Patent Publication Number: US-2004051156-A1

Title: Method of fabricating a high Q - large tuning range micro-electro mechanical system (MEMS) varactor for broadband applications

Description:
TECHNICAL FIELD  
       [0001] This invention relates generally to integrated circuits, and more particularly to Micro Electro-Mechanical System (MEMS) devices.  
       BACKGROUND OF THE INVENTION  
       [0002] In the telecommunications industry, the demand for lightweight portable devices such as personal computing devices, Personal Digital Assistants (PDA&#39;s) and cellular phones has driven designers to reduce the size of existing components. A Q value is a ratio of the power stored in a device to the dissipated power in a device. Due to the need for Q values beyond the capabilities of conventional IC technologies, board-level passive components continue to occupy a substantial portion of the overall area in transceivers of handheld telecommunications equipment, presenting a bottleneck against further miniaturization. For example, discrete components currently occupy approximately 50% of the space in cellular phones.  
       [0003] Recently MEMS devices including resonators, filters, and switches have been developed that offer an alternative set of strategies for transceiver miniaturization and improvement. MEMS devices are high-Q, chip-level, lower power replacements for board-level components that greatly decrease space and area requirements.  
       [0004] One such MEMS device is an RF switch for switching RF signals, shown in a cross-sectional view in FIG. 1. RF drumhead capacitive MEMS switch  10 , disclosed by Goldsmith et al. in U.S. Pat. No. 5,619,061, comprises an insulator  14  such as SiO 2  deposited over a substrate  12 , which may comprise silicon, for example. A bottom electrode  16  is formed on insulator  14  and a dielectric  18  is formed over bottom electrode  16 . Capacitor dielectric  18  typically comprises Si 3 N 4 , Ta 2 O 5  or other suitable dielectric materials, for example. An active element comprising a thin metallic membrane  22  is suspended away from electrode  16  by an insulating spacer  20 . Membrane  22  which serves as a top electrode is movable through the application of a DC electrostatic field between membrane  22  and bottom electrode  16 . Membrane  22 , dielectric  18  and bottom electrode  16  comprise a metal-dielectric-metal capacitor when the MEMS switch  10  is in the “on” position, shown in FIG. 2. In the “off” position shown in FIG. 1, with no voltage applied to membrane  22  and bottom electrode  16 , the capitance value is at a minimum. MEMS switches  10  have low insertion loss, good isolation, high power handling, and very low switching and static power requirements.  
       [0005] A MEMS switch  10  may be designed for use as a varactor. A varactor is a discrete electronic component, usually comprising a P-N junction semiconductor, designed for microwave frequencies, in which the capacitance varies with the applied voltage. Varactors are sometimes referred to as tunable capacitors. Varactors are used in frequency up and down conversion in cellular phone communication, for example. Existing varactors are usually p-n diodes specifically designed for operation in the reverse bias regimes where the capacitance(C j ) of the depletion region is varied to set frequency (   o ) of operation as reflected in Equation 1: 
           o ≈1/( C   J   *R   S   *R   P ) 1/2   Equation 1 
       [0006] where resistances R p  and R s , are the parallel and series resistances of the diode, respectively. Some primary requirements of a varactor are that it have a high quality factor (Q) for increased stability to thermal variations and noise spikes, and a large linear tuning range (TR). High-performing varactors are usually made of GaAs. Unfortunately, these devices use a different processing technology that is not amenable to integration into standard Si-CMOS process.  
       [0007] MEMS devices offer a means by which high Q large tuning range varactors can be integrated in higher level devices such as voltage controlled oscillators and synthesizers using the current Si-CMOS process. The drumhead capacitive switch  10  shown in FIG. 1 may be designed to produce a MEMS varactor. The voltage across the electrodes is varied to pull down and up membrane  22 , which varies the distance D air  between membrane  22  and dielectric  18 , which changes the capacitance of the device  10  accordingly.  
