Abstract:
An organic polymer film patterning method includes the steps of: defining a resist film on a selected area of a substrate; depositing an organic polymer film over the substrate by a plasma CVD process so that the resist film is covered with part of the organic polymer film; and removing the resist film along with the part of the organic polymer film that has covered the resist film.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a method of patterning an organic polymer film and a method for fabricating a semiconductor device by utilizing the patterning method.  
           [0002]    Recently, millimeter wave bands, or ultrahigh frequency bands of 30 GHz or more, are frequency resources that should hopefully be developed for a broad variety of applications including multi-media mobile telecommunications units and radio frequency LANs. To ensure a sufficient gain for an FET in ultrahigh frequency bands like these, the gate electrode of the FET should have its length and resistance both reduced. On top of that, a parasitic capacitance associated with the gate electrode should also be reduced. A T-shaped (or mushroom-shaped) gate electrode would be a best choice so far among various measures for reducing the gate length and gate resistance. So a T-gate electrode is adopted more and more often for that purpose.  
           [0003]    A typical known semiconductor device including a T-gate electrode will now be described with reference to FIG. 3, which schematically illustrates a cross-sectional structure for a known semiconductor device of this type.  
           [0004]    As shown in FIG. 3, the device includes semi-insulating GaAs substrate  51 , epitaxial layer  52  deposited on the substrate  51  and a T-gate electrode  56  formed on the epitaxial layer  52 . The bottom of the T-gate electrode  56  makes a Schottky contact with the surface of the epitaxial layer  52 . A pair of ohmic electrodes  55  is further formed on, and makes an ohmic contact with, the epitaxial layer  52 . For the other parts that are not covered with the T-gate electrode  56  or ohmic electrodes  55 , the upper surface of the epitaxial layer  52  is covered with an interlevel dielectric film  54  of SiO 2 . Also, to electrically isolate the illustrated device from adjacent ones, the epitaxial layer  52  is surrounded with an isolation region  53 .  
           [0005]    A method for fabricating the known semiconductor device will be described next with reference to FIGS. 4A through 4G, which illustrate respective process steps for fabricating the device shown in FIG. 3.  
           [0006]    First, as shown in FIG. 4A, an epitaxial layer  52  is deposited on a semi-insulating GaAs substrate  51  by an MOCVD or MBE process, and an isolation region  53  is defined by implanting dopant ions into a selected region of the substrate.  
           [0007]    Next, as shown in FIG. 4B, an insulating film  54  of SiO 2  is deposited on the epitaxial layer  52  by a CVD process, and then a photoresist  55 , having an opening  55   a  with a width of 0.1 μm, is defined on the insulating film  54 .  
           [0008]    Thereafter, as shown in FIG. 4C, an opening  54   a  is formed in the insulating film  54  by dry-etching the film  54  anisotropically using the photoresist  55  as a mask, and then the photoresist  55  is removed as shown in FIG. 4D.  
           [0009]    Subsequently, as shown in FIG. 4E, parts of the insulating film  54 , where ohmic electrodes will be formed, are removed to form another pair of openings, and then ohmic electrodes  56  are formed on the particular areas of the epitaxial layer  52  that are exposed inside the openings. Next, another photoresist  57  with an opening  57   a  is defined as shown in FIG. 4F.  
           [0010]    Finally, a metal film (not shown) is deposited over the photoresist  57  so that the opening  57   a  is filled with the metal, and then the photoresist  57  is removed along with the excessive metal, thereby forming a T-gate electrode  58  as shown in FIG. 4G.  
           [0011]    This device includes the T-gate electrode  58 , and can have a shorter gate length and reduced gate resistance. However, the insulating film  54  is made of SiO 2  with a dielectric constant of about 4.0, so the gate parasitic capacitance is not so small. That is to say, this device has a large fringe capacitance due to the particular shape of the gate electrode  58  and the material of the insulating film  54 .  
           [0012]    To reduce the fringe capacitance of the gate electrode  58 , the insulating film  54  should preferably be made of a material with a lower dielectric constant (which will be herein called a “low-κ material”). An organic polymer may be used as an alternative material for the insulating film  54 , because an organic polymer has a dielectric constant lower than that of SiO 2 . However, if the above process is performed as it is just by substituting an organic polymer for SiO 2 , then it is difficult to form the opening  54   a  at a desired small size.  
