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This invention relates to a microelectromechanical systems (MEMS) device and its method of manufacture. More particularly, this invention relates to a material and process for forming a MEMS electrical switch on a substrate.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
A thermal electrical switch may be formed, for example, by placing a conductive circuit adjacent to an elastically deformable flexor beam, and heating the conductive circuit by driving a current through it. The conductive circuit may be tethered to the elastically deformable flexor beam by a dielectric tether, such that the current does not flow to the deformable flexor beam from the conductive circuit. The conductive circuit may heat from Joule heating and expand relative to the deformable flexor beam, thus deflecting the deformable flexor beam to which it is tethered. If the elastically deformable flexor beam is coupled to an input terminal carrying an electrical signal, energizing the conductive circuit may deflect the elastically deformable flexor beam to a position in which it is in contact with another terminal, thereby connecting an input terminal to an output terminal and closing a switch. The conductive circuit and deformable component may therefore constitute an electrical switch. Such a switch may be used in, for example, telecommunications applications.
Because the thermal MEMS switch uses electricity and transmits an electrical signal, its various components may be made from conductive materials having specific electrical attributes. In particular, the conductive circuit of this thermal switch may need to have a finite resistivity, in order to carry the current, and be heated by the current. However, the elastically deformable flexor beam needs low resistivity, in order to carry the electrical signal without a large loss. However, in order to simplify the manufacture of such a switch, they are typically made out of the same material, and deposited in the same process step. As a result, the material forming the conductive circuit may not be optimized for its intended function, and the material forming the passive component may also not be optimized for its intended function.
Among the mechanical properties desired for the deformable flexor beam are high elasticity, such that the deformable flexor beam returns to its original position after deflection by the conductive circuit, high yield strength so that it is not permanently deformed by the deflection. Therefore, in addition to the low resistance needed by the flexor beam, it may also be advantageous to use a material with high elasticity and high strength. Among the characteristics desired for the conductive circuit are relatively low strength and somewhat higher resistance. Nonetheless, pure nickel is typically used for both the passive flexor beam as well as for the conductive circuit because of its generally acceptable resistance values and ease of well-known deposition processes, such as electroplating. However, nickel has relatively poor creep and strength characteristics, such that it is far from an ideal material from which to fabricate the switch components, particularly the flexor beam.
It is known that alloying the nickel with impurities such as manganese may be expected to improve the mechanical characteristics, especially in terms of creep and strength. However alloying also tends to raise the resistivity of the material because of increased electron scattering by the alloying material in the lattice or grain boundaries of the crystal matrix. Accordingly, alloys such as NiMn are generally not considered to be appropriate choices for the fabrication of the MEMS electrical switch, because the increased resistance would lead to unacceptable losses in signal strength through the switch.
Materials and processes are described here which address the above-mentioned problems, and may be particularly applicable to the formation of a MEMS thermal switch. The materials and processes described herein use a very small amount of manganese to form a NiMn alloy with less than about 0.1% manganese. It has been found that even this small additional amount of manganese is sufficient to dramatically improve the mechanical properties of the alloy. It has also been found that in contrast to raising the resistivity of the NiMn alloy as would be expected, the small amount of Mn actually reduces the sheet resistance of the NiMn alloy. Accordingly, this alloy may be appropriate for use both as the conductive circuit and the passive flexor beam of the thermal switch.
The NiMn alloy includes at least about 0.001% by weight and at most about 0.1% by weight of manganese and at least about 99.9% by weight of nickel. More preferably, the percentage by weight of manganese in the alloy is about 0.01%. This results in a material with higher recrystallization temperature, improved creep, strength, and elasticity, and with lower resistance than the pure Ni. It is hypothesized that the improved properties result from the migration of the Mn to the grain boundaries of the film, promoting larger grains and therefore lower resistance. The larger grains offset the increased scattering due to the alloying material in the lattice of the nickel, and therefore lead to a lower resistance structure.
The NiMn alloy may be well suited to the MEMS thermal switch application, which requires low creep, high strength, high elasticity and low resistance.
The material may be deposited by plating the alloy from a plating bath having appropriate concentrations of manganese and nickel to create the 0.01% Mn alloy. Because it is appropriate for use both as the conductive circuit as well as the flexor beam, both structures may be plated simultaneously in a relatively simple process flow described herein, to produce a MEMS thermal switch.
These and other features and advantages are described in, or are apparent from, the following detailed description.
