Abstract:
Metal-insulator-metal planar electron emitters (PEES) have potential for use in advanced lithography for future generations of semiconductor devices. The PEE has, however, a limited lifetime, which restricts its commercial applicability. It is believed that the limited lifetime of the PEE is limited by in-diffusion of metal ions from the anode. The in-diffusion may be countered in a number of different ways. One way is to cool the PEE to temperatures below room temperature. This lowers the metal ion mobility, and so the metal ions are less likely to diffuse into the insulator layer. Another way is to occasionally reverse the electrical potential across the PEE from the polarity used to generate the electron beam. This counteracts the electrical driving force that drives the positively charged metal ions from the PEE anode to the PEE cathode.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally to planar electron beam devices, and more particularly to Metal-Insulator-Metal tunneling planar electron emitters for lithography applications. 
     BACKGROUND 
     Optical lithography has dominated the fabrication of integrated circuits for over 30 years. The general process involves back illuminating a mask with optical radiation, reducing the image of the mask with de-magnifying optics, and then imaging the pattern onto a substrate covered with a layer of photo-resist. Then, with the appropriate photo-resist, the substrate surface (covered with photo-resist) is chemically treated to remove those areas of the photo-resist, which were optically illuminated, thereby transferring a de-magnified image of the mask into the photo-resist. Subsequent chemical etching steps may be utilized to complete the process of transferring a de-magnified image of the mask onto the surface of the substrate material. 
     The constant competitive driving force in integrated circuits is for smaller and faster devices. Optical lithography has taken the state of the art down to dimensions where the diffraction of light has become the major limiting variable. Creating features smaller than the wavelength of the illuminating light has led to creative optical techniques such as off-axis illumination and phase-shifting masks. Even so, initially utilizing krypton fluoride excimer lasers at 248 nanometers (nm), and more recently argon fluoride excimer lasers operating at 193 nm, the industry standard today for narrow linewidths utilizing optical lithography hovers somewhere around 80 nanometers, slightly less that ½ the wavelength of the 193 nm illuminating laser. Also, at these short ultraviolet (UV) wavelengths material science issues become a practical limiting factor. For example, there are few materials that have sufficient UV transmission at these wavelengths to be used as either refractive lenses in the de-magnifying relay optics or as substrates for the mask assembly. 
     Given this, several non-optical lithography techniques have been explored by the semiconductor industry. Direct write electron-beam (E-beam) lithography has been researched and commercialized given its potential of wavelengths in the nanometer range. Commercial direct-write E-beam devices are readily available today with resolutions down to 50 nm and slightly below, however, the direct write devices are slow to process a large scale wafer and the search continues for a faster way to utilize the potential resolution available from electron-beam lithography. 
     Planar electron beam lithography has been investigated for over 20 years with limited commercial success. One configuration commonly referred to as an M-I-M (Metal-Insulator-Metal) planar electron beam lithography (PEBL) device has been constructed and has demonstrated partial technical success. The M-I-M devices consist of an insulator material sandwiched between two conducting metal materials. The individual metal and insulator layers may be made sufficiently thin that when an electrical voltage is applied across the device, electrons from the cathode (the metal with the negative electrical potential) may be driven by the electro-static forces to quantum tunnel into and through the first part of the insulating layer, then drift through the remainder of the insulating layer and anode metal (at the positive potential) and exit the device as free particles, essentially an electron gun. Devices of this type have been fabricated in cross-sectional dimensions as large as 1 inch square, and with appropriate sub-micron masking of the output electron beam, this device configuration opens the possibility of projecting patterns at a 1:1 (one-to-one) ratio in the resist-covered substrate. Also, it has been demonstrated that the exposure time to transfer the entire pattern to the photo-resist in this configuration can be as small as {fraction (1/10)} of a second, which may allow for rapid processing of large commercial wafers utilizing a step-and-repeat procedure. This rapid pattern transfer rate may give the emerging planar electron beam emitter a technical/commercial advantage over the traditional electron beam devices that write the pattern sequentially in the resist similar to how a television raster scans the screen with a small pencil beam. 
     However, the useful life of the planar electron emitting devices have historically been sufficiently short so as to limit their applicability to commercially viable manufacturing processes. In view of this, there is a need for a method or technique to prolong the lifetime of planar electron beam emitters for application to electron beam lithography. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention relates to approaches to increasing the lifetime of M-I-M planar electron emitters (PEEs). It is believed that one of the important mechanisms in limiting the lifetime of the PEE is related to the in-diffusion of metal ions from the thin metal anode of the PEE into the insulating layer. Once the metal ions diffuse through the thin insulator to the other metal layer, it becomes impossible to maintain an electric potential between the two metal layers, and so no electron beam can be generated. 