           [0013]    In the above process, the opening  54   a  is formed in the insulating film  54  by dry-etching the film  54  anisotropically using the photoresist  55  with the opening  55   a  as a mask as shown in FIG. 4C. Then, the opening  54   a  of the insulating film  54  will usually be greater in width than the counterpart  55   a  of the photoresist  55 . This is also true even when the insulating film  54  is made of an organic polymer. In that case, the width of the resultant opening  54   a  will be no less than about 0.7 μm, for example. That is to say, the opening  54   a  cannot have a width as small as 0.3 μm or less (e.g., 0.1 μm) according to the known process.  
           [0014]    To avoid this problem, the opening  54   a  may be formed by a lift-off technique, not by using the photoresist  55  having the opening  55   a.  But we found that another problem is caused by doing so.  
           [0015]    [0015]FIGS. 5A and 5B are cross-sectional views illustrating the process steps of forming an opening by a lift-off technique. First, a substrate  61  is prepared, and a fine-line resist pattern  62  is defined on the substrate  61  by a photolithographic technique as shown in FIG. 5A. Next, as shown in FIG. 5B, an organic polymer film  63  is deposited over the substrate  61 . However, since an organic polymer is usually liquid, the resist pattern  62  cannot be lifted off as it is. That is to say, even if the resist pattern  62  is lifted off, the film  63  of the liquid organic polymer will planarize itself after that. As a result, no opening can be formed in the organic polymer film  63 . To form an opening in the organic polymer film  63  by a lift-off technique, the liquid organic polymer should be cured by annealing it at 200° C. or more. However, the resist pattern  62  is usually cured or deformed at about 150° C. Accordingly, it is meaningless to cure the liquid organic polymer by annealing it at 200° C. or more.  
         SUMMARY OF THE INVENTION  
         [0016]    It is therefore an object of the present invention to provide a method of patterning an organic polymer film in such a manner that the film can have an opening of a very small width.  
           [0017]    It is another object of the invention to provide a method for fabricating a semiconductor device so that the gate electrode will have a reduced fringe capacitance.  
           [0018]    An inventive organic polymer film patterning method includes the steps of: defining a resist film on a selected area of a substrate; depositing an organic polymer film over the substrate by a plasma CVD process so that the resist film is covered with part of the organic polymer film; and removing the resist film along with the part of the organic polymer film that has covered the resist film.  
           [0019]    In one embodiment of the present invention, the organic polymer film is preferably a low-κ film with a dielectric constant lower than that of SiO 2 .  
           [0020]    In this particular embodiment, the low-κ film may be made of a cyclobutane derivative.  
           [0021]    More specifically, the cyclobutane derivative is preferably benzocyclobutene (BCB).  
           [0022]    In an alternative embodiment, the low-κ film may also be made of a fluoropolymer.  
           [0023]    In still another embodiment, the organic polymer film is preferably deposited within an inert gas ambient.  
           [0024]    In yet another embodiment, a deposition temperature of the organic polymer film is preferably lower than a temperature at which the resist film starts to degrade.  
           [0025]    An inventive method for fabricating a semiconductor device includes the steps of: defining a resist film on a selected area of a substrate; depositing an organic polymer film over the substrate by a plasma CVD process so that the resist film is covered with part of the organic polymer film; removing the resist film along with the part of the organic polymer film that has covered the resist film, thereby forming an opening in the organic polymer film; and forming a gate electrode on the selected area of the substrate that is exposed inside the opening of the organic polymer film.  
           [0026]    In one embodiment of the present invention, the organic polymer film is preferably a low-κ film with a dielectric constant lower than that of SiO 2 .  
           [0027]    In this particular embodiment, the low-κ film may be made of a cyclobutane derivative.  
           [0028]    More specifically, the cyclobutane derivative is preferably benzocyclobutene (BCB).  
           [0029]    In an alternative embodiment, the low-κ film may also be made of a fluoropolymer.  
           [0030]    In still another embodiment, the organic polymer film is preferably deposited within an inert gas ambient.  
           [0031]    In yet another embodiment, a deposition temperature of the organic polymer film is preferably lower than a temperature at which the resist film starts to degrade.  
           [0032]    In yet another embodiment, the opening preferably has a width of 0.3 μm or less and the gate electrode is a T-gate electrode.  
           [0033]    In the present invention, an organic polymer film is deposited by a plasma CVD process. Therefore, unlike the known liquid organic polymer film, a fine-line opening can be formed according to the present invention in the organic polymer film just by removing the resist film.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a cross-sectional view illustrating a semiconductor device formed by a fabrication process according to an embodiment of the present invention.  
         [0035]    [0035]FIGS. 2A through 2K are cross-sectional views illustrating respective process steps for fabricating the device shown in FIG. 1.  
