by an RF antenna  13  or alternatively by a filament. A linear magnetic filter  15  or a coaxial magnetic filter  17  is used to decrease energy spread of the ions. The plasma ions pass to extraction region  19  of source  12 . Conventional multicusp ion sources are illustrated by U.S. Pat. Nos. 4,793,961; 4,447,732; 5,198,677, which are herein incorporated by reference. U.S. Pat. No. 6,094,012, which is herein incorporated by reference, describes a preferred ion source with a coaxial magnetic filter which has a very low energy spread.  
     [0023] The multicusp plasma generator provides positive ions needed for resist exposure. Normally either hydrogen or helium ions are used for this purpose. The external surface of ion source  12  is surrounded by columns of permanent magnets  21  which form multicusp fields for primary ionizing electron and plasma confinement. The cusp fields are localized near the source wall, leaving a large portion of the source free of magnetic fields. As a result, this type of ion source can generate large volumes of uniform and quiescent plasmas having relatively flat radial density profiles. For example, a 30 cm diameter chamber can be used to form a uniform plasma volume of about 18 cm diameter. Larger uniform plasmas can be generated by using bigger source chambers with well designed permanent magnet configurations.  
     [0024] The plasma of the multicusp source can be produced by either radio-frequency (RF) induction discharge or by dc filament discharge. However for MMRL, an RF driven discharge is preferred since the quartz antenna coil typically used for antenna  13  will not generate impurities and there is no radiation heating of the first electrode of column  22  due to hot tungsten filament cathodes. The discharge plasma will be formed in short pulses, e.g. about 300 ms pulse length, with high or low repetition rates. With a magnetic filter in the source, the axial ion energy spread can be reduced to values below 1 eV. The output current density is high, e.g. greater than 250 mA/cm2, for pulsed operation and the source can produce ion beams of nearly any element.  
     [0025] B. Pattern Generator—Multibeamlet Extraction System  
     [0026] The open end of ion source  12  is enclosed by pattern generator  14  which forms a multibeamlet extraction system. Pattern generator  14  is formed of a spaced pair of electrodes  16 ,  18  and electrostatically controls the passage of each individual beamlet to form a predetermined beamlet pattern to be projected.  
     [0027]FIGS. 2, 3 illustrate a preferred embodiment of a pattern generator—beamlet extractor  14 . First electrode  16  is the plasma or beam forming electrode and is formed of a conductor  31  having a plurality of apertures or channels  32  formed therein. The apertures  32  on the extractor  14  will be arranged to fall within the uniform plasma density region of the source. Second electrode  18  is the extraction or beamlet switching electrode and is formed of an insulator  33  having a plurality of apertures or channels  34  formed therein. Each channel  34  contains an annular conductor  35  which is electrically connected by electrical connection  36  to a programmable voltage source  37  which can apply different voltages to each of the annular conductors  35 . Conductor  31  is also connected to voltage source  37  or to a separate source. Electrodes  16 ,  18  are separated by an insulator  38 . Channels  32 ,  34  are aligned with each other and extend through insulator  38 . Conductor  31 , insulator  38 , and insulator  33  have thickness of L1, L2, L3 respectively. Typical values are L1=20 μm, L2=5 μm, and L3=15 μm, for a total thickness of about 40 μm which is much thicker than the thickness of a typical stencil mask. The diameter of the channels  32 ,  34  through the pattern generator  14  is d1, typically about 1 μm.  
     [0028] In operation, the first electrode is biased negatively, about 30 V, with respect to the ion source chamber wall. A very thin plasma sheath is formed parallel to the first electrode surface. Positive ions in the plasma will fall through the sheath and impinge perpendicular to the electrode with an energy of about 30 eV. Ions will enter the apertures of the first electrode forming multiple beamlets. With such low impact energies, sputtering of the electrode will not occur. In addition, the heating power generated by ions on the electrode is extremely small and will not produce any instability of the extraction system. Because of low incoming energy, ion scattering inside the aperture channels is minimized. The ions will be absorbed on the channel surfaces rather than forming aberrated beams as they leave the apertures.  
     [0029] In the second electrode, if the annular conductors surrounding each aperture channel are also biased at the same potential as the first electrode, then ions will leave the apertures with an energy of about 30 eV. However, if the annular conductors of the second electrode are biased positively with respect to the first electrode, then the flow of ions to the aperture exit will be impeded by the electrostatic field. If this bias voltage is high enough, then the beam output will essentially become zero, i.e. the beam is turned off. Since the voltage on each annular conductor of the second electrode can be independently controlled, each individual beamlet can independently be turned on and off. Thus any desired beamlet pattern can be produced by the pattern generator, and the pattern can easily be switched to a different pattern.  
     [0030] In this multibeamlet extraction system, circular apertures will typically be employed. There will be many apertures, e.g. each with a diameter of about 1 μm and a separation less than 100 nm. These circular patterns will be projected onto the resist on the wafer with a reduction factor of typically 20. The final image size of each beamlet will then be 50 nm with separation less than 5 nm. The material between the image dots will be made so small that they will disappear during the etching process.  
     [0031] C. Acceleration and Beam Reduction Column  
     [0032] The micro-ion-beams leave the apertures of the extractor  14  with an energy of about 30 eV. They will be further accelerated and focussed by a simple all electrostatic acceleration and reduction column (lens system)  22  which is made up of a plurality of electrodes  24 . The final parallel beam can be reduced to different sizes according to the particular lens design. The total length of one accelerator/reduction column is only about 65 cm, and other designs may be even shorter, e.g. about 35 cm. The beam reduction system can be designed with or without beam crossover.  
