Patent Application: US-200913129280-A

Abstract:
the present invention exploits the combination of the amplification , provided by the integration of a fet , with the signal modulation , provided by the mem resonator , to build a mem resonator with built - in transistor . in these devices , a mechanical displacement is converted into a current modulation and depending on the active mem resonator geometry , number of gates and bias conditions it is possible to selectively amplify an applied signal . this invention integrates proposes to integrate transistor and micro - electro - mechanical resonator operation in a device with a single body and multiple surrounding gates for improved performance , control and functionality . moreover , under certain conditions , an active resonator can serve as dc - ac converter and provide at the output an ac signal corresponding to its mechanical resonance frequency .

Description:
a simplified three dimensional drawing is shown in fig1 . the gate g 1 , g 2 structures 1 and 1 ′ are laterally placed and fixed with respect to the substrate . a source region 2 , a drain region 3 and a low doped body region 4 connecting the source and drain form the active mem resonator . the channels 5 , 5 ′ are formed at the lateral interfaces of the body regions 2 , 3 and 4 . the active mem resonator is connected by elastic means 6 to the substrate . along the channel - to - air gap interface , a possible gate stack 7 can be placed . if the drain and source have the same type of doping ( e . g n + or p +), the structure operates a vibrating fet ( enhancement or accumulation transistor : n +− p − n +, p +− n − p +, n +− n − n +, p +− p . p +). if the drain and the source have opposite dopings and the central part is low doped the structure transforms in a p - i - n junction and can be operated as vibrating tunnel fet ( gate overlapped on the central body ) or as a vibrating impact ionization mos ( gate partially overlapped on the central body and high reversed drain voltage applied applied ). fig2 is a detailed top view of the structure shown in fig1 , adding more details about the possible channel stack , which may be placed on one or both sides of an air gap . one or a stack of material 7 , 7 ′ ( e . g . dielectric like silicon dioxide or silicon nitride ) is put at channel - to - air gap interface to improve the characteristics of the device at the channel side . similarly a gate - to - air gap stack 8 , 8 ′ may be formed in the same process step , and maybe be made of the same material , or include conductive material to improve the device characteristics . further , it is possible to fill completely the air gap with material from 8 ( and 8 ′) and 7 ( and 7 ′) to define a solid gap resonator . the material in the gap serves the purpose of electrical isolation and electrostatic coupling between the gate and the channel . it is advantageous to use a material with a high dielectric constant to increase the electrostatic coupling . a solid gap based mem device may include the gap and the gate into its motion ( intrinsic solid gap ) or the solid gap represents a boundary for the motion ( external solid gap ). in the latter case , a strong acoustic impedance miss - match decreases the amount of energy radiating from the channel into the gate region . fig3 is a cross - section of a possible active mem resonator . the material deposited to improve the interfaces 7 , 7 ′ can be deposited in a conformal or a non - conformal way . the simple structure of fig1 can be extended to a higher number of gates 1 , 1 ′, 1 ″ and of channels 5 , 5 ′, 5 ″, 5 ′″, as illustrated in fig4 , to improve the signal gain by the means of elastic connections 9 of different stiffness , different coupling mode between the channels of the active mem resonator are possible . this is may be used to create different frequency characteristics ( e . g . multi - peak filter or single peak resonator ). as illustrated in fig5 the active mem resonator principle can be applied to bulk mode resonator with four gates 1 ′, 1 ″, 1 ′″ four channels 5 ′, 5 ″, 5 ′″ depending on the desired frequency range . other configurations with more gates than illustrated are of course possible in the frame of the present invention . the detection principle can be applied to other resonators using different types of movements , such as flexural or torsional resonators fig6 illustrates a possible fabrication process of the active mem resonator . ( step a ) an etch mask is formed on top of the structural layer used to build resonator . ( step b ) the structures formed previously are etched into the structural layer . ( step d ) a mask for implantation is formed and different regions of the active mem resonator are implanted to foim the source , drain , gate and body regions of the device . ( step e ) the dopants are activated , the resonator is released by sacrificial etching of the material below the resonator and the gate stack is formed . ( step f ) the released structures are protected with a material during the following step , ( step h ) the active mem resonator is released from the protection material . fig7 ( a ) and ( b ) are sem images of active mem resonators . the one illustrated in fig7 ( b ) is working at a frequency of 71 mhz with four independent gates controlling the inversion charge in the four channels placed on the four lateral sides . the center of the resonator acts a fet body and can be either floating ( as seen in fig7 ) or connected to through one or several anchors to an external voltage source . in fig8 the static characteristics measured on an active mem resonator are depicted . the i d v d curve resembles similar curve obtained from conventional cmos circuits , while the inset shows the i d v g characteristics of the same device . the mechanical pull - in and pull - out is clearly visible . a frequency response of an active mem resonator with a signal gain of approx . + 3 db on a 50ω input is shown in fig9 . as for similar conventional mem resonator , the frequency is function of the applied voltages , in case of the active 1viem resonator all gate and drain voltages influence the resonance frequency . fig1 shows several frequency characteristics of an active mem resonator for different drain voltages . the presence of gain in the current invention is of importance and allows several