|Publication number||US20040011432 A1|
|Application number||US 10/196,764|
|Publication date||Jan 22, 2004|
|Filing date||Jul 17, 2002|
|Priority date||Jul 17, 2002|
|Publication number||10196764, 196764, US 2004/0011432 A1, US 2004/011432 A1, US 20040011432 A1, US 20040011432A1, US 2004011432 A1, US 2004011432A1, US-A1-20040011432, US-A1-2004011432, US2004/0011432A1, US2004/011432A1, US20040011432 A1, US20040011432A1, US2004011432 A1, US2004011432A1|
|Inventors||Elizabeth Podlaha, Michael Murphy, Lakshmikanth Namburi|
|Original Assignee||Podlaha Elizabeth J., Murphy Michael C., Lakshmikanth Namburi|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (19), Classifications (19), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
 The development of this invention was partially funded by the Government under contract number DABT 63-95-C-0020 awarded by the Defense Advanced Research Projects Agency; and under grant number F49620-98-1-0476 awarded by the Air Force Office of Sponsored Research. The Government has certain rights in this invention.
 This application pertains to the electrodeposition of metal alloy microstructures, for example the electrodeposition of certain nickel-tungsten and nickel-iron alloys in high aspect ratio microstructures.
 When a metal layer or metal structure is needed in a microdevice, for example to provide structural integrity, or to act as a conductor of electricity or heat, the most common choice to date has been nickel. Nickel is inexpensive, is a good conductor of electricity and heat, and may readily be deposited by electrodeposition. Unalloyed nickel has certain drawbacks, however. It is relatively soft, and has a relatively high coefficient of thermal expansion. There is an unfilled need for metal microstructures that are harder than nickel to improve mechanical performance, corrosion resistance, and wear resistance.
 In many applications, it would be desirable to produce metal microstructures that have a lower coefficient of thermal expansion than that of nickel, to improve dimensional stability in devices that are subjected to varying temperatures. For example, in a LIGA micromolding process, temperatures typically vary during the course of molding by over 100° C., depending on the particular polymer (or loaded polymer) used. Nickel has a relatively high coefficient of thermal expansion, which can make the precise replication of sub-micrometer structures or features difficult. For example, the length of a 500 μm nickel feature varies by about 0.65 μm (or 13 μm/cm) over a temperature change of 100° C. Such dimensional fluctuations can make it difficult to precisely reproduce features; and in extreme cases can even make it impossible to reproduce certain features requiring a very narrow dimensional tolerance. It would be desirable to have a metal mold insert whose dimensions did not vary strongly with changes in temperature, to improve the consistency of the molding and to improve the characteristics of the molded products.
 On the macroscopic scale, it is known that certain alloys of nickel have greater hardness and lower thermal expansion than does elemental nickel, for example alloys of nickel with tungsten (NiW) or with iron (NiFe). The NiFe alloy called “Invar” has a very low coefficient of thermal expansion. On a macroscopic scale, it has been used in precision mechanical clocks, scientific instruments, and thermostats. Invar contains about 36% nickel and about 64% iron. There has been some prior work on making microstructures from these alloys, primarily NiFe. However, to the knowledge of the inventors there has been no prior work that has successfully produced a NiW microstructure taller than about 50 μm, nor an Invar NiFe microstructure taller than about 100-200 μm. Nor, to the knowledge of the inventors, has there been any prior work that has successfully produced high-aspect ratio NiFe microstructures with an Fe content greater than about 45%, while most NiFe alloy microstructures have had an Fe content of about 20% or less. No prior work known to the present inventors has achieved a controllable gradient in the composition of a metal alloy microstructure.
 W. Ehrfeld et al., “Materials of LIGA technology,” Microsystem Technologies, vol. 5, pp. 105-112 (1999) reviewed several alloys and composites that have been used in microdevices, including NiW and NiFe alloys.
 Most of the work that has been done in NiFe electrodeposition in microdevices has focused on their magnetic properties, particularly in alloys having a composition of about 20 wt % Fe (permalloy). See L. Romankiw, “A path: from electroplating through lithographic masks in electronics to LIGA in MEMS,” Electrochimica Acta, vol. 42, pp. 2985-3005 (1997); and V. Kanigicheria et al., “Enhanced adhesion of PMMA to copper with black oxide for electrodeposition of high aspect ratio nickel-iron microstructures,” Microsystem Technologies, vol. 4, pp. 77-81 (1998).
