Patent Publication Number: US-6217403-B1

Title: Gate electrode formation method

Description:
This is continuation of application Ser. No. 08/899,622 filed on Jul. 7, 1997 now U.S. Pat. No. 6,039,621, which is hereby incorporated by reference to this specification. 
    
    
     FIELD OF THE INVENTION 
     The present claimed invention relates to the field of flat panel displays. More particularly, the present claimed invention relates to the formation of a gate electrode for a flat panel display screen structure. 
     BACKGROUND ART 
     In certain flat panel display devices such as, for example, flat display devices utilizing cold cathodes, a gate electrode is required. In such flat panel display devices, an electron emissive cold cathode is disposed between a first electrode (e.g. a row electrode) and a second electrode (e.g. a gate electrode). By generating a sufficient voltage potential between the row electrode and the gate electrode, the electron emissive cold cathode is caused to emit electrons. In one approach, the emitted electrons are accelerated, through openings in the gate electrode, towards a display screen. In such flat panel display devices, it is desirable to have the openings uniformly and consistently arranged with sufficient spacing provided between each opening to avoid overlapping in the gate electrode. 
     With reference now to Prior Art FIG. 1, a side sectional view of a conventional process step used in the formation of a prior art gate electrode is shown. As shown in Prior Art FIG. 1, a first electrode  102  has an insulating layer  104  disposed thereon. In a conventional gate electrode formation process, a non-insulating material is deposited on top of insulating layer  104  to form a very thin non-insulating layer  106  (e.g. on the order of 100 angstroms) of the non-insulating material. 
     With reference now to Prior Art FIG. 2, conventional gate electrode formation processes then deposit spheres, typically shown as  108 , onto very thin non-insulating layer  106 . Because layer  106  is very thin, it is extremely difficult for such prior art gate electrode formation processes to make very thin non-insulating layer  106  continuous. As a result, spheres  108  are not uniformly or consistently deposited across the surface of very thin non-insulating layer  106  in conventional gate electrode formation processes. 
     With reference next to Prior Art FIG. 3, a second layer of non-insulating material  110  is then deposited over the very thin non-insulating layer  106  and over spheres  108 . As shown in Prior Art FIG. 3, second layer of non-insulating material  110  is much thicker than very thin layer of non-insulating material  106 . In such prior art approaches, very thin non-insulating layer  106  together with second non-insulating layer  110  comprise the body of the gate electrode. 
     As shown in Prior Art FIG. 4, after the deposition of second non-insulating layer  110 , spheres  108  and portions of second non-insulating layer  110  which overlie spheres  108  are removed. As a result, regions, typically shown as  112 , of very thin non-insulating layer  106  have second non-insulating layer  110  removed therefrom. 
     Referring still to Prior Art FIG. 4, after the removal of spheres  108  and portions of second non-insulating layer  110  which overlie spheres  108 , an etch step is performed. The etch step is used to form openings through very thin non-insulating layer  106 . As mentioned above, spheres  108  are not uniformly or consistently disposed across the surface of very thin non-insulating layer  106  in conventional gate electrode formation processes. Consequently, conventionally formed openings in second non-insulating layer  110  and very thin non-insulating layer  106  are likewise not uniformly or consistently disposed across the surface of very thin non-insulating layer  106 . In addition to forming openings through second non-insulating layer  110  and very thin non-insulating layer  106 , the etch step of conventional gate electrode formation processes also substantially etches second non-insulating layer  110 . The etching of second non-insulating layer  110  reduces the thickness thereof. Therefore, second non-insulating layer  110  must be deposited to a thickness which is greater than the desired thickness of the gate electrode, so that second non-insulating layer  110  will be of the desired thickness after being subjected to the etch environment. Thus, conventional gate electrode formation processes reduce the thickness of the gate electrode across the entire surface thereof when etching openings through the gate electrode, as shown in Prior Art FIG.  5 . 
     Referring again to Prior Art FIG. 5, as yet another drawback, during etch steps of the above-described gate electrode formation process, the top surface of second non-insulating layer  110  is subjected to the etch environment. In addition to reducing the thickness of second insulating layer  110 , the etch environment induces deleterious effects such as, for example, oxidation at the top surface of second non-insulating layer  110 . Oxidation of the top surface of second non-insulating layer  110  complicates other processes such as the removal of subsequently deposited emitter material. Thus, conventional gate electrode formation processes subject the gate electrode to unwanted etching, and degrade the surface integrity of the gate electrode. 
