Abstract:
An exposure method and apparatus for use in exposing a photoresist on a semiconductor wafer do not employ an aperture for shaping the exposure light. The exposure apparatus includes a light source unit, a reflecting mirror unit having a micro mirror array (MMA) and a control unit that controls the MMA, and a pattern transfer unit that transfers the pattern of a photomask onto the photoresist. The angles of inclination of the respective mirrors of the MMA are adjusted to reflect incident light in a manner that shapes the incident light. Accordingly, it is possible to form a pattern having the highest degree of resolution and optimum depth of focus (DOF) in the shortest amount of processing time.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the photolithography process used in the manufacturing of semiconductor devices. More particularly, the present invention relates to a method of and an apparatus for exposing a semiconductor wafer to transcribe a pattern onto the wafer. 
   2. Description of the Related Art 
   Photolithography is one of the major processes of manufacturing a semiconductor device as it is indispensable to the overall process of forming a particular circuit on a wafer. In general, photolithography includes a series of individual processes such as a process of coating a substrate with a photoresist, an exposure process in which the photoresist is exposed to light of a given wavelength, and a process of developing the exposed photoresist. In the exposure process, light is directed through a mask having a particular pattern to transcribe the pattern onto the wafer. 
   The higher the degree of integration desired for a semiconductor device, the finer are the patterns that must be produced on the wafer. Thus, photolithography is becoming increasingly critical in the overall process of manufacturing semiconductor devices. In particular, the demand for providing more highly integrated semiconductor devices has triggered the need to develop an exposure apparatus and method that can provide a pattern having high degree of resolution and optimum depth of focus (DOF). In general, off axis illumination (OAI) is used in photolithography to secure a high degree of resolution and optimal DOF. 
     FIG. 1  illustrates a conventional projection-type of exposure apparatus. The projection-type exposure apparatus includes a light source unit  110 , an optical lens unit  120 , an aperture  130 , and a pattern transfer unit  140 . The pattern transfer unit  140  includes a plurality of lenses and thus may be regarded as being a part of the optical lens unit  120 . However, in this disclosure, the pattern transfer unit  140  will be referred to separately to be differentiate it from the optical lens unit  120 , in consideration of its dedicated function of transferring the pattern of the photomask  144  thereof onto a wafer. 
   The light source unit  110  includes a light source  112  and oval mirrors  114  that encompass the light source  112 . Light having a particular wavelength is emitted from the light source  112 . The light emitted from the light source  112  in various directions is reflected in one direction by the oval mirrors  114 . 
   The optical lens unit  120  includes a collecting lens  122  and a fly&#39;s eye lens  124 . The collecting lens  122  focuses light emitted from the light source  112  into parallel light rays, and the fly&#39;s eye lens  124  focuses the parallel rays of light such that they will be uniformly incident on a target object. 
   The light passes through the fly&#39;s eye lens  124  and travels toward the aperture  130  before the light reaches a condensing lens  142  of the pattern transfer unit  140 . The aperture  130  has two regions: an open area through which the light passes and a blocking area that blocks the light. The open area of the aperture  130  has a specific shape. In OAI, a vertical component, i.e., the 0 th  order, of incident light is removed using the specific shape of the aperture  130 . Therefore, light passing through the aperture  130  is incident on the photomask  144  via the condensing lens  142  at a predetermined oblique angle rather than at a right angle. 
     FIG. 2  illustrates various types apertures used in a conventional projection-type of exposure apparatus. In  FIG. 2 , the cross-hatched portions denote the blocking regions of the apertures. The conventional illustrated apertures are a circular aperture, a quadrupole type of aperture, a dipole type of aperture, and an annual aperture. However, other types of apertures are also known. 
   Referring again to  FIG. 1 , the light passing through the aperture  130  is condensed by the condensing lens  142 . The condensed light rays are incident on the photomask  144  that bears a mask pattern. Next, the condensed rays of light passing through the photomask  144  pass through a projecting lens  146  and are finally focused on a semiconductor wafer  150  disposed on a wafer holder  160 . 
   However, the aperture  130  must be tailored to the pattern formed on the photomask  144  in order to obtain a pattern having the highest resolution and optimum DOF using OAI. That is, if the photomask  144  is changed to one whose pattern has a different size, shape, and/or spacing, the aperture  130  must be replaced with one that is matched to the new pattern. 