     The approaches of the present invention are directed to reducing the possibility that diffusion takes place and also to reversing the diffusion process. Diffusion is a temperature dependent process; for the first approach cooling the PEE to temperatures below room temperature lowers the metal ion mobility, and so the metal ions are less likely to diffuse into the insulator layer. 
     In the second approach, the electrical potential across the M-I-M PEE is occasionally reversed from the polarity used to generate the electron beam. This counteracts the electrical driving force that drives the positively charged metal ions from the PEE anode to the PEE cathode, thus increasing the length of time taken for the metal ions to diffuse across the insulator layer from the anode to the cathode. 
     In one particular embodiment, the invention is direction to a planar electron emitter system for lithography. The system includes a planar electron emitter having a first electrically conducting layer, a second electrically conducting layer that emits electrons, and an insulating layer disposed between the first and second electrically conducting layers. The second electrically conducting layer emits electrons when held at an electrical potential that is sufficiently positive with respect to an electrical potential applied to the first electrically conducting layer. A source of electric potential is electrically connected to the first and second electrically conducting layers so as to impose an electrical potential across the insulating layer. The source of electric potential is adapted so that a polarity of the electrical potential difference between the first and the second electrically conducting layers is reversible. 
     Another embodiment of the invention is directed to a method of exposing a resist on a wafer using a planar electron emitter. The method includes applying a first electrical potential of a first polarity to the planar electron emitter so that the planar electron emitter emits electrons incident on the resist to expose a first portion of the resist. A second electrical potential of a second polarity, opposite to the first polarity, is applied to the planar emitter so that the planar emitter does not emit electrons. 
     Another embodiment of the invention is directed to a system for lithography, that includes a planar electron emitter having a first electrically conducting layer, a second electrically conducting layer that emits electrons, and an insulating layer disposed between the first and second electrically conducting layers. The second electrically conducting layer emits electrons when held at an electrical potential that is sufficiently positive with respect to an electrical potential applied to the first electrically conducting layer. A temperature control unit is thermally coupled to the planar electron emitter for controlling temperature of the planar electron emitter. 
     Another embodiment of the invention is directed to a method of exposing a resist on a wafer using a planar electron emitter. The method includes applying a first electrical potential of a first polarity to the planar electron emitter so that the planar electron emitter emits electrons incident on the resist to expose a first portion of the resist. The method also includes controlling the temperature of the planar electron emitter at a temperature below an ambient temperature. 
     Another embodiment of the invention is directed to a stepper system for lithography, comprising a planar electron emitter that has a first electrically conducting layer, a second electrically conducting layer that emits electrons, and an insulating layer disposed between the first and second electrically conducting layers. The second electrically conducting layer emits electrons when held at an electrical potential that is sufficiently positive with respect to an electrical potential applied to the first electrically conducting layer. The system also includes a temperature control unit thermally connected to the planar electron emitter for controlling the temperature of the planar electron emitter, a substrate mount for holding a substrate having a resist layer facing the planar electron emitter, and an adjustable stage. The mount is fixed relative to the adjustable stage. The adjustable stage is adapted to move a wafer, when the mount holds a wafer, relative to the planar electron emitter so that successively different portions of resist on the wafer are exposed to electrons emitted from the second electrically conducting layer. 
     Another embodiment of the invention is directed to a stepper system for lithography that includes a planar electron emitter having a first electrically conducting layer, a second electrically conducting layer that emits electrons, and an insulating layer disposed between the first and second electrically conducting layers. The second electrically conducting layer emits electrons when held at an electrical potential that is sufficiently positive with respect to an electrical potential applied to the first electrically conducting layer. The system also includes a voltage source connected to the first and second electrically conducting layers, the voltage source being adapted to apply a first voltage having a first polarity between the first and second electrically conducting layers, and a second voltage having a second polarity opposite to the first polarity between the first and second electrically conducting layers. There is a substrate mount for holding a substrate having a resist layer facing the planar electron emitter, and an adjustable stage, the mount being fixed relative to the adjustable stage. The adjustable stage is adapted to move a wafer, when the mount holds a wafer, relative to the planar electron emitter so that successively different portions of resist on the wafer are exposed to electrons emitted from the second electrically conducting layer. 
