Imported: 10 Mar '17 | Published: 27 Nov '08
USPTO - Utility Patents
A method for fabricating microcavity discharge devices and arrays of devices. The devices are fabricated by layering a dielectric on a first conducting layer. A second conducting layer or structure is overlaid on the dielectric layer. In some devices, a microcavity is created that penetrates the second conducting layer or structure and the dielectric layer. In other devices, the microcavity penetrates to the first conducting layer. The second conducting layer or structure together with the inside face of the microcavity is overlaid with a second dielectric layer. The microcavities are then filled with a discharge gas. When a time-varying potential of the appropriate magnitude is applied between the conductors, a microplasma discharge is generated in the microcavity. These devices can exhibit extended lifetimes since the conductors are encapsulated, shielding the conductors from degradation due to exposure to the plasma. Some of the devices are flexible and the dielectric can be chosen to act as a mirror.
This invention was made with Government assistance under U.S. Air Force Office of Scientific Research grant Nos. F49620-02-C-007 and F49620-03-1-0391. The Government has certain rights in this invention.
The present invention relates to microdischarge devices and, in particular, to methods for fabricating AC-excited devices and arrays.
Microplasma (microdischarge) devices have been under development for almost a decade and devices having microcavities as small as 10 m have been fabricated. (A microcavity is a cavity having a characteristic dimension (diameter, length of a rectangle, etc.) of approximately 500 m or less.) Arrays of microplasma devices as large as 4104 pixels in 4 cm2 of chip area, for a packing density of 104 pixels per cm2, have been fabricated. Furthermore, applications of these devices in areas as diverse as photodetection in the visible and ultraviolet, environmental sensing, and plasma etching of semiconductors have been demonstrated and several are currently being explored for commercial potential. Many of the microplasma devices reported to date have been driven by DC voltages and have incorporated dielectric films of essentially homogeneous materials.
Regardless of the application envisioned for microplasma devices, the success of this technology will hinge on several factors, of which the most important are manufacturing cost, lifetime, and radiant efficiency. A method of device fabrication that addresses manufacturing cost and lifetime while simultaneously providing for large arrays of devices is, therefore, highly desirable.
In a first embodiment of the invention, a microdischarge device is provided that includes a conducting substrate with one or more microcavities opening to a first face of the substrate. The conducting substrate may be, for example, a semiconductor, metal or a conducting polymer. An electrical contact is coupled to a second (or first) face of the substrate, forming a first electrode. A dielectric layer or layers substantially covers the first face of the substrate and the interior surface of the microcavity. A second electrode is disposed distally to the dielectric layer, with the term distally denoting disposition on the side of the dielectric layer opposite from the substrate. A time-varying potential is applied across the first and second electrodes. In some embodiments, the dielectric layer may include a first dielectric layered on a second dielectric. In other embodiments, an additional dielectric layer may be deposited, substantially covering the second electrode.
In another embodiment of the invention, a method for manufacturing a microdischarge device or an array of devices is provided. The method includes providing a conducting substrate with a microcavity. The microcavity includes an opening to a first face of the substrate. The conducting substrate may be a semiconductor, a metal, a conducting polymer, or a polymer coated with, or bonded to, a conducting film or sheet. An electrical contact is connected to a second (or first) face of the substrate, forming a first electrode, and a first dielectric layer is deposited on the first face of the substrate. A second electrode is provided adjacent to the microcavity opening and disposed distally to the first dielectric layer. The microcavity is filled with a specified discharge gas or vapor. In a specific embodiment, the microcavity extends to the second face of the substrate. In some embodiments, a second dielectric layer may be deposited, substantially covering the second electrode as well as the wall of the microcavity. In some embodiments the second electrode may be a screen or a conducting film.
In a further embodiment of the invention, a microdischarge device is provided. The device includes a dielectric substrate with two faces. The substrate is coated with a conducting layer on each face. At least one microcavity is formed in the substrate. This microcavity may extend to the other conducting layer. Electrical contacts are coupled to the conducting layers on each face of the substrate, forming electrodes. A time-varying potential of the appropriate magnitude applied across the electrodes can ignite a microplasma in the microcavity. This microdischarge device may advantageously be flexible in at least one direction.