   In general, 20–30 sheets of photomasks are used to manufacture one integrated semiconductor device, whereas a projection-type of exposure apparatus is equipped with only several apertures. If necessary, an aperture  130  may be detached from the projection-type of exposure apparatus and replaced with a new aperture. 
   Accordingly, a conventional exposure apparatus and method have some disadvantages. First, the shapes of the available apertures are limited. That is, only several types of apertures are available and thus, there is a high probability that none of the available apertures is an optimal match for the pattern of the selected photomask. Accordingly, in most cases, it is difficult to precisely transcribe the pattern of a photomask and form a pattern having the highest resolution and optimum DOF on a wafer using a conventional exposure apparatus and method. 
   Secondly, the conventional exposure apparatus is inconvenient in that the aperture must often be exchanged during the process of manufacturing a semiconductor device. The operation of the exposure apparatus must be temporarily discontinued while the apertures are being exchanged, thereby increasing the total time of the exposure process and consequently lowering the productivity of the manufacturing process. 
   Thirdly, as described above, an aperture has two regions: an open region that allows light to pass through the aperture, and a blocking region that prevents light from penetrating the aperture. In other words, not all the light that is incident on the aperture passes through the aperture to the semiconductor wafer. Accordingly, the conventional exposure process is not marked by energy efficiency meaning that the exposure time must be long to satisfactorily complete an exposure process. Therefore, the overall manufacturing process using the conventional exposure method and apparatus also takes a long time to complete. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is to an exposure method of apparatus which can form a pattern having a high degree of resolution and optimum depth of focus (DOF) on a semiconductor wafer. Another object of the present invention is to provide an exposure method and apparatus which can form a pattern in a relatively short amount of time, i.e., which are characterized by providing a short exposure time. 
   To achieve these objects, an exposure apparatus according to an aspect of the present invention comprises a reflecting mirror unit having a micro mirror array (MMA). The MMA is interposed, along the optical axis of the apparatus, between a light source unit and a pattern transfer unit. The pattern transfer unit includes a photomask bearing a pattern to be transferred to the photoresist. The exposure light from the light source is thus reflected by the MMA through the photomask of the pattern transfer unit. 
   Preferably, the reflecting mirror unit also comprises driving units disposed on the backs of the mirrors of the MMA, respectively, and operative to adjust the angles of inclination of the mirrors. The reflecting mirror unit may also include a control unit for controlling the operations of the driving units. 
   The control unit may include an input section by which information regarding the pattern of a photomask can be input, a processor configured to determine the optimum angles of the respective mirrors of the MMA based on the information input via the input section, and a controller controlling the operations of the driving units based on the determinations made by processor. 
   According to the exposure method of the present invention, the exposure light generated by the light source is reflected along an optical axis through the photomask and towards the photoresist with the micro mirrors. Preferably, the angles of inclination the respective mirrors are established to reflect the incident light at an optimum angle and in an optimum direction, relative to the optical axis, on the basis of information regarding the pattern of the photomask. In this respect, the orientations of the micro mirrors may be adjusted individually, e.g., by the controller of the apparatus. 
   According to still another aspect of the exposure method according to the present invention, information regarding the pattern to be transferred to the photoresist is acquired or otherwise quantified. A desired form for the shape of the exposure light to be directed through the photomask and onto the photoresist is predetermined from the information regarding the pattern of the photomask. The micro mirrors are oriented to shape the exposure light into the desired form. 
   Finally, in all cases the rays of the exposure light are preferably reflected by the MMA onto the photomask at oblique angles relative to the plane of the photomask so as to take advantage of the off-axis illumination technique. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments thereof made with reference to the attached drawings in which: 
       FIG. 1  is a schematic cross-sectional view of a conventional projection-type of exposure apparatus; 
       FIG. 2  is a schematic diagram of various types of apertures that are used in a conventional projection-type of exposure apparatus; 
       FIG. 3  is a cross-sectional view of an exposure apparatus including a micro mirror array according to the present invention; 
       FIG. 4  is a magnified view of a photograph of a portion of the micro mirror array of the exposure apparatus shown in  FIG. 3 ; 
       FIG. 5  is a schematic diagram illustrating the function of the micro mirror array, of the exposure apparatus according to the present invention; 
       FIG. 6  is a flowchart of an exposure method according to the present invention; 
       FIG. 7  is a flowchart of an operation of transferring a pattern onto a wafer in the method illustrated in  FIG. 6 ; and 
       FIG. 8  is a flowchart of an exposure method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully with reference to the accompanying drawings. Also, it should be noted that like reference numerals designate like elements throughout the drawings. 