     As still another drawback, thickness uniformity of the gate film remaining after an etch process crucially depends on the etch uniformity of the etch system employed. In large area panels, such etch non-uniformity is a major concern, because it is extremely difficult to achieve sufficient etch uniformity across the large area panels. The problem of etch non-uniformity is further exacerbated when etching through submicron features. 
     Thus, a need exists for a gate electrode formation method which provides for improved spacing of openings formed through the gate electrode. Another need exists for a gate electrode formation process which does not reduce the thickness of the gate electrode across the entire surface thereof when etching openings through the gate electrode. Yet another need exists for a method which provides a gate electrode having good surface integrity and an undamaged top surface after the formation thereof. 
     SUMMARY OF INVENTION 
     The present invention is comprised of a method which provides for improved spacing of openings formed through the gate electrode. The present invention further comprises a method which does not reduce the thickness of the gate electrode across the entire surface thereof when etching openings through the gate electrode. The present invention also provides a gate electrode having good surface integrity and an undamaged top surface after the formation thereof. 
     Specifically, in one embodiment, the present invention comprises depositing a gate metal over an underlying substrate such that a layer of the gate metal is formed above the underlying substrate. In the present invention, the layer of the gate metal is deposited to a thickness approximately the same as the thickness desired for the gate electrode. Next, the present invention deposits polymer particles uniformly and consistently arranged onto the layer of gate metal. A sacrificial hard mask layer is then deposited over the polymer particles and the layer of the gate metal. In the present invention, the sacrificial hard mask layer is comprised of a material which is not adversely affected/substantially etched during the etching of the gate metal. The present invention removes the polymer particles and portions of the hard mask layer which overlie the polymer particles such that first regions of the layer of the gate metal are exposed, and such that second regions of the layer of the gate metal remain covered by the hard mask layer. After the removal step, the present invention etches through the first regions of the layer of the gate metal such that openings are formed completely through the layer of the gate metal at the first regions. After the openings have been formed, the present invention then removes the remaining portions of the hard mask layer which overlie the second regions of the layer of the gate metal. 
     In one embodiment, the gate metal is comprised of chromium. In such an embodiment, the present invention etches through the above-described first regions of the layer of chromium using a chlorine and oxygen-containing etch environment such that openings are formed completely through the layer of chromium at the first regions. For purposes of the present application, an etch environment refers to the etchants/gases/plasmas used to perform an etch. The present embodiment also exposes the underlying substrate to a fluorine-containing etch environment. In so doing, the present invention forms respective cavities in the underlying substrate beneath the openings formed through the layer of chromium at the first regions of the layer of chromium. After removing remaining portions of the hard mask layer which overlie the second regions of the layer of chromium, the present embodiment enlarges the respective cavities formed in the underlying substrate by exposing the respective cavities to a wet etchant. 
     In still another embodiment of the present invention, the gate metal is comprised of tantalum. In such an embodiment, the present invention etches through the above-described first regions of the layer of tantalum using a fluorine-containing etch environment such that openings are formed completely through the layer of tantalum at the first regions. The present embodiment also exposes the underlying substrate to the fluorine-containing etch environment. In so doing, the present invention forms respective cavities in the underlying substrate beneath the openings formed through the layer of tantalum at the first regions of the layer of tantalum. After removing remaining portions of the hard mask layer which overlie the second regions of the layer of tantalum, the present embodiment enlarges the respective cavities formed in the underlying substrate by exposing the respective cavities to a wet etchant. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrates embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
     Prior Art FIG. 1 is a side sectional view illustrating a conventional step used during the formation of a prior art gate electrode. 
     Prior Art FIG. 2 is a side sectional view illustrating another conventional step used during the formation of a prior art gate electrode. 
     Prior Art FIG. 3 is a side sectional view illustrating yet another conventional step used during the formation of a prior art gate electrode. 
     Prior Art FIG. 4 is a side sectional view illustrating another conventional step used during the formation of a prior art gate electrode. 
     Prior Art FIG. 5 is a side sectional view illustrating another conventional step used during the formation of a prior art gate electrode. 
     FIGS. 6-13 are side sectional view illustrating the formation of a gate electrode in accordance with the present claimed invention. 