     Another embodiment of the present invention is directed to a planar electron emitter system for lithography that comprises a planar electron emitter having a first electrically conducting layer, a second electrically conducting layer that emits electrons; and an insulating layer disposed between the first and second electrically conducting layers. The second electrically conducting layer emits electrons when held at an electrical potential that is sufficiently positive with respect to an electrical potential applied to the first electrically conducting layer. The planar electron emitter has a lifetime in excess of one million exposure shots of approximately 100 msec. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates an embodiment of a stepper system that uses a planar electron emitter according to principles of the present invention; 
         FIGS. 2A-2C  schematically illustrate different embodiments of planar electron emitter; 
         FIGS. 3A-3C  present graphs representing different time-dependent electrical potentials applied to a planar electron emitter according to principles of the present invention; and 
         FIG. 4  schematically illustrates an embodiment of a cooled planar electron emitter according to principles of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is applicable to planar electron beam devices and is believed to be particularly useful in Metal-Insulator-Metal (M-I-M) tunneling electron emitters for lithography applications. In the process of integrated circuit (IC) manufacturing, the lithographic pattern transfer may be the most critical and frequent process step since up to 30 different patterns may be transferred to the same wafer before the final device can be packaged and tested for quality assurance. 
     Planar electron beam lithography (PEBL) has at least two advantages over other next generation lithographic techniques. First, the PEBL technique requires no external ultraviolet, x-ray, or high energy electrons to transfer a pattern from mask to wafer. The electrons needed for pattern transfer are generated within the planar electron emitter (PEE) and the mask is incorporated within the planar electron emitter. And secondly, the entire pattern is transferred simultaneously, whereas competing technologies require “stitching” together a large number of small overlapping patterns. 
     An embodiment of a planar electron beam lithography system  100  is depicted in FIG.  1 . The planar electron emitter (PEE) and integrated mask unit  102  is housed in the mask controller device  104 . The mask controller unit  104  may also include a lifetime monitoring device  106  for recording the operational run time and/or number of exposures on the planar electron emitter  102 . The lifetime monitoring device  106  may also include an ammeter, voltmeter, network analyzer or other suitable device and may measure the impedance or other electrical properties of the planar electron emitter  102 . A temperature control unit  108  may be connected to the planar electron emitter (PEE) and integrated mask unit  102  for controlling the temperature of the PEE and integrated mask unit  102 . The temperature control unit  108  may be connected to receive cooling fluid from a fluid source  110 . The cooling fluid source may supply liquid nitrogen, liquid helium, or other suitable liquid to cool the planar electron emitter (PEE) and integrated mask unit  102 . 
     The electron beam  112  emanating from the planar electron emitter (PEE) and integrated mask unit  102  may be directed by a projection device  114  to image the mask features at a unity (one-to-one) magnification onto a resist surface of the wafer  116 . The projection device  114  may be a magnetic field, operating in concert with an electric field, oriented in such a way so as to focus the electron beam  112  with an effective magnification factor which is variable, depending upon the strength and orientation of the magnetic and electric field. 
     For one particular approach, the projection device  114  includes a pair of Helmholtz coils, wherein the orientation of the magnetic field lines may be perpendicular to the emitting surface of the planar electron emitter (PEE) and integrated mask unit  102 . The projection device  114  may be augmented in the process of “projecting” electrons emitted from the planar electron emitter (PEE) and integrated mask unit  102  by an electric field generated by applying a voltage between the planar electron emitter (PEE) and integrated mask unit  102  and the wafer  116 . The polarity of the voltage between the planar electron emitter (PEE) and integrated mask unit  102  and the wafer  116  may be such that the planar electron emitter (PEE) and integrated mask unit  102  is held at a negative potential in the range of 5 kilo-volts (KV) relative to the wafer  116 . With this polarity, the applied electric field may cause the emitted electrons emanating from the planar electron emitter (PEE) and integrated mask unit  102  to be accelerated in route to the wafer  116 . Also, given the orientation of the magnetic and electric field lines described above, each individual electron&#39;s terminal position upon impinging the wafer  116 , is uniquely determined by its location where the electron exits the surface of the planar electron emitter (PEE) and integrated mask unit  102 . In other words, the angle at which an electron exits the planar electron emitter (PEE) and integrated mask unit  102  (relative to the normal to the surface of the PEE) does not affect the effective magnification of the projection device  114 . This is analogous to the classical geometrical optics situation in imaging a point source of radiation to a point source in the image plane. That is, all the radiation emanating from the point source (independent of angle) is focused to a point source in image space, where the location of the image is uniquely determined by the location of the radiating point source. It is in this spirit that the projection device  114  may be referred to as an “imaging device”. Given this, with the appropriate choice of orientations and strengths of the magnetic and electric fields, it is possible to “image” the radiating surface of the planar electron emitter (PEE) and integrated mask unit  102  onto the surface of the wafer  116  with one-to-one (1:1) magnification. 