   Referring first to  FIG. 3 , the projection-type of exposure apparatus according to the present invention includes a light source unit  310 , an optical lens unit  320 , a pattern transfer unit  340 , and a reflecting mirror unit  330  interposed between the optical lens unit  320  and the pattern transfer unit  340  with respect to the optical axis of the apparatus. The reflecting mirror unit  330  reflects light at a desired angle from light source unit  310  to the pattern transfer unit  340 . Similar to the conventional exposure apparatus shown  FIG. 1 , the light source unit  310  includes a light source  312  and oval mirrors  314 , the optical lens unit  320  includes a collecting lens  322  and a fly&#39;s eye lens  324 , and the pattern transfer unit  340  includes a photomask  344  and a projecting lens  346 . 
   The reflecting mirror unit  330  according to the present invention will be explained in greater detail with reference to  FIGS. 3 through 5 . 
   The reflecting mirror unit  330  includes a micro mirror array (MMA)  332 . Driving units  333  are preferably installed on the backs of the mirrors of the MMA  332 , respectively, so that the angles of the respective mirrors can be adjusted. Also, the reflecting mirror unit  330  may further include a control unit  334  that controls the operations of the driving units  333 . The driving units  333  may be controlled using electrostatic or external electrical signals. 
   As best shown in  FIG. 4 , the MMA  332  includes a plurality of micro mirrors, i.e., a plurality of mirrors disposed side-by-side and whose dimensions are each on the order of microns. For example, the MMA comprises mirrors of 50 μm×50 μm arrayed in two orthogonal directions. Accordingly, all of the light incident on the MMA  332  is reflected. Thus, theoretically, the MMA  332  will not cause energy loss. 
   As mentioned above, a respective driving unit  333  is installed on the back surface of each of the mirrors of the MMA  332  so that the angle of each mirror of the MMA  332  can be easily changed to adjust the angle of reflection and hence, the direction of the light. Accordingly, the reflected light can assume various shapes depending on the angles of the respective mirrors. 
     FIG. 5  illustrates the incident light  500  being reflected at different angles by a mirror of the MMA  332 , and the resulting reflected lights  510   a  and  510   b . That is, the same incident light can be reflected at different angles by adjusting the angles of the respective mirrors of the MMA  332  using the driving units  333 . If the angles of the mirrors are changed appropriately, the angle at and direction in which the reflected light propagates is changed. In this way, the reflected light may be provided with the same characteristics as incident light passing through a conventional aperture having a particular shape, but without the accompanying energy loss. 
   That is, according to the present invention, light rays emitted from the light source unit  310  of  FIG. 3  are incident on the MMA  332  and are all reflected onto the condensing lens  342  of the pattern transfer unit  340 . On the contrary, in the conventional exposure apparatus of  FIG. 1 , a portion of the incident light passing through the aperture is absorbed by the blocking region of the aperture, i.e., does not reach the pattern transfer unit  140  of  FIG. 1 . However, when the MMA  332  is used, the reflection angle and direction of incident light is adjusted so that all of the incident light passes through the pattern transfer unit  340 , whereby the energy of all of the incident light is transferred to the photoresist on the semiconductor wafer  350 . 
   As mentioned above, the reflecting mirror unit  330  preferably includes a control unit  334  that controls the operations of the driving units  333 . The control unit  334  may comprise a switching unit or an automated computer system. 
   If an automated computer system is used as the control unit  334 , the control unit  334  may include an input section into which information regarding the pattern of the photomask  344  is input, a processor that determines the optimum angles of the respective mirrors based on the information input to the input section, and a controller that controls the operations of the respective driving units  333  based on the determination made by the processor. Alternatively, the processor may have a memory device that stores data regarding the optimum angles of the respective mirrors for the photomask  344 . In this case, the control unit  334  is capable of receiving information via the input section thereof, automatically detecting internal data, and controlling the operation of the MMA  332  so as to provide the optimum angle(s) for the respective mirrors. 
   According to the present invention, the shape of reflected light may be embodied, in accordance with the pattern of the photomask  344 , using the MMA  332 . For instance, circular, quadrupole, dipole, or annular forms of light can be obtained as in a conventional exposure apparatus. However, the shape of the reflected light that can be produced according to the present invention may be totally different from that which can be produced in the conventional exposure apparatus. In any case, the reflected light has a comparatively high degree of resolution and optimum depth of focus (DOF). 