       [0008] A problem in MEMS devices is stiction, which is the unintentional adhesion of MEMS device  10  surfaces. Stiction may arise from the strong interfacial adhesion present between contacting crystalline microstructure surfaces. The term stiction also has evolved to often include sticking problems such as contamination, friction driven adhesion, humidity driven capillary forces on oxide surface, and processing errors. Stiction is particularly a problem in current designs of MEMS varactors, due to the membrane  22  possibly adhering to dielectric  18 , resulting in device  10  failure, either temporarily or permanently. To prevent stiction, material and physical parameters, and voltage signal levels of the varactor are designed to avoid contact of membrane  22  with dielectric  18 . Coatings such as Teflon-like materials that resist stiction are frequently applied over dielectric  18 .  
       SUMMARY OF THE INVENTION  
       [0009] The present invention achieves technical advantages as a MEMS varactor designed to operate in a stiction mode. The pull-down electrode or top membrane maintains contact with the underlying dielectric covering the bottom electrode during operation of the varactor. As the voltage across the pull-down electrode and the bottom electrode is varied, the area of the pull-down electrode contacting the dielectric is varied, which varies the capacitance.  
       [0010] Disclosed is a MEMS varactor, comprising a bottom electrode formed over a substrate, a dielectric material disposed over the bottom electrode, and a spacer proximate the bottom electrode. A pull-down electrode is disposed over the spacer and the dielectric material, wherein the varactor is adapted to operate in a stiction mode.  
       [0011] Also disclosed is a method of manufacturing a MEMS varactor, comprising depositing an insulator on a substrate, forming a bottom electrode on the insulator, and depositing a dielectric material over the bottom electrode. A spacer is formed over the insulator, and a pull-down electrode is formed over the spacer and the dielectric material, wherein the varactor is adapted to operate in a stiction mode.  
       [0012] Further disclosed is a method of operating a MEMS varactor, comprising applying a voltage across the bottom electrode and the pull-down electrode to produce a predetermined capacitance across the bottom and pull-down electrode, wherein at least a portion of the pull-down electrode is adapted to contact the dielectric material during operation in a stiction mode.  
       [0013] Advantages of the invention include solving the stiction problems of the prior art by providing a varactor adapted to operate in a stiction mode. The present MEMS varactor is a high Q varactor having a large tuning range. The distance between the dielectric and the membrane may be increased in accordance with the present invention, allowing for a larger tuning range and providing more sensitivity to a change in voltage. A wider range of voltages and capacitances is available with the present MEMS varactor design. Furthermore, the use of Teflon-like coatings on dielectric to prevent stiction of membrane is not required, as in some prior art designs. A wider variety of dielectric materials may be used for dielectric than in the prior art because there is no need for concern about stiction of the membrane to the dielectric. The invention provides an extended tuning range that is not possible with only an air gap for the capacitive medium.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] The above features of the present invention will be more clearly understood from consideration of the following descriptions in connection with accompanying drawings in which:  
     [0015]FIG. 1 illustrates a cross-sectional view of a prior art MEMS capacitive RF switch;  
     [0016]FIG. 2 illustrates a cross-sectional view of the MEMS varactor of the present invention adapted to operate in a stiction mode, with the majority of the membrane above the bottom electrode in contact with the dielectric;  
     [0017]FIG. 3 illustrates a top view of the MEMS varactor shown in FIG. 2;  
     [0018]FIG. 4 shows a model schematic representation of the MEMS varactor having a capacitance across the membrane and the bottom electrode;  
     [0019]FIG. 5 illustrates a capacitance to voltage relationship of the MEMS varactor output capacitance over a range of voltages;  
     [0020]FIG. 6 illustrates a cross-sectional view of the present MEMS varactor with a portion of the membrane in contact with the dielectric;  
     [0021]FIG. 7 illustrates a top view of the MEMS varactor shown in FIG. 6; and  
     [0022]FIG. 8 illustrates a cross-sectional view of the present MEMS varactor with a minimal portion of the membrane in contact with the dielectric and having an increased spacer height, increasing the tuning range of the varactor.  