 Invar compositions have been electrodeposited as thin films, but the conditions used cannot be used to electrodeposit into deep micro-recesses to make high aspect ratio structures, although, to the inventors' knowledge, no one has previously identified the reasons why electrodeposition in deep micro-recesses has been difficult. See generally N. Phan et al., “Electrodeposition of Fe—Ni and Fe—Ni—Co alloys,” J. Appl. Electrochem., vol. 21, pp. 672-677 (1991).
 M. Maksimović et al., “The effect of the periodically changing current on the electrodeposition of Ni—Fe alloys,” vol. 31, pp. 325-334 (1987) reported that electrodeposited Ni—Fe alloys had a variable composition when they were electroplated (on a macroscale) using a pulsating current, a reversing current, or a sinusoidal alternating current superimposed on a direct current.
 On a macroscale, pulsed electroplating techniques have been used to make deposited films of metal brighter and smoother, to alter morphology and grain size. Reviews of the state of the art in pulsed electroplating techniques include J. Dini, section 10 in “Current Modulation Techniques,” M. Schlesinger et al., Modern Electroplating, pp. 81-83 and 99-101 (John Wiley & Sons, 4th Ed., 2000); and F. Mueller, “Pulse Plating & Other Myths,” Plating & Surface Finishing, pp. 54-55 (April 2000).
 Pulsed electroplating has also been reported in a few cases as being used for microdevices, but with pulse time scales comparable to those that have been used for macroscale pulsing; i.e., there does not appear to have been any previous express recognition of the differences between pulsed electroplating on the macroscale and the microscale, particularly into deep microrecesses.
 B. Löchel et al., “Ultraviolet depth lithography and galvanoforming for micromachining,” J. Electrochem. Soc., vol. 143, pp. 237-244 (1996) reported the pulsed plating of microdevices with a NiFe alloy containing 15% Fe, as well as pulsed plating with gold. The pulse mode for gold was conducted at 222 Hz (corresponding to a cycle time of about 4.5 μs) and a duty cycle of 0.44. The NiFe films were apparently less than about 100 μm tall, and there was-no mention of the distribution of the composition in any of the resulting deposits.
 B. Löchel et al., “Fabrication of magnetic microstructures by using thick layer resists,” Microelectronic Eng., vol. 21, pp. 463-466 (1993) reported the pulsed plating of microdevices with a NiFe alloy containing a maximum of 55% iron. The authors described an optimized deposition with a duty ratio of 0.4, a frequency of 250 Hz, and a pulse current density of about 20 mA/cm2. Resist patterns with structure sizes down to 7 μm were reported in a thickness of about 40 μm, or an aspect ratio of nearly 6.
 B. Löchel et al., “Galvanoplated 3D structures of Micro Systems,” Microelectronic Eng., vol. 23, pp. 455-459 (1993) reported the pulsed electrodeposition of Ni/Fe alloys having aspect ratios up to 10 (6 μm feature in a 60 μm resist layer). The authors described an optimized deposition with a duty ratio of 0.5, a frequency of 100 Hz, and a pulse current density of about 60 mA/cm2. Fe/Ni alloys with about 15% iron were reported to be ferromagnetic.
 S. Leith et al., “In-situ fabrication of sacrificial layers in electrodeposited NiFe microstructures,” J. Micromech. Microeng., vol. 9, pp. 97-104 (1999) reported electrodeposition with pulsed electrolyte agitation, followed by etching, to produce a range of layered NiFe magnetic alloys having differing Fe composition (e.g., 20, 48, 55, and 68 mol %), but apparently having low aspect ratios.
 V. Landa et al., “Structural properties of electrodeposited nickel-molybdenum and nickel-tungsten alloys,” Plating and Surface Finishing, pp. 128-133 (May 1987) discusses the structural properties of several electrodeposited (macroscale) nickel-molybdenum and nickel-tungsten alloys. The authors reported that changes in pH had only a slight influence on the tungsten content of nickel-tungsten alloys.