     The wafer  116  may be mounted on a wafer support unit  118 , which in turn may be mounted on an adjustable stage  120 . The adjustable stage  120  may be an X-Y-Z, or X-Y-Z-θ, or X-Y-Z-θ-φ (five degree-of-freedom) adjustable stage which may be driven by a stage controller  121 , which may be part of an overall alignment system  122 . In another approach, the PEE  102  may be adjustable in one or more degrees of freedom to provide movement relative to the wafer. The overall alignment system  122  may be part of a master controller unit  124 , which controls the overall mechanical features of the planar electron beam lithography system  100 . For example, the master controller unit  124  may control the timing and operation of the vacuum system  126 , the alignment system  122 , and the planar electron emitter (PEE) and integrated mask unit  102 . 
     An operational sequence of the planar electron beam lithography system  100  may proceed as follows. The master controller unit  124  may generate a command, or receive a command from the interface unit  128  to initiate a deposition run on the wafer  116 . The interface unit  128  may couple to a computer or other suitable device to issue electronic commands to the master controller unit  124 . The master controller unit  124  may in response to an externally generated command, issue a command to the vacuum system  126  to generate a vacuum on the inner chambers of the planar electron beam lithography system  100  to remove potentially contaminating residual gases in the chamber. 
     Also, the master controller unit  124  may route a command to the temperature control unit  108  to begin delivering cooling fluid first to the temperature getter device  109  and later to the planar electron emitter (PEE) and integrated mask unit  102 . The getter device  109  may also be held at a lower temperature than the planar electron emitter (PEE) and integrated mask unit  102  to preferentially draw potentially contaminating gaseous particle to the temperature getter device  109  and not to the planar electron emitter (PEE) and integrated mask unit  102 . 
     Next, the master controller unit  124  may issue a command to the alignment system  122  and stage controller  120  to orient the wafer  116  in the proper position before initiating electron beam bombardment. The master controller unit  124  may then issue a command to the projection system  114  to set the magnetic and electric field strengths appropriate for the desired one-to-one (1:1) magnification. The command may then be given to apply the necessary voltage signals to the planar electron emitter (PEE) and integrated mask unit  102  to begin electron bombardment on the wafer  116 . The master controller unit  124  may include a timer module to issue repetitive commands to the stage controller  120  to periodically translate the wafer  116  in the horizontal directions X and Y (Z being elevation) in a process commonly referred to in the semiconductor industry as “step and repeat”. 
     The above process may be repeated until the wafer  116  has been processed by all the necessary planar electron emitters (PEE) and integrated mask units  102  to achieve the desired structure in the wafer&#39;s topology. With the original wafer  116  electron beam lithography completed, the master controller may issue a command to the wafer exchanger unit  132  to remove the original wafer  116  and install the next wafer in the queue to be processed. This procedure may be repeated until all waters scheduled for electron beam lithography are processed. 
     Once the entire top surface of the wafer  116  has been exposed to electron bombardment via the step and repeat process, it may be necessary to insert a different planar electron emitter (PEE) and integrated mask unit  102  in the mask controller unit  104 . The additional planar electron emitter (PEE) and integrated mask unit  102  may be necessary to define additional structures in the wafer&#39;s  116  topology. In this event, the master controller  124  may issue a command to the mask exchanger module  130  to remove the original planar electron emitter (PEE) and integrated mask unit  102  and insert the new emitter. 
     It will be appreciated that modifications to the above process steps are anticipated. For example, once the entire top surface of the “first” wafer  116  has been exposed to electron bombardment via the step and repeat process, in contrast to the above, the planar electron emitter (PEE) and integrated mask unit  102  may stay in place and an additional wafer similar to the “first” wafer  116  may be inserted by the wafer exchanger unit  132  for electron beam lithography. This process may be repeated until all scheduled wafers have been processed by the “first” planar electron emitter (PEE) and integrated mask unit  102 . Then, if additional planar electron emitters (PEE) and integrated mask units  102   s  are needed to fully define the topology of the wafers  116   s , additional planar electron emitters (PEE) and integrated mask units  102  may be inserted one-by-one until all of the scheduled wafers  116  have been appropriately processed by selective electron exposure by the planar electron emitters (PEE) and integrated mask units  102 . 