     [0023] Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated.  
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0024] A cross-sectional view of the MEMS varactor  100  of the present invention is shown in FIG. 2. MEMS varactor  100  comprises an insulator  114  deposited over a substrate  112 , and a bottom electrode  116  formed on insulator  114 . A dielectric  130  is formed over bottom electrode  116  to eliminate the possibility of electrode/electrode fusion and for creating a capacitance that is greater than possible with air. Spacer  120  are formed over the insulator  114  for supporting membrane  122  a distance D 1  above insulator  114 . Distance D 1  may be, for example, 0.5-2.0 micrometers. Membrane  122  is also referred to herein as a pull-down electrode or top electrode. Membrane  122  may comprise holes  124  which are used to remove a temporary filler material (not shown) from cavity  126 . Membrane  122  is movable through the application of a DC electrostatic field across membrane  122  and bottom electrode  116 , similar to the operation of the MEMS RF switch  10  previously discussed.  
     [0025] The MEMS varactor  100  of the present invention is adapted to operate in a stiction mode. A stiction mode is defined herein as an active operating mode during which a voltage is applied across membrane  122  and bottom electrode  116 , and the membrane  122  maintains contact with at least a portion of dielectric  130  covering bottom electrode  116 .  
     [0026] The amount of area or portion  132  of membrane  122  that contacts dielectric material  130  is varied to change the capacitance C. The contact portion  132  is varied by changing voltage V across electrodes  122  and  116 . In the stiction mode, the maximum capacitance C max  is achieved when membrane  122  is biased with a voltage V such that membrane  122  makes complete contact at portion  132  to dielectric  130  as shown in FIG. 2. Capacitance C max  may be expressed by Equation 2, 
       C   max≈∈   die A/D die    Equation 2 
     [0027] where A is the cross-sectional area  132  of the electrode  122  in contact with dielectric  130 , ∈ die  is the dielectric constant of the dielectric  130  covering bottom electrode  116 , and D die  is the thickness of the dielectric  130 . The capacitance is reduced by decreasing the membrane  122 /dielectric  130  contact area, shown in FIGS.  6 - 8 , which is accomplished by changing the voltage V. The relationship of capacitance C to area A, where A is varied by changing the voltage V, is a linear relationship. The minimum capacitance C min , expressed in Equation 3, occurs when the membrane  122  is not contacting the dielectric  130 , 
     1/ C   min≈ 1/(∈ air   A/D   air )+1/(∈ die   A/D   die )  Equation 3 
     [0028] where ∈ air  is the dielectric constant of the air and D air  is the thickness of the air space between membrane  122  and top of dielectric  130 . The tuning range TR is reflected by Equation 4 
       TR =( C   max   −C   min )/ C   min ×100%  Equation 4 
     [0029] The tuning range of the MEMS varactor may be extended or reduced by changing the material parameters, e.g. the materials of dielectric  130  and distances D air  and D die , of Equations 2 and 3, for example.  
     [0030]FIG. 3 illustrates a top view of the MEMS varactor shown in FIG. 2, with a circular region  132  of membrane  122  in contact with dielectric  130  in a maximum amount, giving a maximum capacitance value C max  for the varactor  100 . FIG. 4 shows a model schematic representation of the MEMS varactor  100  having a capacitance C between the membrane  122  and the bottom electrode  116  for a voltage signal V input to either electrode  122 ,  116  of the varactor  100 . FIG. 5 illustrates the capacitance to voltage relationship of the MEMS varactor  100  over a range of voltages, for example, a range of voltage signals from 3 to 10 volts produces a capacitance ranging from 13 to 25 pF in the stiction mode. These voltages and capacitances are exemplary and may vary with air gap distances D 1  and dielectric material properties. FIG. 6 illustrates a cross-sectional view of the present MEMS varactor with a portion  136  of membrane  122  in contact with dielectric  130 , membrane portion  136  being smaller than membrane portion  132  shown in FIG. 2. FIG. 7 illustrates a top view of the MEMS varactor  100  shown in FIG. 2, with circular portion  136  of membrane  122  in contact with dielectric  130 . FIG. 8 illustrates a cross-sectional view of an alternate embodiment of the present MEMS varactor  200  with a minimal portion  138  of membrane  122  in contact with dielectric  130  and having an increased spacer  120  height D 2 , increasing the tuning range of the varactor  200 . Increasing the distance D 2  to greater than 2 micrometers also provides more sensitivity to a change in voltage signal V.  