   An example of a method of determining the optimum shape of the aperture is described in the commonly assigned Korean Patent Application No. 2002-0035173 entitled “Simulation Method and Apparatus for Designing Aperture for Exposure Apparatus, and Recording Medium for Recording the Simulation Method”. The contents of the above Korean patent application are hereby incorporated by reference. Similarly, data regarding the optimum shape of the reflected light in accordance with the present invention can be obtained using the simulation method described in the Korean patent application. The data may be stored in the control unit  334  and used when determining the angles of reflection based on the information input to the control unit  334 . 
   Hereinafter, a process of transferring a pattern onto a semiconductor wafer using the aforementioned exposure apparatus will be described with reference to  FIGS. 3 ,  6 , and  7 . 
   Referring first to  FIGS. 3 and 6 , light emitted from the light source  312  is reflected by the oval mirrors  314  onto the optical lens unit  320 . The incident light passes through the collecting lens  322  and the fly&#39;s eye lens  324  of the optical lens unit  320 , and is then incident on the MMA  332  of the reflecting mirror unit  330  (step  610 ). 
   After step  610 , the light incident on the MMA  332  is reflected onto the pattern transfer unit  340  (step  620 ). The shape of the reflected light is controlled by the control unit  334 . For instance, the control unit  334  transmits signals to the driving units  333  of the respective mirrors of the MMA  332  individually in order to establish the direction and angle of reflection of the light. 
   Subsequently, the light reflected from the MMA  332  passes through the pattern transfer unit  340 . As a result, the pattern of the photomask  344  is transferred to the photoresist on the semiconductor wafer  350  (step  630 ). More specifically, referring to  FIG. 7 , the light illuminated by the MMA  332  is condensed by the condensing lens  342  ( 631 ). Next, the condensed light is incident on the photomask  344  that bears a particular (mask) pattern (step  632 ). The light is incident on the condensing lens  342  at an oblique angle to take advantage of the effect provided for by using off-axis illumination (OAI). The light passing through the photomask  344  is then transmitted onto the photoresist on the semiconductor wafer  350 , via the projecting lens  346  ( 633 ). 
   A more detailed description of the exposure method according to the present invention will now be made with reference to  FIGS. 3 and 8 . 
   First, information on the pattern of the photomask  344  is input into the control unit  334  via the input section thereof (step  810 ). Next, the processor of the control unit  334  compares the input information with existing data stored therein, thereby correlating the pattern of the photomask with an optimum shape for the light that is to illuminate the mask, and determines the optimum angles of the respective mirrors based on the comparison. The controller of the control unit  334  then operates the driving units  333  to position the respective mirrors of the MMA  332  at the optimum angles (step  820 ). Subsequently, light emitted from the light source  310  is incident on the MMA  332 . The MMA  332  reflects the incident light in a particular shape that is best suited to the pattern of the photomask  344 , and the pattern of the photomask  344  is thereby transferred onto the photoresist on the semiconductor wafer  350  (step  830 ). 
   An exposure apparatus and method according to the present invention has many advantages. First, a pattern having the highest degree of resolution and optimum DOF can be formed on a semiconductor wafer by establishing appropriate orientations of the individual mirrors of the MMA. For example, the mirrors can be individually adjusted to set the direction and angle of the reflected light in such a way that the mask pattern is optimally transferred to the photoresist. 
   Secondly, an exposure apparatus according to the present invention does not require an aperture. Thus, there is no need to exchange apertures and stop the operation of the exposure apparatus. Accordingly, the method of the present invention can be conducted more productively than the conventional exposure method. 
   Thirdly, the semiconductor wafer can be illuminated with practically all of the light emitted by the light source. Thus, the intensity of the light that irradiates the photoresist is higher than the conventional method in which some of the light is blocked by an aperture. For this reason, exposure time according to the present invention is shorter than in the conventional exposure method. 
   Lastly, in an exposure method according to the present invention, information regarding the pattern of the photomask is input to a control unit and then an MMA is controlled to reflect incident light at a particular angle and in a particular direction. Accordingly, the exposure method and apparatus of the present invention can be used in photolithography to fabricate a pattern having a higher degree of resolution and DOF than can be achieved using a conventional exposure method and apparatus. 
   Finally, although the present invention has been described above in connection with the preferred embodiments thereof, various changes can be made to the preferred embodiments as will be apparent to those of ordinary in the art. All such changes are thus seen to be within the true spirit and scope of the present invention as defined by the appended claims.