         [0036]    [0036]FIG. 3 is a cross-sectional view illustrating a semiconductor device formed by a known fabrication process.  
         [0037]    [0037]FIGS. 4A through 4G are cross-sectional views illustrating respective process steps for fabricating the device shown in FIG. 3.  
         [0038]    [0038]FIGS. 5A and 5B are cross-sectional views illustrating the process steps of forming an opening by a lift-off technique.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is in no way limited to the following illustrative embodiments.  
         [0040]    [0040]FIG. 1 schematically illustrates a cross-sectional structure for a semiconductor device formed by a fabrication process according to an embodiment of the present invention.  
         [0041]    As shown in FIG. 1, the device includes buffer layer  2 , channel layer  3 , spacer layer  4 , doped layer (not shown)  5 , Schottky layer  6  and cap layer  7  that have been stacked in this order on a semi-insulating GaAs substrate  1 . The thicknesses of the layers  2 ,  3 ,  4 ,  6  and  7  are 1 μm, 20 nm, 5 nm, 30 nm and 100 nm, respectively. These layers  2  through  7  are formed by an epitaxy process, so will be herein called an “epitaxial layer”  12  collectively.  
         [0042]    The buffer layer  2  is made of undoped GaAs and buffers lattice misfit between the epitaxial layer  12  and substrate  1 . The channel layer  3  is made of undoped In 0.2 Ga 0.8 As and provides a channel where carriers move. The spacer layer  4  is made of undoped Al 0.25 Ga 0.75 As. The doped layer  5  is formed by planar doping just one atomic layer of Si ions, which are n-type dopant ions, at a dose of 5×10 12  cm −2 . The Schottky layer  6  is made of undoped Al 0.25 Ga 0.75 As. And the cap layer  7  is made of n + -GaAs.  
         [0043]    A pair of ohmic electrodes  8  exists on the cap layer  7 , while a gate electrode  9  has been formed on the Schottky layer  6 . The other parts of the epitaxial layer  12 , which are not covered with the ohmic electrodes  8  or gate electrode  9 , are covered with an organic polymer film  11  made of benzocyclobutene (BCB). The film  11  is made of an organic polymer with a dielectric constant κ lower than that of SiO 2 , and will be herein called a “low-κ film”.  
         [0044]    As shown in FIG. 1, the gate electrode  9  is formed in T-shape. Accordingly, the gate resistance at the upper part thereof with an increased width is lower than usual, while the gate length at the lower part thereof with a reduced width is shorter than usual. The organic polymer film  11  has an opening so that the gate electrode  9  can make a Schottky contact with the Schottky layer  6  therethrough. To attain the reduced gate length, the opening has a width of 0.3 μm or less (e.g., about 0.1 μm). Those parts of the insulating film located near the gate electrode  9  are made of an organic polymer with a relatively low dielectric constant. Thus, the device shown in FIG. 1 can have a reduced gate parasitic capacitance. That is to say, since the dielectric constant of the organic polymer film  11  is lower than that of the normal insulating film of SiO 2 , a smaller fringe capacitance is formed between the gate electrode  9  and epitaxial layer  12 . Around the outer periphery of the ohmic electrodes  8 , an isolation region  10  has been defined to electrically isolate the illustrated device from adjacent ones.  
         [0045]    A method of patterning an organic polymer film and a method for fabricating a semiconductor device according to this embodiment of the present invention will now be described with reference to FIGS. 2A through 2K. FIGS. 2A through 2K are cross-sectional views illustrating respective process steps for fabricating a semiconductor device according to this embodiment.  
         [0046]    First, as shown in FIG. 2A, buffer, channel, spacer, doped, Schottky and cap layers  2 ,  3 ,  4 ,  5 ,  6  and  7  are deposited in this order on a substrate  1  of semi-insulating GaAs by an MOCVD or MBE process, thereby forming an epitaxial layer  12 . It should be noted that the substrate  1  with the epitaxial layer  12  will sometimes be called a “substrate” in the following description.  
         [0047]    Next, as shown in FIG. 2B, an isolation region  10  is defined by implanting dopant ions into a predetermined region of the substrate. Then, a resist film  13  with a width of 0.1 μm is defined on the epitaxial layer  12  by a lithographic technique. The resulting device does not have to be electrically isolated by the isolation region  10 , but may have a mesa structure by etching away predetermined parts of the epitaxial layer  12 . The width of the resist film  13  will eventually define the gate length. Accordingly, the width of the resist film  13  may be set appropriately depending on a desired gate length of the resultant semiconductor device. The thickness of the resist film  13  is preferably about 1 μm.  