     The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     With reference to FIG. 6, a side sectional view illustrating a starting step of the present invention is shown. In the present embodiment, a first electrode  600  (e.g. a row electrode) has a layer  602  of dielectric material disposed thereover. In the present embodiment, dielectric layer  602  is comprised of, for example, silicon dioxide. The present invention is, however, well suited to the use of various other dielectric materials. Additionally, although not shown in FIG. 6, the present invention is also well suited for use in an embodiment which includes a resistive layer disposed between row electrode  600  and dielectric layer  602 . Such a resistive layer is not shown in FIG.  6  and subsequent figures for purposes of clarity. In the present embodiment, dielectric layer  602  forms an underlying substrate for supporting a gate electrode. Thus, for purposes of the present application, dielectric layer  602  is referred to as the “underlying substrate”. 
     Referring still to FIG. 6, gate metal is deposited over underlying substrate  602  such that a layer  604  of the gate metal is formed above underlying substrate  602 . In the present invention, layer  604  of the gate metal is deposited to a thickness approximately the same as a desired thickness of the gate electrode to be formed. That is, unlike prior art gate electrode formation processes, the present invention does not require depositing gate metal to a thickness which is greater than the intended/desired thickness of the gate electrode being formed. In the present embodiment, layer  604  of the gate metal is deposited to a thickness in the range of approximately 300-1000 angstroms. By depositing the gate metal to such a thickness, the present invention achieves a gate metal layer  604  having consistent thickness and uniformity across the entire surface thereof. Hence, the present invention eliminates the very thin and discontinuous metal layers associated with conventional gate electrode formation processes. In one embodiment of the present invention, layer  604  of the gate metal is formed of chromium. In another embodiment, layer  604  of the gate metal is formed of tantalum. Although such metals are specifically recited, the present invention is not limited to the use of only chromium or tantalum. 
     Referring now to FIG. 7, the present invention then deposits polymer particles or “spheres”  700  onto layer  604 . In the present embodiment, the deposition of polymer particles  700  is accomplished using, for example, an electrophoretic deposition. 
     Referring again to FIG. 7, after the deposition of particles  700 , the structure (i.e. row electrode  600 , underlying substrate  602 , layer  604 , and newly deposited particles  700 ) is then dried. 
     With reference still to FIG. 7, due to the thick (e.g. 300-1000 angstroms), and hence less resistive, and continuous nature of layer  604 , the present invention provides for improved uniformity in the spacing of particles  700 . That is, the present invention improves the uniformity of particle spacing compared to conventional gate electrode formation processes. 
     Referring now to FIG. 8, after the deposition of particles  700 , the present invention deposits a sacrificial “hard mask layer”  800  over polymer particles  700  and layer  604 . In the present invention, hard mask layer  800  is comprised of a material which has a significantly lower etch rate than the gate metal when subjected to a plasma etch environment used to etch the gate metal. That is, the sacrificial hard mask layer of the present invention is comprised of a material which is not adversely affected/substantially etched during the etching of the gate metal or other layers of the present structure. In the present embodiment, hard mask layer  800  is comprised of aluminum. Although aluminum is recited as the material of hard mask layer  800  in the present embodiment, the present invention is also well suited to the use of various other materials such as, for example, nickel, chromium, and the like. The choice of the hard mask layer is dependent upon the material comprising the various layers of the structure (i.e. the material comprising the row electrode, the resistive layer, the dielectric, the gate electrode, and the like). Additionally, in the present embodiment, hard mask layer  800  has a thickness of approximately 200-1000 angstroms. 
     With reference next to FIG. 9, the present invention then removes particles  700 . As a result, portions of hard mask layer  800  which overlie polymer particles  700  are also removed. Thus, as shown in FIG. 9, first regions, typically shown as  900 , of layer  604  are exposed, and second regions of layer  604  remain covered by remaining portions of hard mask layer  800 . In the present embodiment, polymer particles  700  are removed by immersing the structure in a bath of deionized water and subjecting the structure to mechanical stripping using, for example, sonic vibrations. More specifically, in one embodiment, the structure is disposed to sonic transducers, and vibrated at a frequency range needed to remove particles having a specific size range, and with a power range of approximately 50-200 watts for a period of approximately 5 minutes. The structure is then subjected to sonic transducers, and vibrated at a frequency range needed to remove particles having a specific size range, and with a power range of approximately 50-200 watts for a period of approximately 5 minutes. It will be understood that the present invention is also well suited to varying the parameters of the sonic particle removal process. 