Various exemplary details are described with reference to the following figures, wherein:
a-3d are SEM photomicrographs of samples of pure nickel and samples of 0.01% NiMn, showing the grain structures of the films;
The systems and methods described herein may be particularly applicable to a MEMS thermal switch. However, it should be understood that this embodiment is exemplary only, and that the material disclosed herein may be used in any application requiring structures having good mechanical characteristics as well as low resistance.
Similarly, applying a voltage between terminals 230 and 240 may cause heat to be generated in circuit 220, which drives flexor beam 210 in the direction 265 shown in
In order for MEMS thermal switch to open reliably, it is required for flexor beams 110 and conductive circuit 120 along with flexor beam 210 and conductive circuit 220 to return to nearly their original positions upon reversal of the sequence described above. If the structures do not return to nearly their original positions, the switch may fail to open properly or fail to close properly, by having contact 170 interfere unintentionally with the motion of contact 270 during the opening or closing operation.
The material of flexor beam 110 or 210 and conductive circuit 120 or 220 is all typically the same, plated nickel, because of its advantageous conducting properties, and ease of deposition. However, nickel also has some disadvantageous attributes, in particular, plastic deformation and/or creep, such that flexor beam 110 or 210 may not return to its original position upon the cessation of drive current through the conductive circuit 120 or 220, respectively. Accordingly, the use of pure nickel may undermine the reliability of MEMS thermal switch 1000. However, as described above, flexor beams 110 and 210 may also carry an electrical signal which is transmitted from an input electrode 155 to an output electrode. In order to transmit the signal without significant losses, the resistance of the flexor beam may need to be a low as possible. Accordingly, a conductor with very low creep, high strength, high elasticity and low resistance, and which is easy to manufacture may be needed to form the flexor beam 110.
It has been determined by the inventors, that the addition of a small amount of manganese (Mn) to nickel to form a NiMn alloy provides much improved mechanical properties, without increasing the sheet resistance of the alloy. In fact, adding a small amount (less than about 0.01% by weight) of manganese actually lowers the sheet resistance as shown in Table 1, below. This is particularly surprising in view of published resistance measurements (T. Farrell et al., J. Phys. C, 1968, Ser. 2, vol. 1, pp. 1359-1369) of a NiMn alloy having 0.5% Mn. The published results indicate that the 0.5% Mn alloy has about a 10% higher resistivity (Ice-point resistivity=7.02 μOhm-cm) than the pure Ni (Ice-point resistivity=6.31 μOhm-cm). Furthermore, this reference states that for dilute alloys, the residual resistance is proportional to the impurity concentration. In other words, the resistance of a NiMn film with any concentration of manganese is expected to be at least as high as the pure metal, and the amount by which it is higher depends on the concentration of the impurity Mn.
However, in contrast to the expectations set forth in the published literature, the resistance of NiMn at very low concentrations (much less than 0.1% by weight) of the impurity metal is actually lower than that of the pure metal. Experimental results summarizing the resistance values for NiMn films where the Mn concentration is on the order of 0.01% is shown in Table 1, below. Unless otherwise stated, all of the measured values of the films presented hereinafter refer to a 0.01% NiMn alloy or a pure Ni sample.
According to Table 1, the sheet resistance of the NiMn alloy is lower than that of pure Ni, in all cases, before and after baking. The baking step may have the effect of annealing the smaller grains into larger grains, thus reducing the resistance. In fact for the pure metal Ni, the sheet resistance after baking drops from about 22 ohms/square to about 19 degrees centigrade after a 350 degree bake for the pure nickel sample. In contrast, the 0.01% NiMn alloy has a sheet resistance of about 17 ohms/square, and remains relatively constant after baking. This data suggests that the NiMn alloy grains start out relatively large, and do not change dramatically with further annealing. Therefore, in all cases, the sheet resistance of the NiMn alloy films is at least about 10% lower than the sheet resistance of the pure Ni film.
Thus, it is hypothesized that the lowering of the sheet resistance of the alloy occurs as a result of the larger grain size of the alloy compared to the pure material. The larger grains are readily evident in SEM cross sections taken of the material; Exemplary SEM cross sections of the 0.01% NiMn alloy and pure Ni are shown in
The data in
In addition to the lower resistance, other mechanical properties of the material may be enhanced, or at least not appreciatively degraded as a result of the addition of the alloying manganese to the pure nickel metal. Among the other mechanical properties of interest for the NiMn alloy are its Young's modulus and hardness. These properties, and their comparison to pure Ni are shown in
As mentioned above, improving creep and raising the recrystallization temperature is a primary motivation for alloying of the nickel.