In another embodiment of the invention, a method is provided for manufacturing a flexible microdischarge device. The method includes providing a dielectric substrate coated with a conducting layer on each face; forming a microcavity in the first face of the substrate; and connecting an electrical contact to each conducting layer. The microcavity is then filled with a specified discharge gas or vapor. In a specific embodiment of the invention, the method further includes the ability to flex the device in at least one direction.
As used in this description and in any appended claims, layers may be formed in a single step or in multiple steps (e.g., depositions). One layer or structure may be formed or layered on another structure or layer without being directly adjacent to, or in contact with, the other structure or layer.
In certain embodiments of the invention, a microcavity discharge device is obtained by overcoating with a dielectric layer (or layers) a first face of a conducting (or semiconducting) substrate into which a microcavity is formed. A first electrode is disposed distally to the dielectric layer, adjacent to the microcavity opening. The first electrode is overcoated with another dielectric layer. Electrical contact is made to the substrate, forming a second electrode. The microcavity (or microcavities) is backfilled with a discharge gas or vapor and then sealed. A microplasma discharge can be ignited in the microcavity when a time-varying (AC, pulsed DC, etc.) excitation potential of the appropriate magnitude is applied between the two electrodes. These devices can have considerably extended lifetimes as compared to conventional devices because electrode erosion can be negligible and the dielectric coatings will, if chosen properly, be resistant to chemical erosion and, if desired, reflective over a particular wavelength region. Further, when the substrate is a semiconductor, the substrate may be precisely fashioned by VLSI processing techniques.
In an embodiment of the invention, a process is provided for fabricating a microdischarge device. A flow diagram for the process is shown in FIG. 1. FIG. 2 illustrates the structure of an exemplary device 200 fabricated by the process. A conducting (or semiconducting) substrate 210 is provided that includes a microcavity 212. The substrate includes first 214 and second 216 faces and the microcavity 212 opens to the first face of the substrate. In some embodiments, the microcavity may also extend to the second face 216 of the substrate. The substrate may be a semiconductor, such as a p-type wafer of Si (6-8 -cm), (where is resistivity), a metal or a metal/polymer structure. In a specific embodiment of the invention, an inverted pyramidal microcavity is etched in the substrate by wet processing. In other embodiments of the invention, a wide variety of microcavities 212 having different cross-sectional geometries can be fabricated. A first dielectric layer 220 (typically 1 m or more in thickness), which may be Si3N4 or SiO2, is formed 110 on the first face 214 of the substrate including the interior surface of the microcavity. An electrical contact 225 is connected 120 to the second face (backside) 216 of the substrate, forming a first electrode. If the first dielectric layer 220 has been formed over the entire substrate, the substrate is first etched on the second face to permit electrical contact to the substrate. A second electrode 240 is then provided 130, adjacent to the microcavity opening and distal (with respect to the substrate) to the first dielectric layer 220.
In a specific embodiment of the invention, the second electrode may be a conductor, such as Ni, deposited on the first dielectric layer, adjacent to the microcavity opening and a second dielectric layer 250, such as silicon nitride, may be formed 160 on the second electrode, encapsulating the electrode and thereby prolonging the life of the device. In another specific embodiment of the invention, this second electrode can take on a variety of forms (screen, conducting polymer or semiconductor film, etc.). Electrodes in these forms can also be covered by one or more dielectric layers to further prolong the life of the device. Irrespective of the microcavity geometry chosen, minimizing the surface roughness of the cavity walls and the final dielectric surface within the microcavity is an important consideration.
In another specific embodiment of the invention, an additional dielectric layer 230, such as polyimide, may be deposited 170 on the first dielectric layer before the conductor, which forms the second electrode, is deposited. An additional dielectric layer, such as silicon nitride, can be deposited onto any of the above described structures of the device, if desired.
After any of the device structures described above are formed, the device may be evacuated by a vacuum system and may be heated under vacuum to de-gas the structure. Subsequently, the microcavity (or microcavities) in the device (or array of devices) may be backfilled 180 with a desired gas or vapor, or a mixture of gases and vapors. It is then generally desirable to seal the device or array by one of a variety of well-known processes such as anodic bonding, lamination or sealing with glass frit or epoxy. A microplasma discharge may be ignited in the cavity by applying a time-varying (AC, pulsed DC, bipolar, etc.) excitation potential 260 between the electrodes.