 C. Huang et al., “Study of stress reducers in nickel-tungsten electroforming baths,” Plating and Surface Finishing, pp. 79-83 (December 1999) reported that various stress reducers such as sodium benzene sulfonate and phthalimide reduced stress in several electrodeposited (macroscale) nickel-tungsten alloys.
 We have discovered metal alloy microstructures, made of alloys such as NiW, NiFe, and other alloys, and a method of making those structures, where the microstructures may have heights up to 500 μm or greater, and where the alloy composition may optionally have a controllable gradient if it is desired to impart different properties to different parts of a structure. The novel microstructures may be designed to be harder than conventional nickel microstructures, or to have lower coefficients of thermal expansion, or both. The appropriate use of pulsed electrodeposition allows the production of the novel microstructures. We have also discovered appropriate electrolyte and pulse conditions to make Invar and Invar-like NiFe compositions. While both types of alloyed microdevices have improved hardness and reduced coefficients of thermal expansion, we expect that the NiW alloys will be primarily useful where increased hardness is important, for example in micro-gears and other microdevices with moving parts that would benefit from increased hardness at points of contact; while the NiFe alloys will be primarily useful where a small coefficient of thermal expansion is desirable. The new techniques are especially useful for plating alloys into deep recesses of a microstructure. Prior techniques have not been particularly effective at plating alloys into deep recesses. We believe that we have discovered one of the principal obstacles to plating alloys into deep recesses—an obstacle that had not previously been recognized. We have also discovered a pulsed electroplating technique for overcoming this previously unrecognized obstacle. The novel technique may also be used in electrodepositing other metal alloys into deep recesses of microstructures.
 For present purposes, a “deep recess” may be considered to be one having a depth of about 50 μm or greater, preferably about 200 μm or greater, most preferably about 500 μm or greater, and having an aspect ratio greater than about 1, preferably greater than about 5, most preferably greater than about 10.
 The electrodeposition of metals creates local zones of high pH. In those conventional, macroscale, electrodeposition systems in which hydroxyl ions are generated as a side reaction or as a reaction product, convection eddies and other fluid movement tend to sweep away the hydroxyl ions, so that the production of hydroxyl does not adversely affect electrodeposition. We have discovered, however, that in the deep recesses that are typical of high aspect ratio microstructures (e.g., 200 μm, 500 μm, or even deeper), these eddies and fluid movements may be insufficient to remove hydroxyl anions or other reaction or side reaction products. The build-up of hydroxide ions or other products interferes with the electrodeposition. High pH can, for example, cause precipitation from the electrolyte, and the resulting precipitate layer can effectively stop further electrodeposition. We have discovered that proper combinations of electrolyte and pulsed electrodeposition (relatively short current or “on” times, relatively long relaxation or “off” times) can prevent the build-up of excess hydroxyl, even in deep recesses, allowing electrodeposition of much higher structures than is otherwise possible. The relaxation times used in our pulse plating are typically an order of magnitude or more longer than the relaxation times that have been used in conventional pulsed electroplating techniques. In conventional pulsed electroplating on a macroscale, pulsed current promotes small, scattered sites of nucleation, and thus smoother films, or it permits the more efficient transport of reactants, or both. Although transport of reactants is also a consequence of the pulsed electrodeposition used in the present invention, the principal reason for its use is the transport of unwanted reaction products such as hydroxyl ions away from the site of electrodeposition, thereby allowing electrodeposition to continue longer and more efficiently than it otherwise would.
 In practicing the novel process, the duty cycle is preferably less than about 0.2, more preferably less than about 0.125, most preferably less than about 0.1. The on time is preferably greater than about 0.1 second, most preferably greater than about 1 second. Depending upon the particular setup, the on time will often preferably be less than about 60 seconds, as that is often approximately the time that it takes to reach steady state.
 Although the initial embodiments of this invention have been the pulsed electrodeposition of microdevices formed of NiW or NiFe alloys, the novel method may also be used in other applications where it is desired to electroplate metals or metal alloys into deep microrecesses, and where the ability to electrodeposit would otherwise be limited by the buildup of high pH or by other reaction products or side reaction products. For example, we have begun also using the novel technique to electrodeposit NiFeW. We also plan to use the novel technique to electrodeposit W with any one or more of the metals Ni, Fe, and Co, in varying proportions of the different metals.