new architectures and applications . possible architectures include active filers ( fig1 ), where the filtering and the amplification is achieved with a single device , active mechanical mixer - filters , which include three functionalities ( mixing , filtering and amplification ) in one device ( fig1 ) and novel oscillator architectures ( fig1 and 14 ) without the needed for a separate feed - back amplifier . the following architectures ( filter , mixer - filter , oscillator ) even though greatly benefiting from the gain , can also be realized in a more traditional way using the vibrating body transistor as a highly sensitive device without gain . a possible layout of tuning fork filter based on an active mem resonator is shown in fig1 ( a ). in the given example the signal is applied on gate 1 , which is “ inactive ”, that means it does not contribute to the output current . such an active filter can be represented by multiple springs and masses , see fig1 ( b ). fig1 ( c ) is a schematic representation of the mode shapes the systems can assume . the active memfet is amplifying the input signal in the pass - band of the filter transfer function . the active mem resonator filter comprises at least a resonator with a mechanical filter comprising coupled and / or uncoupled active mem resonators placed in a topology to create the desired filter shape and input / output impedance , achieving signal amplification in the structure . the combination of active and inactive vibrating body fets increase the design flexibility and are important to achieve a given mode shape in the output current . the spectrum of an active mem resonator used as mixer - filter is shown in fig1 ( a ). the setup used for the measurement is shown in fig1 ( b ), where both signals to be mixed ( lo and rf ) are applied on both gate electrodes . the bias voltage on one of the electrodes is negative , to account for the phase difference between the two channels . fig1 ( c ) is the filter transfer function of the active mem resonator mixer , memorizing the maximum output at each frequency when sweeping lo in a narrow range . the black curve is an overlay of a part of the 40 mhz spectrum of fig1 ( a ). in the active mem resonator mixer - filter configuration , the filter envelope is given by the mechanical design of the active mem resonator and can be of higher order , compared to the resonator . the mixing occurs when the difference of the two signals ( rf and lo ) to be mixed corresponds to the resonance frequency ( if ) of the resonator . the frequency if can be generated with different configurations : ( iii ) rf and lo on separate gates , making use of surface potential in small vibrating body transistor . depending on the exact realization of the active mem resonator , different circuits for an oscillator without external amplifier are possible . fig1 shows such a device , were the ac signal generated in the drain current is converted into a voltage signal and feed back to the gate . fig1 the frequency spectrum of such an active mem resonator based oscillator without external amplifier is shown in fig1 . in an active mem resonator oscillator , the oscillator circuit loop includes an amplification and / or amplitude control circuit , where the circuit may serve different purposes , such as a reducing the start - up time of the oscillator , limiting the amplitude of the oscillator and / or amplification of the signal to sustain the oscillation . in one embodiment , the oscillator circuit loop may not include an amplification and / or amplitude control circuit in the signal loop , such that the gain of the active mem resonator sustains the oscillations . the layout is chosen such that the current signal is converted on a passive element such as the input impedance of the active mem resonator in a voltage signal and applied on the gate of the active resonator . in another embodiment , no loop is needed to sustain the oscillation , such that under specific bias conditions , the device starts to self - oscillate without an external excitation , a sustaining amplifier or a loop connection . such self - oscillation occurs in vibrating body fets with gain and is a simple layout for an oscillator based on an active mem resonator . mass - sensing is given as an example of a resonant sensor based on a active mem resonator . due to the current based read - out robust signal processing is possible . the mass sensing can be done with a functionalization layer ( fig3 , 7 ′) to directly influence the key parameters of the active mem resonator . the quantity to be analyzed can be frequency , q , signal gain or a combination of all relevant parameters . the physical quantity to be sensed can be of different origin ( e . g . temperature pressure , acceleration and mass ), when its influence on the active resonator resonance frequency or quality factor is known . the internal amplification provides a current based signal , which is robust to noise and other perturbations whereby the interfacing with integrated silicon circuits would be much easier in current detection than in capacitive detection . the surface passivation as described above is important for electrical isolation and bio - sensing applications . as mentioned previously , sio2 surface passivation is a standard of fet technology and was the key for the cmos technology . it is necessary and additionally allows at the thermal compensation of the silicon material properties . surface functionalization is used for resonant sensors : in this case , the surface becomes sensitive to one specific particle , which can then be detected . the sensing of chemicals ( molecules in gas or liquids ) implies preferably a surface treatment , to ensure a molecule specific detection . sensing of physical quantities does not need a modification of the device ( temperature pressure , acceleration and mass ), but the design can be optimized for the given quantity to be measured . of course , all the examples given above should be regarded as illustrative and not construed in a limiting fashion . the present invention may be applied to active devices with and without the presence of gain . also equivalent constructions may be envisaged in the frame of the present invention . 1 . d . grogg , h . c . tekin , n . d . badila - 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