In a specific embodiment of the invention, wet chemical etching of Ge yields microcavities that are trapezoidal (slanted sidewalls and a flat bottom) or triangular (i.e., pyramidal microcavities) in cross-section. Furthermore, trapezoidal cross-section microcavities can be formed in Si as well.
Microdischarge devices have been fabricated using the process of FIG. 1, both as single pixels and as arrays. The arrays are considerably more uniform (in emission intensity) and better behaved as compared to earlier devices which were generally DC-excited and did not have a dielectric coating on the semiconductor electrode. FIGS. 3 and 4 show voltage-current (V-I) characteristics for 1010 pixel arrays of 5050 m2 inverted pyramid Si devices in which the Si electrode is overcoated with a dielectric. However, the second dielectric layer 250 has not been added.
The data of FIG. 3 are for operation with 600 Torr of Ne whereas, in FIG. 4, the Ne pressure is 700 Torr. Note that no external ballast is required. Data are shown for several different frequencies of a bipolar voltage waveform and electrical breakdown occurs (where data for a given excitation frequency fork) at about the same value of voltage, 190 V. Notice that, for a constant voltage, the RMS current increases with AC frequency, which means that the brightness of the array can be controlled with the excitation frequency. The labeling at the top of the figure indicates the average power dissipated per pixel. These values range from 0.5 to 1.25 mW/pixel which translates to 500 W to 1 kW for an array consisting of 1 million pixels. It is expected that these power dissipation levels can be lowered by approximately an order of magnitude by coating the anode with dielectric layer 250, as well. The data for a higher Ne pressure (700 Torr, FIG. 4) show a slightly lower power dissipation/pixel, a favorable result since operating the arrays at one atmosphere is optimal for packaging reasons.
Preliminary lifetime data for arrays having 27 pixels and operating at 5 kHz in Ne are shown in FIG. 5. After 100 hours of continuous operation, the output power drops to 71% of its initial output but it appears that most of this decline is due to poisoning of the gas by the exposed electrode. More sophisticated cleaning of the pixels before operating and covering the remaining electrode will likely extend the array lifetime dramatically.
FIG. 6 shows optical micrographs of microdischarge device arrays fabricated as described in connection with FIG. 1. A 200200 pixel array comprises pixels having inverted square pyramidal microcavities, produced by wet chemical etching in p-type Si wafers (6-8 -cm). A cross-sectional diagram of a single pixel is illustrated by FIG. 2. At the upper surface of the Si substrate, the dimensions of the pyramidal microcavity are 5050 m2. The tip of each pyramid microcavity lies 35 m below surface 214 of the Si substrate 210. After depositing 2 m of silicon nitride over the entire Si structure by plasma-enhanced chemical vapor deposition, an opening is etched in the dielectric on the backside of the substrate to accommodate an electrical contact. Around the perimeter of the microcavity aperture, 10 m of a dry etchable polyimide and 0.2 m of Ni are deposited, the latter of which serves as a second electrode. Depositing a second film of silicon nitride, typically 2-5 m in thickness, over the microcavity and substrate surface completes the device. After evacuating a finished array to 107 Torr and backfilling with Ne, an AC (sinusoidal) voltage with a frequency of 5-20 kHz is imposed between the Si substrate and Ni electrode. All experiments have been carried out at room temperature with Ne at a static pressure.
Voltage-current (V-I) characteristics for a 200200 pixel array (again, (50 m)2 devices), operating in 500-900 Torr of Ne and excited at a frequency of 10 kHz, are shown in FIG. 7. All voltages are expressed in terms of their peak-to-peak (p-p) values and the dashed horizontal line indicates the voltage (1 V) at which the array ignites when pNe=900 Torr. Measurements of the dependence of the array operating voltage on pNe show a monotonic decline with increasing pressure for 500pNe900 Torr, regardless of whether the driving frequency is 5, 10, or 15 kHz. For 500 Torr of Ne, the peak-to-peak operating voltages range from 725 to 785 V for 5 and 15 kHz, respectively, but fall to 660-680 V for pNe=900 Torr. Although only a slight variation in the radiative output power is observed over this pressure range, maximum radiative efficiency occurs for pNe at about 700 Torr and an excitation frequency of 10-15 kHz.