FIG. 1 depicts an electron micrograph of typical NiW structures 500 μm tall produced by the novel method.
FIG. 2 depicts the measured weight percentage of tungsten along the length of a 500 μm structure electrodeposited with pulses of constant current.
FIG. 3 depicts the measured weight percentage of tungsten along the length of a 300 μm structure deposited into a 500 μm recess with ramping current pulses.
FIG. 4 depicts an SEM micrograph of Invar NiFe deposits prepared in accordance with the present invention.
 Electrodeposition of NiW into Deep Recesses
 We have successfully grown NiW deposits 500 microns tall without cracks, and have successfully controlled the deposit concentration gradient where a gradient is desired. FIG. 1 depicts an electron micrograph of typical NiW structures 500 μm tall, electroplated into recesses in poly (methylmethacrylate) (PMMA); the PMMA was subsequently dissolved away. Since, to our knowledge, the electrodeposition of NiW microstructures over 50 microns tall had not previously been reported, we have been able to controllably electrodeposit this alloy to a depth ten times greater than has been reported in prior literature. We used relatively long relaxation times in pulsed electrodeposition, to allow unwanted reaction products (particularly hydroxide) more time to diffuse from the recesses, thereby enhancing the ability to electrodeposit alloy. During the relaxation periods (no electric current), reactants are also replenished near the electrode surface, permitting higher reaction rates during the pulses of current. We determined relaxation times based on a mass transport analysis and the depth of the recess. In a deep, narrow recess, all else being equal, the preferred relaxation time is approximately proportional to the depth of the recess. By contrast, in control experiments run without pulsed current, at the same applied current density, essentially no deposits were formed due to an accumulation of hydroxyl ion inside the recess, and the resulting precipitation of the electrolyte (data not shown).
FIGS. 2 and 3 demonstrate that the novel process allows the controlled electrodeposition of an alloy composition with either a relatively uniform composition (FIG. 2), or a progressively increasing concentration of tungsten (FIG. 3). FIG. 2 depicts the measured weight percentage of tungsten along the length of a 500 μm structure electrodeposited with pulses of constant current. FIG. 3 depicts the measured weight percentage of tungsten along the length of a 300 μm structure deposited into a 500 μm recess with ramping current pulses. (In the system shown in FIG. 3, the deposition was interrupted for analysis before the growth was complete.) The micropatterned recesses depicted in these particular figures were produced by x-ray synchrotron lithography, although the novel electrodeposition technique may be applied to deep recesses produced by other lithographic or non-lithographic techniques. The composition of cross-sections of the deposited material was measured by WDS x-ray analysis with an electron microprobe. In the former case, a uniform concentration profile was found to result from constant pulsed currents. By contrast, a graded alloy was formed by ramping the current while pulsing. The graded alloy had the added advantage that the processing time was significantly reduced (by roughly 33% in this example) due to the increase in current as the deposition progressed.
 Pulsing at higher current increases the concentration of tungsten in the alloy, and thus increases its hardness. Also, if such an alloy (or for that matter a NiFe alloy) is to be bonded to nickel, adhesion can be improved by having a higher nickel concentration near the nickel substrate, and progressively ramping up the concentration of W (or Fe) as one moves away from that substrate. However, it is preferred that the current density should not exceed 20 mA/cm2 at the start of the deposition at the bottom of a 500 μm microrecess. If the current density is too high, then the side reaction (generation of hydrogen) can become too rapid, requiring an extremely long “off” time and introducing potential coalescence of gas bubbles. Large gas bubbles, like precipitation products, can block the electrode surface and terminate (or strongly interfere with) electroplating. In the case of deeper or shallower recesses, the maximum current density should be scaled linearly as dictated by the limiting current, which is inversely proportional to the boundary layer thickness. The boundary layer thickness, in turn, may be estimated as the depth of the microrecess.
