Abstract:
The present invention generally relates to methods for increasing the lifetime of MEMS devices by reducing the landing velocity on switching by introducing gas into the cavity surrounding the switching element of the MEMS device. The gas is introduced using ion implantation into a cavity close to the cavity housing the switching element and connected to that cavity by a channel through which the gas can flow from one cavity to the other. The implantation energy is chosen to implant many of the atoms close to the inside roof and floor of the cavity so that on annealing those atoms diffuse into the cavity. The gas provides gas damping which reduces the kinetic energy of the switching MEMS device which then should have a longer lifetime.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to introducing a gas into a micro cavity to enhance the lifetime of a micro-electromechanical system (MEMS) devices. 
     2. Description of the Related Art 
     A digital variable capacitor (DVC) utilizing MEMS technology operates by having a switching element of the MEMS device move between a state of high capacitance and a state of low capacitance. In a state of high capacitance, the switching element is in a position adjacent an RF electrode. In a state of low capacitance, the switching element is in a position adjacent to another electrode spaced from the RF electrode, or more specifically, away from an insulating layer that is disposed on the RF electrode. The switching element may also be moved to ground whereby the switching element is adjacent neither the RF electrode nor the other electrode. 
     During the lifetime of the MEMS device, the switching element cycles between the various states (i.e., high capacitance, low capacitance and ground). For a cycle, the switching element moves from the ground state to either the high or low capacitance state. After the cycle is completed, and before the next cycle, the switching element returns to the ground state. Then, a new cycle begins whereby the switching element moves to either a high or low capacitance state, or remains in the ground state. This corresponds to a physics movement of a plate which either touches the insulating layer overlying the RF electrode or touches the roof of the cavity in which it is housed. 
     There are only a finite number of times that the switching element can move before the MEMS device fails. With each movement of the switching element, the MEMS device accumulates a finite amount of damage which, given enough total cycles, results in failure. The magnitude of the finite damage to the MEMS device or the roof of the cavity or the insulating layer overlying the RF electrode is proportional to the impact speed of the fast moving switching element as the switching element is brought into contact therewith. In the above example the contact can cause material to be ejected from the exposed surfaces which then enter between the plates of the capacitor, reducing the closest distance that the ground MEMS plate can make to the insulating layer over the RF electrode. Thus, the maximum possible capacitance is reduced. 
     Therefore, there is a need in the art for increasing the lifetime of MEMS devices in DVCs by reducing the impact velocity of the switching element in a MEMS device as the switching element makes contact with various surfaces within the device cavity. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to methods for increasing the lifetime of MEMS devices by reducing the impact velocity of a switching element in the MEMS device. Rather than leaving the encapsulated MEMS device in a vacuum cavity, atoms are implanted into the cavity to introduce an inert gas into the cavity after the cavity has been sealed in the vacuum state. Introducing gas into the cavity causes gas damping and thin film damping which reduces the final impact speed. 
     In one embodiment, a method of MEMs fabrication comprises fabricating a MEMs device, the MEMs device having a cavity sealed by an encapsulating layer, implanting atoms into one of more of the encapsulating layer and another layer bordering the cavity and annealing the MEMs device to release the atoms into the cavity and pressurize the cavity. 
     In another embodiment, a MEMs device includes a first cavity having a switching element therein movable between a first position and a second position, a second cavity disposed adjacent the first cavity, a channel connecting the first cavity to the second cavity and atoms implanted into a portion of a boundary of the second cavity. 
     In another embodiment a MEMs device or set of MEMs devices are housed in a cavity under high vacuum. For example, the MEMs device may be a DVC consisting of a conducting beam that is electrically grounded and movable from a position close to a radio frequency (RF) electrode (i.e., a high capacitance state) that may be on the substrate and that is coated with a thin layer of insulator. The conductive beam is pulled into the high capacitance state by applying a voltage to the electrodes adjacent the RF electrode which is also coated with a thin insulator. The conductive beam can then be pulled to the roof of the cavity when a voltage is applied to an electrode (i.e., a pull-up electrode) above the conductive beam. An insulator under the pull-up electrode prevents the voltage on the pull-up electrode leading to current flow to the conductive beam (i.e., a low capacitance state). The low capacitance state moves the grounded conductive beam away from the RF electrode leading to the low capacitance state for the RF electrode. Typically voltages of between 10V and 30V are applied to move the conductive beam between the low and high capacitance states. These high voltages cause the conductive beam to accelerate across the cavity and land on the insulating material with velocities that can be greater than 1 m/s. These devices have to switch many hundreds of millions of times and if the impact velocity is too great, eventually material wears off the surfaces leading device failure. By introducing a gas into the cavity, the air under the conductive beam needs to move out the way as the beam makes contact with the insulating material. The gas has to flow laterally through an ever decreasing gap leading to an increased pressure in the narrow gap between the conductive beam and the surface it is approaching. This pressure causes a declaration force which slows down the conductive beam as the conductive beam lands reducing the impact damage. 