     A cross sectional view of one embodiment of a planar electron emitter (PEE) and integrated mask unit  200 A in the Metal-Insulator-Metal configuration is shown schematically in FIG.  2 A. The metal substrate  202 A may be composed of aluminum or other suitable electrically conducting material such as a metal, and serves as the source of electrons for the electron emitter. The conducting substrate  202 A may also be referred to as the cathode of the PEE  200 A, and is typically formed over a structural substrate, for example a silicon or sapphire structural substrate. 
     A thin insulating layer  204 A may be formed on the substrate  202 A, for example by over-coating or by chemical treatment. The thin insulating layer  204 A may include aluminum oxide (Al 2 O 3 ) or other suitable insulating material. In the configuration shown in  FIG. 2A , a pattern layer  206 A (composed of the resist remaining after a step of chemical etching) has been fabricated on the exposed surface of the insulator layer  204 A by standard lithographic techniques. The pattern layer  206 A has been over-coated with a thin metal overlay  208 A. The thin metal overlay  208 A may be composed of gold or other suitable metal. Typical thickness dimensions for the individual layers may be;
         conducting substrate  202 A thickness: 1 micron   insulating layer  204 A thickness: 100 Angstroms   metal overlay  208 A thickness: 100 Angstroms
 
It will be appreciated that the thickness of each layer may be greater or less than these typical dimensions. Furthermore, the PEE may be made using different materials. The conducting substrate may be formed from many different types of electrically conducting materials, including metals and conducting semiconductor materials. For the purposes of this description, the first “Metal” in “Metal-Insulator-Metal” is assumed to include electrically conducting semiconductors. For example, the conducting substrate may be formed from electrically conducting silicon, the insulating layer may be formed from silicon dioxide and the thin metal overlay may be aluminum.
       

     To initiate electron emission from the planar electron emitter (PEE) and integrated mask unit  200 A, an electrical potential (voltage)  210 A is applied between the conducting substrate  202 A (negative lead) and the metal overlay  208 A (positive lead). For the dimensions and materials shown above, a 5-volt potential may be sufficient to commence electron emission. With the applied voltage  210 A, electrons from the surface of the metal substrate  202 A may receive a sufficient driving force to quantum tunnel into and then drift through the remaining thickness of the insulating layer  204 A. Upon exiting the insulating layer  204 A, those electrons that propagate into the resist left behind in the lithographic process to define the mask  206 A are absorbed by the resist material. The remaining electrons that propagate through the insulating layer  204 A and exit in regions where the resist was etched away, continue on, propagating through the metal overlay  208 A and exit the planar electron emitter (PEE) and integrated mask unit  200 A as free-space propagating electrons  216 A. The free space propagating electrons  216 A are subsequently incident on the resist layer on the wafer, so as to selectively expose portions of the resist layer. 
     Another embodiment of a planar electron emitter (PEE) and integrated mask unit  200 B in the Metal-Insulator-Metal configuration is shown schematically in  FIG. 2B. A  conducting substrate  202 B is composed of aluminum or other suitable electrically conducting material and serves as the source of electrons (cathode) for the electron emitter. The conducting substrate  202 B may be over-coated or chemically treated to form a thin insulating layer  204 B, for example made from aluminum oxide (Al 2 O 3 ) or other suitable insulating material. In the configuration shown in  FIG. 2B , a mask  200 B may be formed by chemically etching the exposed surface of the insulating layer  204 B prior to overcoating with the metal overlay  208 B. 
     In this configuration, the preferential spatial absorption and/or scattering of electrons occurs due to the spatially non-uniform quantum tunneling barrier of the insulating layer  204 B. For example, the insulating layer  204 B, prior to chemical etching, may be deposited or grown in sufficient thickness to absorb and/or scatter electrons that enter the insulating layer  204 B from the metal substrate  202 B. The subsequent chemical etching of the insulating layer  204 B may be carded out until the etched thickness areas are sufficiently thin (similar to the thickness described above in  FIG. 2A ) such that electrons propagate through the insulating layer  204 B only in those areas “thinned” by chemical etching. 