 The pulse times and current densities used in the examples depicted in FIGS. 2 and 3 were the following for the constant concentration deposit: 17.5 mA/cm2 for 15 s, followed by a relaxation time of 45 s at no current (duty cycle=0.25) on square posts 180 μm×180 μm×500 μm. The deposition rate was 100 μm per day. The average composition was 3.3±0.4 wt % W. We have also carried out a deposition at 10 mA/cm2, 15 s on and 75 s off (duty cycle=0.17). In the latter case, the average composition was 5.5±1.1 wt % W. The increase in W concentration is believed to have been more strongly influenced by the longer relaxation time than by the change in current density.
 A ramped-pulse current was produced with a change in current density from 5 to 15 mA/cm2 in steps of 0.75 mA/cm2 every 4 hours (i.e, 5, 10, 15 mA/cm2 each 8 hours). The applied current duration was 10 s, followed by a relaxation time of 30 s (duty cycle=0.25). The average growth rate was 118 μm per day, and the composition was continuously graded, as depicted in FIG. 3. Less frequent current adjustments resulted in a more discontinuous concentration profile, but with the advantage of a faster growth rate. For example, changing the pulsed current density in the following order: 5, 10, 15, and 20 mA/cm2 for 8, 8, and 48 hours, respectively, produced an average deposition rate of 118 μm per day.
 We have also measured the hardness of these microposts, and found that the microposts containing 3.3 wt % W had a hardness of 640 Knoop at the top, and 580 Knoop along the sidewalls, or about three times the hardness of unalloyed Ni electrodeposited from a sulfamate electrolyte, which is the present state of the art in LIGA processing.
 The electrolyte used in the examples depicted in FIGS. 2 and 3 contained 0.25 M sodium citrate, 0.4 M sodium tungstate, 0.2 M nickel sulfate at pH=10 adjusted with ammonia, at a plating bath temperature of 70° C.
 The technique of this invention may be used, for example, to make microstructure alloys comprising between about 60% and 99% by weight nickel; and between about 1% and about 30% tungsten, preferably between about 1% and about 6% by weight tungsten. The technique may be used to make microstructures between about 50 μm and about 2000 μm long, preferably between about 50 μm and about 500 μm; and having an aspect ratio greater than about 1.
 Electrodeposition of NiFe into Deep Recesses
 The Invar composition is of particular interest, because macroscopic Invar materials are known to have a low coefficient of thermal expansion. Thus Invar microstructures will also be useful in micro-molding applications in which high dimensional stability is desired. We have successfully deposited Invar and Invar-like alloys into deep micro-recesses by using long, constant current pulses, and long relaxation times when no current was applied. The electrolyte concentrations were modified from those of Phan et al. (1991), who reported electrodeposits of thin films of Invar. We found that the process was aided both by adding iron to the electrolyte, and by eliminating chloride.
FIG. 4 depicts an SEM micrograph of the Invar NiFe deposits. Plating conditions that we found to produce a uniform concentration of iron and nickel along the length of the electrode were an applied current density of 8.5 mA/cm2 for 10 s, followed by a relaxation with no current for 35 s (duty cycle=0.22). The plating rate was 50 μm per day.
 The electrolyte used in these examples contained 0.72 M nickel sulfamate, 0.155 M ferrous sulfate, 0.5 M boric acid, 0.001 M sodium lauryl sulfate, 0.011 M ascorbic acid and 0.008 M saccharin, pH=2.0 adjusted with sulfamic acid. The bath temperature was 40° C. It is preferred to add iron to the electrolyte, e.g., ferrous sulfate, to inhibit the formation of microcracks. The concentration of iron we used was less than that reported by N. Phan et al. (1991). Saccharin and ascorbic acid help to decrease the internal stress in the plated structure, and to minimize the formation of ferric ions in the electrolyte, respectively.
 We found that the concentration of ferrous sulfate was important in achieving the desired iron alloy concentration of ˜64%. We found an optimum electrolyte concentration of ferrous sulfate to be about 0.155M. Because iron species in the electrolyte can inhibit the reduction of nickel and thus change the composition of the deposit, the concentration of iron in the electrolyte can be particularly important. We found that at a slightly lower iron concentration (0.147 M), the NiFe microposts contained only 58 wt % Fe. We also found that using ferrous sulfate helped in making high quality structures, rather than the ferrous chloride that was typically used in earlier deposition techniques directed to making thin films. The sulfate reduced stress in the deposited structures as compared to the chloride. Deposits made with the chloride typically show undesirable microcracks. Without wishing to be bound by this theory, it is believed that sulfate leads to the incorporation of small amounts of sulfur into the alloy, reducing stress and cracking.