     Ion implantation of non reactive gases such as argon, nitrogen, or helium is available in semiconductor processing fab and is used to inject dopant into semiconductor substrates. A high voltage is used to accelerate ions to a very high velocity, these can be combined with a beam of electrons to then neutralize their charge before they enter the substrate. The high velocity ensures that most of the atoms come to rest some distance below the surface of the semiconductor. In this embodiment the ion acceleration would be adjusted to deposit most of the ions at a depth comparable to the thickness of the top layers of the cavity. There will be a distribution of depths where the atoms will come to a halt, and this will mean a large percentage of those atoms injected will come to rest either just above the cavity or just below. This may be an issue if there is a switching element in the cavity as the ions will then come to rest in the switching element itself, which may cause damage or a change in stress in the top surface leading to curvature of the switching element. To get around this issue a second cavity will be provided next to the MEMs cavity and joined by a pipe-like channel. An extra layer of masking material can be placed over the cavity containing the MEMs device and then be removed from over the empty cavity. The atom implanting process then implants atoms into the removable layer above the MEMs cavity (i.e., the masking material), but into the cavity and regions closely spaced just above and below the empty cavity. Subsequent annealing will then cause the gas to come out of the empty cavity walls and enter the empty cavity where pressure equalization will lead to gas diffusing into the cavity containing the switching element. The final stage of the process is to remove the masking material which has protected the MEMs cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1A  shows a MEMS device in a cavity connected to a neighboring empty cavity. 
         FIG. 1B  shows the MEMS device and cavities of  FIG. 1A  after addition of the masking layer. 
         FIG. 1C  shows the MEMS device and cavities of  FIGS. 1A and 1B  after the implantation process. 
         FIG. 1D  shows the MEMS device and cavity of  FIG. 1C  after annealing and removal of the masking layer. 
         FIG. 2A  shows the typical atom concentration versus depth in a solid substrate after high energy implantation. 
         FIG. 2B  shows the typical atom concentration versus depth in the cast of a cavity. 
         FIG. 2C  show the subsequent distribution of atoms shown in  FIG. 2B  after annealing. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
     DETAILED DESCRIPTION 
     The present invention generally relates to methods for increasing the lifetime of MEMS devices by reducing the impact velocity of a switching element in the MEMS device. Rather than leaving the encapsulated MEMS device in a vacuum cavity, atoms are implanted into the cavity to introduce an inert gas into the cavity after the cavity has been sealed in the vacuum state. Introducing gas into the cavity causes gas damping and thin film damping which reduces the final impact speed. 
       FIG. 1A  shows a MEMS device  100  in the ground state according to one embodiment. The MEMS device  100  includes a substrate  102  having a plurality of electrodes  104 A- 104 E formed therein. Two electrodes  104 B,  104 D are referred to as ‘pull-in’ electrodes because the electrodes  104 B,  104 D are used to pull the switching element  108  towards electrode  104 C. Electrode  104 C is an RF electrode. Electrodes  104 A,  104 E provide the ground connection to the switching element  108  through vias filled with electrically conductive material  110 . An electrically insulating layer  106  is formed over the electrodes  104 B- 104 D. In one embodiment, insulating layer  106  may comprise silicon dioxide, silicon nitride, or combinations thereof. 
     The switching element  108  may comprise an electrically conductive material such as titanium nitride or an alloy of aluminum titanium nitride. In one embodiment, the titanium nitride may be coated with a thin layer of electrically insulating material. The switching element  108  is shown to have a bottom layer  112  and a top layer  114  that are connected by one or more posts  116 . It is to be understood that the switching element  108  is contemplated to have other arrangements as well. Additionally, it is to be understood that both the top and bottom layer  112 ,  114  of the switching element  108  are contemplated to comprise titanium nitride having a thin layer of electrically insulating material thereon, but other materials are contemplated as well. The switching element  108  is disposed within a cavity  118  and movable within the cavity  118  between the low capacitance, high capacitance and ground states. 
     Above the switching element  108 , another electrode  120 , sometimes referred to as a ‘pull-up’ or ‘pull-off’ electrode, is present. A thin layer  122  of electrically insulating material is disposed between the electrode  120  and the cavity  118  such that the layer  122  bounds the cavity  118 . The cavity  118  is sealed with a capping layer  124  that encapsulates the cavity  118 . 
     Next to the cavity  118  containing the switching element  108  is a separate cavity  130 . Cavity  130  is connected to cavity  118  containing the switching element  108  by the pipe or channel  131 . Cavity  130  can be used to introduce atoms into the cavity  118 , which would reduce the impact velocity of the switching element  108  in the cavity  118 . The atoms may be injected into the cavity  130  by an ion implantation process. 