An exemplary method for fabricating the MEMS switch 1000 with the NiMn alloy will be described next. Particular attention will be given to the formation of the conductive circuit and flexor beam portions 120 and 110 of the MEMS switch 1000, as was shown in
A second exemplary step in fabricating the compact MEMS switch 1000 is illustrated in
The gold features 640, 645 and 460 may then be electroplated in the areas exposed by the photoresist, to form gold features 640, 645 and 460 and any other gold structures needed. The photoresist is then stripped from the substrate 620. The thickness of the gold features 640, 645 and 460 may be, for example, 1 μm.
A NiMn plating process used to produce a low resistivity NiMn beam 400 is described below. The method may be practiced using standard thin film electroplating equipment. The NiMn plating bath contains nickel sulfamate, manganese sulfamate, boric acid and a wetting agent in an aqueous solution. The wetting agent may be any standard commercially available nickel sulfamate wetting agent. The plating bath may be prepared having the composition set forth in Table 2:
The acidity of the plating bath may be important because it, along with temperature, current and concentration of the plating bath, it may affect the deposition rate of the NiMn alloy from the plating process. The pH of the plating bath may be adjusted by adding a small amount of sulfamic acid solution to the bath, as described below.
The following steps may be taken to prepare the plating bath of the composition set forth above:
According to the plating parameters set forth in Table 3, the plating bath is first heated to a temperature of about 51 degrees centigrade and the substrate 620 is submerged in the plating bath. The flow rate of the solution through the plating bath is set to be about 2.5 gal/min. Upon submerging the substrate 620 in the plating bath, a current density of about 8 mA/cm2 is applied between the electrodes of the plating apparatus until the desired thickness is achieved. The nominal plating rate under these conditions may be about 6 microns per hour. Alternatively, an alternating current waveform may be used to plate the NiMn from the plating bath. The plating results in the deposition of a beam about 13 μm tall of the NiMn alloy as cantilever 400 over the previously formed seed layer 630 and sacrificial layer 680. The alloy composition of the resulting cantilever 400 may be less than about 0.01% manganese and at least about 99.99% nickel. The weight % of the manganese in the NiMn alloy may be adjusted for different applications, by, for example, adjusting the Ni:Mn ratio of the plating bath from the specification of 73, as set forth above, to a lower number for a larger proportion of Mn, for example. This process was used to form the NiMn alloy material for the data shown in
Although not shown, it should be understood that dielectric tether 250, flexor beam 210 and drive loop 220 are formed in a manner similar to that described above for dielectric tether 450 and conductive cantilever 400.
The resulting MEMS device 1000 may then be encapsulated in a protective lid or cap wafer. Details relating to the fabrication of a cap wafer may be found in co-pending U.S. patent application Ser. No. 11/211,625, incorporated by reference herein in its entirety. Further details regarding the sealing of the cap wafer and the MEMS device 1000 in a hermetic seal may be found in U.S. patent application Ser. No. 11/211,622 incorporated by reference in its entirety.
It should be understood that one gold feature 645 may be used as an external access pad for electrical access to the MEMS switch 1000, such as to supply a signal to the MEMS switch 1000, or to supply a voltage to the terminals 130 or 140 in order to energize the drive loop 120 of the MEMS switch 1000, for example. The external access pad 645 may be located outside the bond line which will be formed upon the bonding of a cap layer to the substrate 620. Alternatively, electrical connections to MEMS switch 1000 may be made using through wafer vias, such as those disclosed in co-pending U.S. patent application Ser. No. 11/211,624, incorporated herein by reference in its entirety.
While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. While the embodiment described above relates to a microelectromechanical thermal switch, it should be understood that the techniques and materials described above may be applied to any of a number of other microelectromechanical devices, such as valves and actuators. Furthermore, details related to the specific design features and dimensions of the MEMS thermal switch are intended to be illustrative only, and the invention is not limited to such embodiments. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.
Number | Name | Date | Kind |
---|---|---|---|
5764056 | Mao et al. | Jun 1998 | A |
6859304 | Miller et al. | Feb 2005 | B2 |
6869690 | Hodgens et al. | Mar 2005 | B1 |
6902827 | Kelly et al. | Jun 2005 | B2 |
20020105396 | Streeter et al. | Aug 2002 | A1 |
20030222337 | Stewart | Dec 2003 | A1 |
20030228096 | Parker et al. | Dec 2003 | A1 |
20040031691 | Kelly et al. | Feb 2004 | A1 |
20070018077 | Puscasu et al. | Jan 2007 | A1 |
20080124565 | Carlson et al. | May 2008 | A1 |
Number | Date | Country | |
---|---|---|---|
20070222004 A1 | Sep 2007 | US |