APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 13
24 SEPTEMBER 2001
Independently addressable subarrays of silicon microdischarge devices: Electrical characteristics of large „30Ã30… arrays and excitation of a phosphor S.-J. Park and J. G. Edena) Department of Electrical and Computer Engineering, Laboratory for Optical Physics and Engineering, University of Illinois, Urbana, Illinois 61801
J. Chen and C. Liu Department of Electrical and Computer Engineering, Microelectronics Laboratory, University of Illinois, Urbana, Illinois 61801
共Received 29 May 2001; accepted for publication 10 July 2001兲 Large arrays 共up to 30⫻30兲 of microdischarge devices having separately addressable subarrays have been fabricated in Si and operated continuously in Ne, Ne/Ar, and Ne/Xe gas mixtures at pressures up to 800 Torr. Eight 3⫻3 arrays fabricated on the same substrate operate simultaneously at voltages as low as 210 V in 400 Torr of Ne and exhibit lifetimes beyond 19 h, or approximately 1 order of magnitude larger than those for earlier arrays in which all devices have a common anode. Four 15⫻15 arrays have also been tested and, when operated in Ne/Ar or Ne/10% Xe gas mixtures, generate intense fluorescence in the green from a phosphor over an area of 16 mm2 . © 2001 American Institute of Physics. 关DOI: 10.1063/1.1401791兴
Although microdischarges were first studied by White1 and have been at the heart of the plasma display panel since its inception in the 1960s by Bitzer and Slottow,2 the recent adaptation of microfabrication and semiconductor processing techniques to the design and realization of microdischarge devices in Si and metal/polymer structures has given rise to a new family of photonic devices.3– 6 Specifically, very large scale integration 共VSLI兲 fabrication processes allow for precise optical array structures to be explored, a capability that is particularly attractive for microdisplays and sensors. Recently, we reported the fabrication and characteristics of arrays of microdischarge devices having square pyramidal Si cathodes.7 Early arrays had a single polymer dielectric and arrays larger than 6⫻6 suffered from lifetimes of only a few minutes and the preferential ignition of devices at the perimeter of the array. The subsequent introduction8 of a composite dielectric and a revised fabrication process prolonged the lifetime of 10⫻10 arrays to 2 h and improved array reliability and ignition characteristics significantly, but extending this design to yet larger arrays was problematic. This letter describes the performance of arrays of pyramidal cathode microdischarge devices, as large as 30⫻30, in which the array comprises several subarrays excited independently. The size and lifetimes of these arrays are at least an order of magnitude larger than those reported previously.7,8 Specifically, 30⫻30 arrays operate reliably at Ne gas pressures up to 800 Torr and voltages as low as 300 V. Back illumination of a green phosphor with a 30⫻30 array through a sapphire window produces intense fluorescence over an area of 16 mm2 . Eight 3⫻3 arrays have been operated in Ne and Ne/Ar gas mixtures at voltages as low as 210 V for an a兲
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operating current of ⬃9 mA. Results to date suggest that this design can be extended to considerably larger arrays. Much of the procedure for fabricating the pyramidal cathode Si microdischarge arrays has been described previously.7,8 All of the devices studied in these experiments have 50 m square cathodes and composite dielectrics 共⬃0.9 m SiO2 /0.1–0.2 m Si3N4 /8 m polyimide兲 as in Ref. 8 共with the exception of the thinner Si3N4 film兲, but the Ni anode is patterned, and the device layout arranged, so as to yield subarrays that can be excited individually. The interdevice separation within subarrays is fixed at 50 m. Voltage–current (V – I) characteristics for as many as eight 3⫻3 subarrays of microdischarge devices fabricated on the same Si substrate and excited simultaneously are pre-
FIG. 1. V – I characteristics for 3⫻3 arrays of pyramidal cathode microdischarge devices fabricated on the same Si substrate. Data are shown for as many as eight 3⫻3 arrays and all results are for 50 m square devices spaced by 50 m within the 3⫻3 array. The data were obtained for a Ne pressure of 400 Torr and an external ballast of 250 k⍀.