     To initiate electron emission from the planar electron emitter (PEE) and integrated mask unit  200 B, an electrical potential (voltage)  210 B is applied between the metal substrate  202 B (negative lead) and the metal overlay  208 B (positive lead). For the dimensions and materials shown above, a 5 volt potential may be sufficient for electrons to be emitted. With the applied voltage  210 B, electrons from the surface of the metal substrate  202 B may receive a sufficient driving force to quantum tunnel into the insulating layer  204 B with sufficient velocity to propagate the entire thickness of the insulating layer  204 B. And, those electrons transiting the insulating layer  204 B in the “thinned” region  205 B of the insulating layer  204 B, continue on, propagating through the metal overlay  208 B and exit the planar electron emitter (PEE) and integrated mask unit  200 B as free-space propagating electrons  216 B. The electrons  216 B emitted from the metal overlay  208 B are used for illuminating and exposing the resist layer on the substrate. 
     Another embodiment of a planar electron emitter (PEE) and integrated mask unit  200 C in the Metal-Insulator-Metal configuration is shown schematically in  FIG. 2C. A  conducting substrate  202 C may be composed of aluminum or other suitable electrically conducting material and serves as the source of electrons for the electron emitter. The substrate  202 C may be over-coated or chemically treated with a thin insulating layer  204 C which may be aluminum oxide (Al 2 O 3 ) or other suitable insulating material such as silicon dioxide. The surface of the insulating layer  204 C may be over-coated with metal overlay  208 C. In the configuration shown in  FIG. 2C , a mask unit  200 C may be formed by at least two different methods. A voltage may be applied between the conducting substrate  202 C and the metal overlay  208 C using a power supply  210 C of some sort. 
     In the first method of fabricating the mask unit  200 C, the metal overlay  208 C, prior to chemical etching, may be deposited or grown in sufficient thickness to absorb and/or scatter all electrons which may be entering the metal overlay  208 C from the insulating layer  204 C. The subsequent chemical etching of the metal overlay  208 C may be carried out until the etched thickness areas are sufficiently thin (similar to the thickness described above in  FIG. 2A ) such that electrons may propagate through the metal overlay  208 C only in those areas “thinned” by chemical etching. 
     In a second method of fabricating the mask unit  200 C requires no direct chemical etching of the exposed metal overlay  208 C surface. Instead, a layer of resist may be applied to the exposed surface of the metal overlay  208 C, and the desired mask unit  200 C structure may be fabricated in the resist by standard lithographic techniques. 
     In a third method of fabricating the mask unit  200 C, a thin layer of metal overlay  208 C is deposited over the insulating layer  204 C. Regions of the metal overlay  208 C are protected by portions of resist, while other portions of the metal overlay  208 C are exposed. In a second metal overlaying step, additional metal is grown at the exposed regions, so that the thickness of the metal overlay  208 C at those regions protected by the resist remain thin, while those portions  212 C that were exposed during the second metal overlaying step are relatively thick, advantageously too thick to permit passage of electrons that have tunneled through the insulator layer  204 C from the conducting substrate  202 C to have been grown to a greater thickness. 
     In a fourth method of fabricating the mask unit  200 C, a thick layer (typically at least 500 to 1000 Angstroms thick) of metal overlay  208 C is deposited over the insulating layer  204 C. The metal overlay  208 C is then coated with resist material and treated via standard lithographic techniques to define mask geometries similar to those described above in the third method of fabrication. The metal overlay  208 C is then chemically etched in those regions un-protected by resist, down to the insulating layer  204 C. A thin layer of metal overlay is then deposited in the “etched wells” and the resist material may then be chemically removed. Or as an alternative, the resist material may be chemically removed prior to overcoating with a second layer of metal. In this embodiment, the metal overlay  208 C may have two layers of metal in the “thick” region and 1 layer of metal in the “thinned” region. In both cases, electrons propagating through the metal overlay  208 C may be totally absorbed and/or scattered in the thick regions and may propagate through and exit as free particles only in those areas of the metal overlay  208 C that have been sufficiently thinned. 