     [0033] A portion of the acceleration and reduction column  22  may be made up of an Einzel lens system which includes a pair of split electrodes. The two Einzel electrodes can be used to steer the beamlets by applying suitable voltages. This feature is important for circuit stitching purposes when the field of exposure is smaller than the chip size. By applying different voltages on the segments of the split electrodes, one can steer or scan the beam very fast, as fast as several cm in tens of nanoseconds, in the x or y direction.  
     [0034] D. Multiplex Addressing System  
     [0035] In the universal pattern generator, each aperture of the pattern generator is independently controlled to pass a beamlet. A wire to each control electrode is provided. However, as the number of apertures increases, e.g. an M×N array, MN wires are needed, creating a difficult fabrication problem.  
     [0036] Using a multiplex addressing approach, an M×N array only requires M+N wires (instead of MN wires). An additional layer of control electrodes is added, separated from the first layer by an insulator. All apertures in each row of the first layer are connected to a single row address line. All apertures in each column of the second layer are connected to a single column address line. By using the combination of row and column address lines, each aperture can be controlled. The electrodes of the second layer can be split electrodes for beamlet steering.  
     [0037] The electrode structure for multiplex addressing is shown in FIG. 4. The different conductive layers or electrodes (E 1 ), (E 2 ), (E 3 ) formed of conductors  40  separated by insulators  42  are used. The first electrode  44  is the plasma or beam forming electrode, similar to electrode  31  in FIGS. 2, 3. The single switching electrode  35  of FIGS. 2, 3 is replaced by a pair of control electrodes  46 ,  48 . The first control electrode  46  may be connected to the row address line and the second control electrode  48  may be connected to the column address line. Aperture  45  is formed through electrodes E 1 , E 2 , E 3 , through which an ion beam is extracted.  
     [0038] As shown in FIG. 5, an array  50  of universal mask extraction apertures  52 , each with an electrode structure as shown in FIG. 4, is connected to a plurality (e.g. 7) of row address lines  54  (X 1  . . . X 7 ) and column address lines  56  (Y 1  . . . Y 7 ).  
     [0039] Different writing schemes can be used with the MMRL technique. The entire patternable surface can be filled with switchable apertures. But since each switching element requires an electrical connection, the number of connectors would be 10 12  for a 10 6 ×10 6  aperture arrangement. A more realistic scheme is to combine the switching with either beam or mechanical scanning of the wafer.  
     [0040] There is another way of reducing the number of connections to the pattern generator. By adding another layer to the pattern generator, it is possible to perform simple X-Y addressing via multiplexing as illustrated in FIGS. 4, 5 and in FIGS. 6A, B. FIG. 6A shows a sequence of pulses being applied to (three) row address lines  54 ; pulses can similarly be applied to column address lines  56 . FIG. 6B shows an exploded view of the layer structure for a multiplexed addressing system. Conductor layer  50 , containing a plurality of apertures  51 , is the plasma or beam forming electrode (corresponding to electrode  44  in FIG. 4). The next layer is an insulator layer  52 , which also includes a plurality of apertures  51 . Next is the row (X) addressing layer  54 , which is formed of an insulator and also includes a plurality of apertures  51 . Each aperture  51  on layer  54  includes an electrode structure similar to electrode  46  in FIG. 4, with all electrodes in a row connected to a row address line  55 . The next layer is an insulator layer  56 , which also includes a plurality of apertures  51 . Finally is the column (Y) addressing layer  58 , which is formed of an insulator and also includes a plurality of apertures  51 . Each aperture  51  on layer  58  includes an electrode structure similar to electrode  48  in FIG. 4, with all electrodes in a column connected to a column address line  59 . The layers  50 ,  52 ,  54 ,  56 ,  58  are assembled together with all apertures  51  aligned so that ion beamlets can be extracted. Each beamlet is addressed by a combination of row and column.  
     [0041] In this case, for an array of 10 6 ×10 6  apertures, only 2×10 6  connections would be required. Either X or Y voltages can be used to turn the beam off. The bias voltage required to turn the beam off is 1-3 V more positive with respect to the source potential. The only time the beam is on is when X and Y are below the source potential as shown in FIG. 7. Although it seems that the first switching electrode is more effective in switching the beam off, the multiplexing method can also be used.  
     [0042] The same setup can be used for electron beam switching. The polarity of the power supplies was reversed to extract electrons. Source operation and discharge conditions remain the same with argon used as the working gas. However, other source gases can also be used selectively. FIG. 8 shows the electron beam current for the on and off conditions. Under the same conditions, the electron beam current is higher than the ion beam current.  
     [0043] The MMRL system uses a pattern generator which electrostatically produces and manipulates, i.e. switches on and off, a plurality of micro-ion beamlets which are coupled to a beam reduction and acceleration column. A compact addressing scheme uses multiplexed row and column lines to control each of the beamlets. Beam demagnification factors of up to 50 or more can be achieved with simple all-electrostatic accelerator columns. The system can provide economic and high throughput processing.  
     [0044] Thus the invention provides method and apparatus for ion beam projection lithography which could be used in semiconductor manufacturing with minimum feature sizes of 100 nm or less. Multicusp ion sources with magnetic filters produce uniform plasma volumes larger than 20 cm in diameter. By employing a patterned beamlet switching system, in which each beamlet is individually controlled, as the extractor for the ion source, a beam with a desired feature pattern is produced without requiring a separate mask for each pattern. The beam with selected pattern is then passed through a compact all electrostatic column to demagnify the feature pattern to a desired level.  
     [0045] Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.