0003-6951/2001/79(13)/2100/3/$18.00 2100 © 2001 American Institute of Physics Downloaded 09 Apr 2002 to 128.174.97.173. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
Appl. Phys. Lett., Vol. 79, No. 13, 24 September 2001
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FIG. 3. Lifetime data for eight 3⫻3 arrays operating in 800 Torr of Ne with 86 k⍀ of external ballast. The relative output intensity of the arrays in the 300–1000 nm spectral region over a ⬃20 h time period is shown.
imide dielectric alone. Notice, too, that the minimum operating voltage, regardless of the number of subarrays, is ⬃210 V. We expect this value to fall as the dielectric thickness is reduced and the cathode design is optimized.
FIG. 2. 共Color兲 Photographs of an array comprising eight 3⫻3 subarrays of (50 m) 2 devices: 共Top兲 scanning electron micrograph of a portion of the structure; 共Middle兲 optical micrograph of the entire array; 共Bottom兲 micrograph of the array operating in 500 Torr of Ne at a voltage and current of 340 V and 0.49 mA, respectively.
sented in Fig. 1 for a Ne gas pressure of 400 Torr and an external ballast of 250 k⍀. A single 3⫻3 array has an ignition voltage similar to that for 3⫻3 arrays reported recently7 and having solely a polymer dielectric but this new array draws ⬃4 mA. As progressively more 3⫻3 arrays are energized, the V – I characteristic maintains essentially the same functional form and the current rises, but does not scale linearly with the number of devices in operation. Also, individual subarrays as well as larger ensembles exhibit a positive differential resistivity over the entire current range studied. Initially as high as ⬃80 k⍀, the differential resistance tapers off to a value of ⬃3 k⍀ for currents above ⬃30 mA. As noted previously,8 the introduction of the composite dielectric dramatically lowers the differential resistance of the plasma V – I characteristic relative to that for the poly-
FIG. 4. 共Color兲 共Top兲 30⫻30 array, comprising four 15⫻15 subarrays, operating in 400 Torr of Ne; 共Bottom兲 green emission produced when a phosphor is backilluminated through a thin sapphire substrate by a 30⫻30 array operating in a gas mixture of 500 Torr of Ne and 50 Torr of Xe. Downloaded 09 Apr 2002 to 128.174.97.173. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 79, No. 13, 24 September 2001
Photographs of an array structure consisting of eight 3⫻3 subarrays are shown in Fig. 2. A scanning electron micrograph of a portion of the array is at the top of the figure. As noted earlier, all of the individual devices are 50 m square and the interdevice spacing within a subarray is also fixed at 50 m. The middle image of Fig. 2 is an optical micrograph showing the entire structure, viewed normal to its surface. A photograph, recorded with a telescope and a charge coupled device camera and all eight arrays operating in 500 Torr of Ne at a voltage and current of 340 V and ⬃0.5 mA, is at the bottom of Fig. 2. At higher gas pressures, the total current drawn by the multiple arrays increases considerably 共see Fig. 1兲. All of the devices produce stable glow discharges and spatially uniform emission in Ne. Also, similar behavior is observed for Ne/Ar gas mixtures. Because of the Penning ionization of Ar, however, the emission spectrum of 150 Torr Ne/150 Torr Ar mixtures, for example, is dominated by Ar lines, giving rise to bluish–white fluorescence. The introduction of subarrays has a dramatic impact on device lifetime. Representative radiant output data for eight 3⫻3 arrays operating in 800 Torr of Ne with 86 k⍀ of external ballast are illustrated in Fig. 3. Maximum intensity over the 300–1000 nm spectral region 共measured with a calibrated pin diode兲 is reached 3– 4 h following startup. After ⬃19 h of continuous operation, the output intensity of the array has fallen to ⬃70% of its peak value. The average lifetimes of these new arrays represent an order of magnitude increase over those of earlier