Returning to FIG. 6, several photographs of one of the 4104 (200200) pixel arrays fabricated to date are presented. The left-hand portion of the figure shows (top) the completed structure, fabricated in a 50 mm (2 inch) diameter Si (100) wafer, and the lower photograph is that of an array operating in 700 Torr of Ne at an excitation frequency of 10 kHz, and drawing 30 mA of current. A prominent characteristic of these arrays, exemplified by the optical micrographs on the right side of FIG. 6, is the pixel-to-pixel emission uniformity. Acquired with a telescope and a CCD camera, these images show portions of the array in operation and at two values of magnification. A false color image of the intensity contour produced by a 1713 pixel segment of a 200200 array is illustrated in FIG. 8. A neutral density filter inserted into the optical path to the CCD camera ensured that the image is not saturated. In the lower portion of FIG. 8 is a superposition of several lineouts from the digital image at the top of the figure. These lineouts, obtained from columns and rows of pixels, show that the pixel-to-pixel peak emission intensity is reproducible and uniform to within 10%.
FIG. 9 shows the variation of the device and array power consumption with the number of pixels. Results are given for a single device as well as 1010, 5050, and 200200 pixel arrays. The right-hand ordinate and open points (i.e., o) indicate the total power dissipated by the array (or single pixel), whereas the solid points (associated with the left-hand ordinate) present the data as normalized to the emitting area of the array. Although the total power consumed by an array grows with its size, the power dissipated per pixel is almost two orders of magnitude larger for a single pixel than that for the 200200 array. Current flow in the thin (0.2 m) Ni film anode presently in use is apparently a factor in this result and for those applications requiring large arrays and greater optical output power/pixel, employing a thicker, possibly electroplated, anode will be of value.
AC excitation of microdischarge devices offers several clear advantages with respect to DC-driven microplasma arrays, particularly when device lifetime is of primary importance. Since at least one dielectric layer (such as those in FIG. 2) provides a physical barrier between the plasma and electrode, erosion of the cathode by ion bombardment or the anode by electron sputtering is minimized.
Herein, and in any appended claims, we refer to a body as flexible if it can be bent without fracture in a radius of curvature no larger than 10 cm. A fracture will denote a break in a body due to deformation. Upper and lower and above shall be relative terms for convenience of description and without limitation as to the orientation of that body in space.
In another embodiment of the present invention, a process is provided for forming a microcavity discharge device that can flex in one or more directions. Exemplary structures 1000, 1001 for the device are shown in FIG. 10, and FIG. 11 is a flow diagram for the process 1100. A first electrically conducting film or substrate 1010 is provided 1110 and is then overcoated 1120 with a first dielectric layer 1020. The first dielectric layer includes a first face 1022 and a second face 1024. A second electrically conducting film 1030 is deposited 1130 on the first face 1022 of the first dielectric layer. A microcavity 1040 is then formed 1140 with an opening to the first face 1032 of the second conducting layer. The microcavity may be formed by microdrilling, chemical etching, laser machining, etc. Also, photolithographic and etching or photoablation techniques well known in the VLSI and MEMs communities may be used to fabricate arrays of microcavities. The microcavity will generally extend at least to the first dielectric layer 1020, as shown in FIG. 10A and, in some embodiments, may extend to the first conducting film 1010, as shown in FIG. 10B, or even extend completely through substrate 1010. Electrical contacts 1050, 1052 may be connected 1150 to each of the first and second conducting films, forming first and second electrodes. A second dielectric layer 1070 may be deposited 1160 on the face 1032 of the second conducting layer that is distal to the first dielectric layer, and on the interior surface of the microcavity. The materials and thicknesses of the layers may be such that the device is flexible. In some embodiments, flexibility is limited to flexing in a direction perpendicular to the faces of the layers, while in other embodiments such flexibility may also include the direction transverse to the faces of the layers.
In another specific embodiment of the invention, instead of depositing a conducting layer on the dielectric layer 1020, a second electrode is disposed above the first face 1022 of the first dielectric layer 1020. The second electrode may be coated with a dielectric layer. In other embodiments of the invention, one or more of the dielectric layers may be replaced by other non-conducting layers or structures.