     With reference still to FIG. 9, in another embodiment of the present invention, particles  700  are removed by subjecting particles  700  to a high pressure fluid spray in conjunction with a brushing (contact or non-contact) of particles  700 . 
     Referring next to FIG. 10, the present invention then etches through first regions  900  of layer  604  such that openings, typically shown as  1000 , are formed completely through layer  604 . In an embodiment where layer  604  is comprised of chromium, a chlorine and oxygen-containing etch environment is used to form openings  1000 . In such an embodiment, the structure is subjected to a plasma etch environment comprising: a power of 500 watts; a bottom electrode bias of 20 watts; a temperature of 60 Celsius; and a pressure of 10-20 milliTorr for a period of approximately 40 seconds. In an embodiment where layer  604  is comprised of tantalum, a fluorine-containing etch environment (e.g. CHF 3 /CF 4 ) is used to form openings  1000 . In such an embodiment, the structure is subjected to a plasma etch environment comprising: a power of 400 watts; a bottom electrode bias of 80 watts; a temperature of 60 Celsius; and a pressure of 15 milliTorr for a period of approximately 160 seconds. The present invention is, however, well suited to varying the parameters of the plasma etch environment. 
     Referring still to FIG. 10, during the etching of openings  1000 , hard mask layer  800  of the present invention protects the underlying top surface of layer  604  from the plasma environment. Thus, unlike conventional gate electrode formation processes, the present invention protects the top surface of layer  604  from, for example, oxidation. Hence, in the present invention, the condition of the top surface of layer  604  does not complicates other processes such as the removal of subsequently deposited emitter material. Therefore, the present invention provides a gate electrode with an undamaged top surface and which has good surface integrity. 
     With reference now to FIG. 11, the present invention then etches through a substantial amount of the thickness of underlying substrate  602 . In an embodiment where layer  604  is comprised of chromium and a chlorine and oxygen-containing etch environment was used to form openings  1000 , the structure is then subjected to another etch environment which contains fluorine (e.g. CHF 3 /CF 4 ). The fluorine etch environment is used to etch cavities  1100  in underlying substrate  602 . In the present invention the change from the chlorine and oxygen-containing etch environment to the fluorine containing etch environment is made without breaking the vacuum of the etch environment. In an embodiment where layer  604  is comprised of tantalum and a fluorine-containing etch environment was used to form openings  1000 , the same fluorine etch environment is used to etch cavities  1100  in underlying substrate  602 . 
     With reference again to FIG. 11, during the etching of cavities  1100 , hard mask layer  800  continues to protect the underlying top surface of layer  604  from the plasma environment. Thus, unlike conventional gate electrode formation processes, the present invention protects the top surface of layer  604  from, for example, oxidation. 
     Referring now to FIG. 12, the present invention then removes remaining portions of hard mask layer  800  which overlie the second regions of layer  604 . Thus, hard mask layer  800  protects the top surface of layer  604  during the etching of both layer  604 , and underlying substrate  602 . As a result, unlike prior art gate electrodes, the top surface of a gate electrode formed according to the present invention remains in pristine condition even after numerous etch steps. In the present embodiment, hard mask layer  800  is removed using a selective wet etch comprised of approximately 10 percent sodium hydroxide. Hard mask layer  800  can also be removed using various other etchants, however. 
     With reference next to FIG. 13, after the removal of hard mask layer  800 , the present invention removes the remaining underlying substrate  602  and enlarges cavities  1100  formed in underlying substrate  602  by exposing cavities  1100  to a wet etchant. Hence, a gate electrode and corresponding underlying cavities have been formed by the present embodiment of this invention. By eliminating many of the disadvantages associated with conventional gate electrode formation processes, the present invention increases, yield, improves throughput, and reduces the costs required to form a gate electrode. Alternately, it is conceivable, for certain types of materials, that hard mask layer  800  can be removed during the wet etch (i.e. during the enlargement) of the cavities. 
     The present invention further comprises a method which does not reduce the thickness of the gate electrode across the entire surface thereof when etching openings through the gate electrode. The present invention also provides a gate electrode having good surface integrity and an undamaged top surface after the formation thereof. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.