         [0048]    Subsequently, as shown in FIG. 2C, an organic polymer film  11  is deposited, by a plasma CVD process, to a thickness of 200 nm over the epitaxial layer  12  so as to cover the resist film  13 . In this embodiment, the organic polymer film  11  is formed by a plasma CVD process. Accordingly, the organic polymer film  11  can be deposited at such a temperature as not degrading the resist film  13  thermally. Normally, the resist film  13  thermally degrades at 150° C. or more, for example. So the temperature of the substrate may be set to less than 150° C., for example. To carry out the deposition process easily and at a low cost, a coating technique, by which a liquid organic polymer is applied onto the substrate, is most preferable. However, this embodiment of the present invention intentionally uses a plasma CVD process, which is more complicated and less cost effective than the coating technique but which can be performed at such a temperature as not degrading the resist film  13  thermally. Also, unlike the known coating process, the organic polymer film  11  can be deposited by the plasma CVD process of this embodiment to a substantially uniform thickness (i.e., about 200 nm) over the epitaxial layer  12  as well as over the side and upper surfaces of the resist film  13 . Parts of the organic polymer film  11 , located at the corners between the resist film  13  and epitaxial layer  12 , are tapered toward the upper surface of the epitaxial layer  12 .  
         [0049]    The organic polymer film  11 , deposited by the plasma CVD process, is a low-κ film with a dielectric constant lower than that of SiO 2  (i.e., from about 4.0 to about 4.5). In the illustrated embodiment, the low-κ film  11  is made of benzocyclobutene (BCB), or a cyclobutane derivative, and has a dielectric constant of about 2.7. Examples of other applicable cyclobutane derivatives include divinylsiloxane benzocyclobutane (DVS-BCB) and perfluorocyclobutane (PFCB) with a dielectric constant of about 2.3.  
         [0050]    To deposit the organic polymer film  11  at an even lower temperature by a plasma CVD process, a plasma is preferably created from an inert gas such as Ar gas inside the deposition chamber with the film material (e.g., BCB) sprayed into the chamber. According to this technique, the energy of the resultant Ar plasma can be given to the particles of the sprayed material. Thus, even if the temperature of the substrate is as low as about 100° C., the organic polymer film  11  still can be deposited thereon. At that low temperature, the deposition process can be carried out while preventing the thermal deformation of the resist film  13  with much more certainty.  
         [0051]    Preferred conditions for the plasma CVD process we carried out include a substrate temperature of 150° C. or less and an in-chamber total pressure between 0.1 Torr and 1 Torr (i.e., between about 13.3 Pa and about 133.3 Pa) during the deposition process. We laid down these conditions because of the following reasons. Firstly, when the in-chamber total pressure was more than 1 Torr, the organic polymer film  11  still could be deposited successfully, but was dissolved unintentionally in an organic solvent (e.g., ethyl alcohol) when the resist film  13  was lifted off. Secondly, where the in-chamber total pressure was less than 0.1 Torr, the organic polymer film  11  could not be deposited as intended. Thirdly, if the substrate temperature during the deposition process was higher than 150° C., then the resist film  13  cured and could not be lifted off.  
         [0052]    Thereafter, as shown in FIG. 2D, the resist film  13  is lifted off, thereby forming an opening  11   a  in the organic polymer film  11 . That is to say, when the resist film  13  is removed, part of the organic polymer film  11 , which has covered the side and upper surfaces of the resist film  13 , is also peeled off along with the resist film  13 . At the bottom of the opening  11   a,  the upper surface of the epitaxial layer  12  is exposed. The width of the opening  11   a  is almost equal to the width of the resist film  13 , i.e., about 0.1 μm. It should be noted that the side faces of the opening  11   a  are tapered according to this embodiment. In this manner, by applying the lift-off technique to the organic polymer film  11  that has been formed by the plasma CVD process, the opening  11   a  can have a very small width.  
         [0053]    A field effect transistor can be formed by performing known process steps after that. Specifically, a transistor can be formed in the following manner.  
         [0054]    For example, a photoresist  14  with openings for forming ohmic electrodes  8  is defined on the organic polymer film  11  as shown in FIG. 2E. Next, as shown in FIG. 2F, the organic polymer film  11  is dry-etched with a mixture of CF 4  and O 2  gases while being masked by the photoresist  14 . In this manner, openings  11   b  are formed.  