In a specific embodiment of the invention, the overall thickness of the device is approximately 50 m and the characteristic dimension of the microcavity opening is approximately 50-100 m.
After any of the device structures described above are formed, the device may be evacuated by a vacuum system and may be heated under vacuum to de-gas the structure. Subsequently, the microcavity (or microcavities) in the device (or array of devices) may be backfilled 1170 with a desired gas or vapor. It is then generally desirable to seal the device or array (depending upon the materials chosen) by one of a variety of well-known processes such as anodic bonding, lamination or sealing with epoxy. A microplasma discharge may be ignited in the cavity by applying a time-varying (AC, pulsed DC, etc.) excitation potential (of the appropriate magnitude) between the electrodes.
A number of these devices have been fabricated and characterized. FIG. 12 is an optical micrograph of arrays of devices fabricated by the process of FIG. 11. A prominent characteristic of these arrays is, as noted previously in connection with another microcavity discharge device structure, the pixel-to-pixel emission uniformity. Another attractive aspect of this device structure is its robust behavior. Tests have shown, for example, that layer 1030 in FIG. 10A can be scratched, microcavity 1040 can be of irregular cross-section from pixel to pixel, and the microcavity extending completely through film (or substrate) 1010, and yet the devices will still work well. The presence of dielectric layer 1070, typically 1 m or more in thickness, is a key factor in this result. FIG. 13 is a graph showing the peak-to-peak turn on voltage for a 1212 pixel array of the device of FIG. 10B as a function of the pressure of the discharge gas. Each microcavity is approximately 100 m in diameter and results are shown for three values of AC (sinusoidal) excitation frequency. FIG. 14 shows the voltage-current (V-I) characteristics for the 1212 arrays of FIG. 13 at a Ne pressure of 700 Torr. The microcavities are filled with Ne gas at 700 Torr and discharges are excited at several AC frequencies. FIG. 15 is a graph of voltage-current (V-I) characteristics for the 1212 pixel arrays of the device of FIG. 10B, but for a Ne gas pressure of 900 Torr.
Returning to FIG. 2, in another embodiment of the invention, the first and second dielectric layers, 220 and 250, can be chosen for their optical, as well as electrical (and protective) characteristics. If the microdischarge in microcavity 212 is producing one or more wavelengths of interest and extracting that light from the microcavity efficiently is important, the materials chosen for layers 220 and 250, as well as their thicknesses, can be selected to act as a mirror. Dielectric mirrors comprising a stack of dielectric films in which the index of refraction alternates from high to low are well-known in the art. Mirrors manufactured for lasers generally have an odd number of dielectric layers and a structure denoted (HL)nH where H denotes a layer of the high index dielectric, L a layer of the low index dielectric, and n the number of pairs of layers. The dielectric mirror used for layers 220 and 250 can be chosen to have appropriate indices of refraction and layer thicknesses so that high reflectivity is realized at the wavelength(s) (and viewing angle) of interest. In addition to silicon dioxide, other dielectrics having good electrical properties that are also useful for mirrors are silicon nitride, polyimides, and titanium dioxide. At a wavelength of 500 nm, the indices of refraction of SiO2, Si3N4, polyimide, and TiO2 films are 1.45, 2.0, 1.6-1.8 (depending upon the polyimide structure) and 2.62, respectively. The two layer stack (layers 220 and 250 of FIG. 2) will yield significant reflectivities in the visible when a suitable dielectric is used. If higher reflectivity is desired, one or more additional dielectrics can be deposited onto layer 250 in FIG. 2. It should also be noted that the reverse is also truenamely, the first and second (as well as additional) dielectric layers can be designed to suppress the emission of a specific range of wavelengths from the microcavity. The design of multi-dielectric layer reflective or bandstop coatings is well known in the optics community. Similar techniques can be used for the flexible microcavity discharge devices described above, such as the device shown in FIG. 10.
Similarly, it is of course apparent that the present invention is not limited to the aspects of the detailed description set forth above. Various changes and modifications of this invention as described will be apparent to those skilled in the art without departing from the spirit and scope of this invention as defined in the appended claims.