     It is an object of the present invention to increase the useful life of planar electron emitters of the type described above. Experimental PEEs of the type described above have been fabricated, tested, and found to have a limited lifetime both in a pulsed and continuous mode of operation. It is believed that the failure mechanism may involve the in-diffusion of metal atoms from the metal overlay region, the anode, into the insulating region, which contributes to “shorting out” the device, resulting in a catastrophic failure. In response to this, different methods have been developed to increase the useful life of the REE. The methods are designed to modify the mobility characteristics of the in-diffusing metal atoms penetrating into the insulating layer. The first approach comprises periodically reversing the polarity of the voltage applied between the conducting substrate and metal overlay. This can be better understood by referring to the voltage vs. time diagram shown in FIG.  3 A. Applied voltage is represented on the vertical axis and time on the horizontal axis. A positive voltage represents a forward bias condition for the planar electron emitter, for example, the negative terminal of a battery or other electrical power source, connected to the conducting substrate, the cathode and the positive terminal connected to the metal overlay, the anode. A certain time period later, the voltage polarity is reversed, and during this period of reverse biasing, it is believed that at least some of the metal ions that have in-diffused into the insulating layer, driven by electric field forces, diffuse in the reverse direction back into the metal overlay region, thereby re-establishing the original Metal-Insulator-Metal (M-I-M) configuration. For the sake of functionally naming the configuration depicted in  FIG. 3A , and to identify the differences in the upcoming alternative methods, this configuration is referred to as the “completely symmetric” configuration for the reverse bias approach, in that both the magnitude of the reverse bias voltage and the time interval of the reverse bias are equal to their forward bias counterparts. This procedure has been tested experimentally, and has been shown to increase the useful life of the planar electron emitter by a factor of three-fold. 
     Another technique utilizing the “reverse bias” technique is illustrated in the timing diagram shown in FIG.  3 B. This configuration is the “asymmetric time interval” approach to applying the reverse bias. Although the magnitudes of the forward and reverse bias voltages are shown to be equal, the time interval for applying the reverse bias is shown to be approximately twice as long as for the forward bias case, although other ratios can be readily applied. Another perspective on this approach may be understood if one thinks of it as the “asymmetric energy” approach. The asymmetric energy approach incorporates the concept of an “engineering safety factor” to increase the probability that metal ions which may have diffused into the insulating layer during the forward bias condition, are driven back into the metal overlay region during the reverse bias time interval. 
     Another alternative technique utilizing the “reverse bias” technique is illustrated in the timing diagram shown in FIG.  3 C. This configuration is the “asymmetric voltage” approach to applying the reverse bias. Although the magnitudes of the forward and reverse time intervals are shown to be equal, the magnitude of the reverse bias voltage is larger than the forward bias case. In the illustrated embodiment, the positive voltage time was about one half of the reverse voltage time. It will be appreciated that other ratios can be readily applied. Similar to the “asymmetric time interval” approach discussed above, the approach incorporates the concept of an “engineering safety factor” (again, via the asymmetric energy argument) to increase the probability that metal ions which may have diffused into the insulating layer during the forward bias condition, are driven back into the metal overlay region during the reverse bias time interval. 
     It will be appreciated that different shapes of waveforms may be applied to the planar electron emitter (PEE) and integrated mask unit  102 . For example, rather than being applied as a square wave, as illustrated, the voltage may be applied with a sinusoidal or triangular waveform, or with some other type of waveform. Furthermore, the asymmetries in time and voltage may be reversed, with the longer times and greater voltages being applied in the forward bias direction. 
     It will also be appreciated that the type and magnitude of the reverse bias techniques described above may evolve over time. For example, the information gathered by the lifetime monitor, for example the internal impedance of the planar electron emitter PEE as the PEE ages, may be used to increase the life of the PEE by appropriately altering (e.g., increasing the time duration of reverse biasing or increasing the magnitude of the reverse bias voltage, or both) as the PEE ages. 
     The length of time the positive voltage is applied to the PEE depends on the desired duration of the electron beam. For example, under some conditions, a period of 100 msec time may be sufficient to expose the resist on the wafer. Furthermore, it may take a few 100&#39;s of msec to step the wafer to the next position. In such a case, the positive voltage may be applied to the PEE for a pulse length of 100 msec, while the negative voltage is applied during the at least a portion of the period that the stepper system needs to align the PEE to the next region on the wafer to be exposed. In other approaches, the voltage applied to the PEE may be stepped, and take on a number of different values during an exposure step-and-repeat cycle. 
     Another approach to extending the useful life of planar electron emitters centers on decreasing the mobility of the metal atoms to migrate (diffuse) into the insulating layer, in other words reducing the diffusion coefficient. Diffusion is a temperature dependent process. In many areas, the temperature dependence of the diffusion coefficient approximates an exponential behavior of the following form;
 
 D≅ constant· e   −(ΔE/KT)   (1) 
 
Where D is the diffusion coefficient, K is Boltzman&#39;s constant, T is the absolute temperature of the diffusing species, ΔE is the activation energy of the diffusion process, and the constant depends on the particulars of the materials. As shown is equation (1), the diffusion coefficient can become exceeding small as the temperature approaches absolute zero, in other words the potentially in-diffusing metal atoms may be nearly “frozen in place” at extremely low temperatures.