arrays7,8 in which the devices shared a common anode. Arrays as large as 30⫻30 and comprising four 15⫻15 subarrays have been fabricated and tested to date. A photograph of a 30⫻30 array of (50 m) 2 microdischarge devices operating in 400 Torr of Ne is shown by the upper portion of Fig. 4. An output power of 75 W is measured for this array in a solid angle of 4.5⫻10⫺2 sr when the discharge voltage and current 共drawn by the entire structure兲 are 1130 V and 3.48 mA, respectively. The maximum output power observed from the four arrays 共for a total current of 14.5 mA兲 is 0.25 mW 共again, in a solid angle of ⬃5⫻10⫺2 sr兲, which corresponds to an efficiency consistent with the well-known value for Ne (10⫺4 – 10⫺5 ). Also, although improvement in the spatial uniformity of the emission generated by the arrays is necessary, these arrays are more than 1 order of magnitude larger than those previously operated successfully. In an effort to explore the potential of this integrated structure for display applications, a film of a green phosphor (Zn2SiO4 :Mn) was deposited onto a 250 m thick sapphire wafer 共cut to 6 mm⫻4 mm兲 which was mounted ⬃1.1 mm
above a 30⫻30 array. Back illumination of the phosphor through the sapphire substrate yields intense green fluorescence when the microdischarge array is operated with Ne/Ar or Ne/Xe Penning gas mixtures. A photograph of the emission produced when the gas mixture in the microdischarge array is 500 Torr Ne and 50 Torr Xe is given by the lower half of Fig. 4. With 250 k⍀ of ballast and a voltage and current of 760 V and 5.6 mA, respectively, the green power radiated into a solid angle of 5⫻10⫺2 sr was measured to be 14 W. The emission from this structure is quite bright, even when viewed from a distance of several meters. Tests conducted to date have produced stable green emission on a continuous basis for more than 2 h. It is evident that multicolor phosphor pixels can readily be excited by microdischarges in this structure. In summary, arrays of microdischarges as large as 30⫻30 and consisting of subarrays excited independently, have been fabricated and tested. Patterning the anode and ballasting the subarrays, either individually or as a group, improves the reliability and performance of the entire array. Operating at pressures up to and beyond 1 atm at voltages as low as 210 V at present, the arrays described in this letter already exhibit lifetimes beyond 19 h and produce strong fluorescence from phosphors. These devices provide spatial resolution superior to that offered currently by plasma display panel pixels and microdischarge array fabrication appears to be readily scalable in emitting surface area from the value of 16 mm2 reported here, to at least several tens of cm2 . Furthermore, the ability to integrate this new photonic source with multichannel photodetectors and control electronics suggests the suitability of microdischarge arrays for chemical sensing applications. The technical assistance of K. Collier and discussions with C. M. Herring and J. J. Ewing are gratefully acknowledged. This work was supported by the U.S. Air Force Office of Scientific Research under Grant Nos. F49620-00-1-0372, F49620-99-1-0106, F49620-99-1-0317, and AF ETA 0043. A. D. White, J. Appl. Phys. 30, 711 共1959兲. See, for example, R. L. Johnson, D. L. Bitzer, and H. G. Slottow, IEEE Trans. Electron Devices ED-18, 642 共1971兲 and references cited therein. 3 J. W. Frame, D. J. Wheeler, T. A. DeTemple, and J. G. Eden, Appl. Phys. Lett. 71, 1165 共1997兲. 4 J. W. Frame and J. G. Eden, Electron. Lett. 34, 1529 共1998兲. 5 S.-J. Park, C. J. Wagner, C. M. Herring, and J. G. Eden, Appl. Phys. Lett. 77, 199 共2000兲. 6 C. J. Wagner, S.-J. Park, and J. G. Eden, Appl. Phys. Lett. 78, 709 共2001兲. 7 S.-J. Park, J. Chen, C. Liu, and J. G. Eden, Appl. Phys. Lett. 78, 419 共2001兲. 8 S.-J. Park, J. G. Eden, J. Chen, C. Liu, and J. J. Ewing, Opt. Lett. 共in press兲. 1 2
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