         [0055]    Subsequently, as shown in FIG. 2G, another photoresist  15 , having openings that define the locations and shapes of the ohmic electrodes  8 , is defined on the organic polymer film  11  and epitaxial layer  12 . Then, an ohmic metal, e.g., an Ni/Au/Ge alloy, is deposited by an evaporation technique over the substrate and then the photoresist  15  with the excessive metal is lifted off, thereby forming ohmic electrodes  8  as shown in FIG. 2H.  
         [0056]    Thereafter, as shown in FIG. 2I, still another photoresist  16  is defined over the substrate to form a recess under the bottom of the opening  11   a  by partially etching the epitaxial layer  12  away (or the cap layer  7  more exactly). Then, using the photoresist  16  as a mask, that part of the cap layer  7  is removed to form an opening  12   a  as shown in FIG. 2J. As a result of this recess etching process, that part of the cap layer  7  no longer exists and the Schottky layer  6  is exposed at the bottom of the opening  12   a.  It should be noted that the threshold voltage of the resultant field effect transistor is controllable by the size of that particular part of the cap layer  7 . For that reason, the conditions of the recess etching process may be determined appropriately according to the threshold voltage of the semiconductor device (or field effect transistor) to be fabricated.  
         [0057]    Finally, a metal film is deposited over the substrate by an evaporation technique, and then the photoresist  16  is lifted off along with the excessive metal. As a result, a field effect transistor, including a T-gate electrode  9 , is formed as shown in FIG. 2K. The width of the gate electrode  9  at the bottom is defined by the width of the opening  11   a,  and is also about 0.1 μm.  
         [0058]    According to this embodiment, even though the insulating film  11  near the gate electrode  9  is made of an organic polymer, the gate length can be shortened to 0.3 μm or less (e.g., about 0.1 μm). Thus, the radio frequency characteristics (including f T , fmax and noise characteristic) of the transistor greatly improve. In addition, according to this embodiment, the gate electrode  9  can be formed by a lift-off technique. That is to say, the organic polymer film  11  can be patterned easily just by inverting the pattern for the resist film  13 . In this manner, the width of the resist film  13  can be reflected on the resultant gate length very accurately. Stated otherwise, variation in gate length can be minimized. In contrast, if the gate electrode is formed by dry etching as in a known process, then the feature size of the resultant pattern is subject to change because some variation is normally inevitable for a dry etching process. As a result, the gate length also varies unintentionally.  
         [0059]    Also, the organic polymer film  11 , surrounding the gate electrode  9 , is a low-κ film with a dielectric constant of about 2.7, which is much lower than that of SiO 2 . Thus, according to this embodiment, a field effect transistor with a very small fringe capacitance (i.e., a parasitic capacitance associated with the gate electrode  9 ) can be formed. A field effect transistor with that small fringe capacitance can operate at a much higher speed. Accordingly, an ultrahigh frequency field effect transistor, which is effectively applicable to cultivating the millimeter wave bands, is realized. Specifically, where the organic polymer film  11  was made of BCB, the fmax value, a typical index representing the radio frequency characteristics of a device, could be as high as 170 GHz, which is much higher than 140 GHz obtained by a device with the known insulating film of SiO 2 . That is to say, the reduction in gate capacitance improves the radio frequency characteristics, or increases the gain.  
         [0060]    In the foregoing embodiment, the organic polymer film  11  is made of BCB. Alternatively, to further reduce the fringe capacitance, the organic polymer film  11  may also be made of a fluoropolymer with a dielectric constant of 2.1. We confirmed that the organic polymer film  11  of a fluoropolymer can be deposited at a substrate temperature of 100° C. or less and at an in-chamber total pressure of 0.5 Torr or 260 Torr during the deposition process.  
         [0061]    In the foregoing illustrative embodiment, the present invention has been described as being applied to a field effect transistor including a T-gate electrode. However, the present invention is not limited to any particular method of forming such a semiconductor device, but is broadly applicable to any semiconductor device fabrication process that needs forming a fine-line opening in an organic polymer film. Furthermore, the present invention does not have to be implemented as a method for fabricating a semiconductor device, but may be realized as a method of patterning an organic polymer film by forming a very small opening in it. A supporting or underlying substrate for the organic polymer film to be patterned does not have to be the GaAs substrate used for the foregoing embodiment, but may be any other semiconductor substrate of GaN, SiC or Si, an insulating substrate made of glass, for example, or an SOI substrate.  
         [0062]    According to the present invention, an organic polymer film is deposited by a plasma CVD process, and can be patterned into any desired shape by forming a very small opening in it. In addition, now that it is possible to form an opening of such a small size in an organic polymer film, the fringe capacitance of the resultant gate electrode can be reduced considerably. Thus, the present invention realizes a field effect transistor operating in millimeter wave bands.