 
     In order to test the planar electron emitter&#39;s (PEE&#39;s) potential increase in useful life at low temperatures, experimental devices were fabricated and tested while being subjected to cooling at liquid nitrogen temperatures—approximately 77 K. The liquid nitrogen cooled devices out-lived their non-cooled counterparts on average by a factor of thirty- to forty-fold. Given this, it may be possible to extend the useful life of the planar electron emitters (PEE&#39;s) even further by subjected them to even lower temperatures by using, say, liquid helium temperatures—around 4 K. 
     One embodiment of a planar electron emitter (PEE) with integrated temperature control unit  400  is shown schematically in FIG.  4 . The planar electron emitter (PEE)  401  is shown in the Metal-Insulator-Metal (M-I-M) configuration, where the conducting substrate  402  may be attached to a mounting unit  403 . The mounting unit  403  may be a thermoelectric (TE) cooler or other type of heat extracting device to assist in cooling the planar electron emitter (PEE). The planar electron emitter (PEE)  401  is shown with the insulating layer  404  sandwiched between the conducting substrate  402  and metal overlay  408  similar to the configuration depicted in FIG.  2 A. The PEE  401  may be cooled to any desired temperature below room temperature, for example 20K below room temperature, below 100 K below room temperature, or further. 
     The mounting unit  403  may in turn be attached to a fluid cooling unit  410  having a series of fluid coolant ducts  412  for delivery of coolants such as liquid nitrogen, liquid helium, or other appropriate coolants. The cooling ducts are closed off form the vacuum chamber in which the wafer is positioned. The fluid cooling unit  410  may also have a temperature getter  414  device placed strategically to be at a lower temperature than the planar electron emitter (PEE)  401 , so as to preferentially attract airborne contaminates in the vacuum chamber to the getter  414  and not to the PEE  401 . 
     In one embodiment, the getter  414  device may be situated in the direct flow path of one (or more) of the fluid coolant ducts  412 , whereas the planar electron emitter (PEE)  401  may not be in the direct path of a fluid coolant duct  412 . In this configuration the getter  414  device may be cooled sooner than the planar electron emitter (PEE)  401  and may therefore preferentially attract airborne contaminants to its surface and not the planar electron emitter (PEE) during the initial start-up of the instrument. 
     In another embodiment, the fluid coolant unit  410  may time sequence the onset of delivery of cooling fluid to the individual fluid cooling ducts  412 . In this embodiment, the fluid coolant unit  410  may initially only route cooling fluid to the appropriate cooling ducts  412  delivering coolant fluid directly in the vicinity of the getter  414  device. As before, the getter  414  device may be cooled sooner than the planar electron emitter (PEE)  401  and may therefore preferentially attract airborne contaminants to its surface and not the planar electron emitter (PEE) during the initial start-up of the instrument. 
     It will be appreciated that other methods may be employed to preferentially cool the getter  414  device prior to cooling the planar electron emitter (PEE)  401 , and that alternative geometries for the getter device  414  may be employed, for example a ring shield around the PEE  401 . Also, the present invention contemplates further, that during the operation of the instrument, it may be useful to maintain the temperature of the getter  414  device at a temperature lower than the planar electron emitter (PEE)  401 . This may prove to be beneficial if the vacuum chamber becomes leaky or malfunctions and allows contaminants to enter the chamber after the onset of electron bombardment. 
     It will be appreciated that other approaches to controlling the temperature of the PEE  401  and temperature getter  414  may be employed. 
     In another embodiment, the present invention contemplates employing the “reverse bias” procedure(s) described earlier, in simultaneous concert with the low temperature method(s) described above. All possible combinations and permutations of the reverse bias and low temperature methods of extending the lifetime of the PEE are contemplated. 
     It will be noted that operation of a PEE without either reverse biasing or cooling leads to a lifetime on the order of 30,000-40,000 exposure shots of 100 msec each. A thirty-fold increase in the lifetime in the lifetime of the PEE, by cooling to liquid nitrogen temperatures, leads to a lifetime of approximately 1 million shots, which is in desired range for a commercial device Reverse biasing leads to a further increase in device lifetime. Accordingly, the inventions described here enable the PEE to be a serious contender for next generation lithographic procedures. As noted above, the present invention is applicable to lithographic techniques and is believed to be particularly useful in the manufacture of semiconductor components having feature sizes of 50 nm or less. The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.