     [0031] There are many preferred and alternate configurations for the present varactor  100 ,  200  adapted to operate in a stiction mode. A first voltage signal applied across the bottom electrode and the pull-down electrode produces a first capacitance, and a second voltage signal applied across the bottom electrode and the pull-down electrode produces a second capacitance, where the first and second voltages are different.  
     [0032] Although preferably the pull-down electrode  122  maintains contact with the dielectric material  130  over a range of voltage signals, the varactor  100 ,  200  may also be operated in a non-stiction mode in an alternate embodiment. In this embodiment, the tuning range of the varactor may be increased if the membrane starts at the undeformed (no voltage signal applied) position and then is deflected so that it makes contact with bottom electrode. The height of the membrane is varied over the air gap until it makes contact partially, then fully with the bottom electrode. In this embodiment, a larger tuning range is achievable. However, the varactor may not be reliably operated across the entire tuning range if the membrane permanently sticks, in which case the varactor would then operate only in the stiction mode.  
     [0033] The invention also includes a method of manufacturing a MEMS varactor  100 ,  200  comprising depositing an insulator  114  on substrate  112 , forming bottom electrode  116  on insulator  114  and depositing dielectric material  130  over bottom electrode  116 . Spacer  120  are formed over insulator  114 , and pull-down electrode  122  is formed over spacer  120  and dielectric material  130 , wherein the varactor  100 ,  200  is adapted to operate in a stiction mode. At least a portion  132 ,  136 ,  138  of pull-down electrode  122  contacts dielectric material  130  in a stiction mode.  
     [0034] The invention also includes a method of operating a MEMS varactor  100 ,  200 . The method comprises applying a voltage signal V across bottom electrode  116  and pull-down electrode  122  to produce a predetermined capacitance C across bottom  116  and pull-down  122  electrode, wherein at least a portion  132 ,  136 ,  138  of pull-down electrode  122  is adapted to contact dielectric material  130  during a stiction mode.  
     [0035] The novel MEMS varactor  100 ,  200  of the present invention achieves technical advantages by providing a high Q varactor having a large tuning range and increased sensitivity. MEMS varactor  100 ,  200  solves the stiction problems of prior art MEMS varactors by being adapted to operate in a stiction mode. The distance D 1 , D 2  between the dielectric and the membrane may be increased in accordance with the present invention, allowing for a larger tuning range. A wider range of voltages and capacitances is available with the present MEMS varactor design compared with the prior art. Furthermore, the use of Teflon-like coatings on dielectric  130  to prevent stiction of membrane  122  is not required as in some prior art designs. A wider variety of dielectric materials may be used for dielectric  130  than in the prior art because there is no need for concern about stiction of the membrane  122  to the dielectric  130 . The invention provides an extended tuning range that is not possible with only an air gap for the capacitive medium. Furthermore, the MEMS varactor  100 ,  200  preferably comprises silicon rather than GaAs, and may comprise metals that maintain low insertion loss and good isolation of the MEMS varactor  100 ,  200 .  
     [0036] While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications in combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. For example, although membrane portions  132  and  136  in contact with dielectric  130  are shown in a top view as being circular, other shapes for contact membrane portion  132 ,  136  are anticipated, for example, square, oval rectangular, or any other geometrical shape. The MEMS varactor  100  may be designed to also operate in a non-stiction mode, wherein membrane  122  is not in contact with dielectric  130 , as well as the stiction mode described herein. It is therefore intended that the appended claims encompass any such modifications or embodiments.