 A sample of Ni—Fe Invar microposts prepared in accordance with the novel method was sent to an outside laboratory for testing (Stork Technimet, New Berlin, Wisconsin). Under a 300° C. heat treatment, the coefficient of thermal expansion of the microposts was comparable to that of bulk Invar. At 265-300° C., the NiFe microposts had a measured coefficient of thermal expansion of 6.24 μm/m-° C. For comparison, the reported value for bulk Invar 36 from www.matweb.com (Carpenter) is 4.18 μm/m° C. We will also measure the coefficient of thermal expansion of the NiFe microposts at 20° C. (following preliminary heat treatment). We expect the value to be close to that for bulk Invar 36, about 1 μm/m-° C.
 The technique of this invention may be used, for example, to make microstructure Invar or Invar-like alloys comprising between about 58% and 70% by weight iron; and between about 30% and about 42% nickel; preferably about 64% iron and about 36% nickel by weight. The technique may be used to make microstructures between about 50 μm and about 2000 μm deep, preferably between about 50 μm and about 500 μm; and having an aspect ratio greater than about 1.
 “Super Invar” microstructures may also be made in accordance with this technique. A “Super Invar” alloy comprises by weight about 32% Ni, about 58%-65% Fe, and about 3%-10% Co. The concentration of cobalt in the alloy may be adjusted by adjusting the concentration of cobalt in the electrolyte bath, for example as cobalt sulfate. The concentration of cobalt in solution will not, in general, be directly proportional to the resulting concentration of cobalt in the deposited alloy, due to interaction with other metal ion reduction reactions.
 Different authors have sometimes used different definitions of “duty cycle.” Unless context clearly indicates otherwise, as used in the Claims below, and as used in the present specification, “duty cycle” refers to the ratio of (on time) to (on time plus off time) in a pulsed electrodeposition process. For example, a pulsed electrodeposition process having an on time of 10 seconds and an off time of 40 seconds has a duty cycle of 0.20 under this definition.
 The complete disclosures of all references cited in this specification are hereby incorporated by reference. Also incorporated by reference are the following instances of the inventors' own work, none of which is admitted to be prior art to this application: P. Datta et al., “Design and Fabrication of Thermomechanical Microactuator for High Temperature Applications,” poster presented at HARMST (High Aspect Ratio Microsystem Technology) 2001 Conference in Baden-Baden, Germany (Jun. 17-19, 2001); L. Namburi et al., “Electrodeposition of Ni—W alloys into deep recesses,” Abstract, Joint 200th Meeting of the Electrochemical Society and the 52nd annual meeting of the International Society of Electrochemistry (San Francisco, Calif., Sept. 3, 2001); L. Namburi, “Electrodeposition of nickel/tungsten alloys into deep recesses,” M. S. Thesis (Louisiana State University, Baton Rouge, La., submitted October 2001); and P. Datta et al., “A Microfabricated Recurve Bimetallic Actuator,” abstract for Micromachining and Microfabrication Process Technology VIII Conference, to be held January 2003, abstract submitted prior to the filing date of the present application, but, to the inventors' knowledge, not believed to have been published as of the filing date of the present application. In the event of an otherwise irreconcilable conflict, however, the present specification shall control.
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|U.S. Classification||148/336, 205/557, 148/426, 205/255|
|International Classification||C25D5/18, C22C19/03, C25D3/56, C22C38/08, C22C30/00|
|Cooperative Classification||C25D3/562, C22C19/03, C22C38/08, C25D5/18, C22C30/00|
|European Classification||C22C38/08, C22C19/03, C25D3/56B, C25D5/18, C22C30/00|
|Sep 4, 2002||AS||Assignment|
Owner name: BOARD OF SUPERVISORS OF LOUISIANA STATE UNIVERSITY
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:PODLAHA, ELIZABETH J.;MURPHY, MICHAEL C.;NAMBURI, LAKSHMIKANTH;REEL/FRAME:013265/0832;SIGNING DATES FROM 20020717 TO 20020812