     The MEMS device  100  may be formed as follows according to one embodiment. The substrate  102  is patterned by forming a mask thereover and etching the substrate  102  to form the trenches into which the electrically conductive material forming the electrodes  104 A- 104 E will be formed. Thereafter, the mask is removed and the electrically conductive material is deposited into the trenches to form the electrodes  104 A- 104 E. The electrically insulating layer  106  is then deposited thereover. Another mask is then formed over the electrically insulating layer  106  so that vias may be formed that will be filled with electrically conductive material  110 . The vias are etched into the insulating layer to expose the electrodes  104 A,  104 E, and then the mask is removed. The electrically conductive material  110  is then deposited in the vias. 
     A sacrificial material is then deposited over the electrically insulating layer  106 . The switching element  108  is then formed in the sacrificial material and additional sacrificial material is formed over the switching element  108 . The sacrificial material forms the boundary of the cavities  118 ,  130  that are to be formed. An electrically insulating layer  122  is then formed over the topmost sacrificial layer followed by the electrode  120 . A hole is formed through the capping layer  124 , electrode  120  and insulating layer  122  to expose the sacrificial material. Etchant is introduced to the cavity  118  to remove the sacrificial material and thus form cavity  118  and cavity  130 . Within cavity  118 , the switching element  108  is free to move in response to electrical current applied to the electrode  120  or the pull-in electrodes  104 B,  104 D. An additional encapsulating layer may be deposited thereover to seal the cavities  118 ,  130 . After the sealing, atoms may be implanted into cavity  130  through an ion implantation process. 
     In order to implant the atoms into the cavity  130 , a mask  140  is formed over the capping layer  124 .  FIG. 1B  shows the MEMS device  100  having cavity  118  and cavity  130  adjacent thereto. A mask  140  is formed over the capping layer  124 . The mask  140  has an opening  142  therethrough to expose the capping layer  124  over the cavity  130 . The masking material may comprise optical resist, polyimide or any other masking material that can absorb the accelerated atoms. The masking material is patterned using optical lithography and etching so as to be removed over the neighboring empty cavity  130  and thus form the mask  140 . 
     Once the mask  140  has been formed, atoms may be implanted through the mask  140  into the cavity  130 .  FIG. 1C  shows the MEMS device  100  in which atoms have been implanted into cavity  130 . The implanted atoms may include argon, helium, nitrogen, combinations thereof or any other material that forms a gas at ambient temperatures. In one embodiment, the implanted atoms comprise a gas that will not react with the materials in the cavity  118 . The implanted atoms are marked in regions  150  and  151 . 
     Following the implantation of the atoms, the MEMS device  100  can be annealed to diffuse the atoms into cavity  130 , channel  131  and cavity  118 . The annealing may occur at a temperature up to about 450 degrees Celsius. The mask  140  is also removed. The annealing may occur either before or after removal of the mask  140 .  FIG. 1D  shows the MEMS device  100  after annealing and after removal of the mask  140 . Due to the annealing, the gas molecules will have diffused into the cavity  118 . While not shown, it is contemplated that some sacrificial material may remain within the cavity  130  and have the atoms implanted therein during the implantation process. 
       FIG. 2A  shows the typical atom concentration versus depth in a solid substrate after high energy implantation. The vertical axis shows the distance down into the substrate for a solid. The x horizontal axis shows the density of implanted atoms. 
       FIG. 2B  shows the typical atom concentration versus depth in the cast of a cavity. The depth of the top of the cavity is marked as  1  and the depth of the bottom of the cavity is marked as  2 . Because the cavity provides hardly any opportunity for the injected atoms to lose energy, the atoms that make it through to the cavity traverse the cavity and are imbedded at the bottom of the cavity. Thus the distribution is broadened by almost the thickness of the cavity. Some atoms may be reflected by the bottom of the cavity and provide some pressure. 
       FIG. 2C  show the subsequent distribution of atoms shown in  FIG. 2B  after annealing. The concentration gradient of implanted atoms leads to the diffusion of the atoms close to the cavity surface into the cavity providing a pressure of atoms that can reduce the impact velocity of the switching MEMs device. 
     In one embodiment, during the fabrication of the MEMs device  100 , some of the sacrificial material may remain in cavity  130 , so that the implanted atoms come to a halt in the cavity region. If this sacrificial material is porous or the atoms have a fast diffusion time, then they will be released into the joined cavities more quickly and more efficiently. The cavity  130  may also have material deposited in the cavity  130  during the fabrication stage that is not removed from the cavity during release, but will stop the implanted atoms easily, and allow them to diffuse out quickly on heating. 
     The invention has been described with respect to one capacitor in a cavity being part of many capacitors in a digital variable capacitor, but it is understood that the invention is applicable to cavities having multiple MEMS devices where the switching of the device causes impacts which limit the life of the device. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.