US 20100171569 A1
The present invention exploits the combination of the amplification, provided by the integration of a FET (or any other active device with two or more terminals), with the signal modulation, provided by the MEM resonator, to build a MEM resonator with built-in transistor (hereafter called active MEM resonator). 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.
1. An active micro-electro-mechanical resonator comprising a vibrating body transistor with a source (2), a drain (3) and a low doped body region (4) connecting the source and the drain, and at least two fixed gates (1,1′,1″), wherein the transistor cooperates with each gate.
2. A resonator as defined in
3. A resonator as defined in
4. A resonator as defined in
5. A resonator as defined in
6. A resonator as defined in
7. A resonator as defined in
8. A resonator as defined in
9. A resonator as defined in
10. A resonator as defined in
11. A resonator as defined in
12. The resonator of
13. A resonator as defined in
14. A resonator as defined in
15. An active MEM resonator filter comprising at least a resonator as defined in
16. An active MEM resonator mixer-filter comprising at least a resonator as defined in
17. An active MEM resonator oscillator comprising at least a resonator as defined in
18. An active MEM resonator oscillator comprising at least a resonator as defined in
19. An active MEM resonator oscillator comprising at least a resonator as defined in
20. An active MEM resonator sensor comprising at least a resonator as defined in
The present invention concerns the field of vibrating micro electro mechanical systems (MEMS) and transistors, in particular the combination of both to improve the performances of MEM resonators.
The present invention exploits the combination of the amplification, provided by the integration of a FET (or similar active device), with the signal modulation, provided by the MEM resonator, to build a MEM resonator with intrinsic signal gain (hereafter called active MEM resonator). Depending on the active MEM resonator dimensions and under certain bias conditions it is possible to selectively amplify an applied signal.
The principle of such device operating in a Double Gate configuration has been fully validated for the first time by the inventors of the present application in a 2008 publication , which is incorporated by reference in its entirety in the present application and is in total contrast with the device reported in previous publications [2-5] and the patent application WO 2007/135064, where the gate of a transistor is vibrating, offering key advantages for the intrinsic signal gain, scaling of the device and a larger range of applications. More specifically, in this prior art publication, in all the configurations disclosed, each individual transistor is coupled to a single gate only.
The current device is based on the Single Gate device published in 2007 , which is incorporated by reference in its entirety in the present application, and the body of the transistor is the vibrant part. The present invention is however clearly distinct as it uses two or more electrodes to modulate the current in one transistor. The advantages of this configuration over the state of the art are a reduction of the number interconnections needed (simplification of the fabrication), an increase of the electrostatic control on the FET body region and an increase of the resulting current modulation through.
The increase of the electrostatic control on the transistor body can be obtained by two or more gates placed in the same plane, increasing the number of active channels. Furthermore, supplementary gates can be placed in parallel planes below and above, increasing the potential control on one or more channel by coupled action of some or all gates.
Depending on the exact geometry, the type of transistor and its mode of operation, it can be more advantageous to operate the gates in a coupled voltage mode or with independent voltages.
In contrast to the state of the art, the present invention integrates the vertical transistor into the mode shape of the mechanical displacement. As a consequence and unlike the structure presented in WO 2007/135064 , a stress may be induced in the channel region of the transistor, which modulates the conductivity of the channel (piezoresistive effect). The effective mass and mobility of the carriers in the channel change with the stress, which is a function of the vibration amplitude and as a consequence, the total current Ids in the transistor is modulated by a combination of the field effect (number of carriers in the channel) and the piezoresistive effect (mobility and mass of the carriers). The stress-component(s) in the channel region may be uniaxial or biaxial (along or perpendicular) to the current flow.
Moreover, the number of interconnections in WO 2007/135064 , is higher because each individual transistor is coupled to a single gate only and each has a source and a drain, all needing individual interconnections to the respective contact. The proposed invention reduces the number of contact lines by combining multiple channels into one transistor, thus simplifying the electrical interconnect schematic while maximizing the transistors current modulation capability.
The body of the transistor described in this invention can be surrounded by one or more stacked surface layers (7,7′) to control the surface conduction in a similar way to a solid-state-transistor. It is common to use a gate oxide at the channel surface to increase the performance of the transistor. A channel stack (7,7′) can include dielectric materials to increase the electrostatic coupling (e.g. high-k materials, . . . ) and conductive materials (metals, silicon, . . . ) to create a floating gate further optimizing the transistor. For some device structures vibrating at very small amplitudes (usually in the order of nm), the dielectric materials can completely close the air-gap, transforming the device into a vibrating transistor with solid-gap.
Other layers in the channel stack (7,7′) include surface treatments for sensing applications.
The present invention is not limited to resonators, but extends to resonant and non-resonant embodiments of transistor based motion detection using two or more gates as is useful in the field of MEMS and NEMS sensors (Accelerometers, gyroscopes, . . . ). Such a vibrating body transistor can be used in an open-loop or closed loop configuration, below, at or above its mechanical resonance frequency. Applications of special interest include, but are not limited to hysteric switches with two or more gates or mechanical memories using a single transistor and two or more gates.
We propose the extension of the vibrating FET principle to any other two-terminal or multi-terminal gated device, where the device body is suspended and vibrates, inducing the modulation of the output current such as:
In all these cases the vibrating structure is the device body made on a semiconductor material or made on a hero-structure and two or more fixed gates can be placed around the device body, being separated from it by air-gap or by solid-gap insulators.
The signal transmission parameters of such devices are well beyond what is currently possible for conventional capacitively transduced passive MEM resonators [8-13], where a change in the resonator to electrode spacing under a constant bias voltage generates a current in both the resonator and the electrode. This current depends on the geometry of the device and is usually rather low. Especially the dependence on the electrode surface makes scaling of capacitive transduced passive MEM resonators difficult without strongly decreasing the signal transmission parameters. Depending on the active MEM resonator and the air gap dimensions, signal gain can be obtained for low voltages (16 V demonstrated) when connected to a state of the art 50Ω RF circuits. Low power consumption of the active MEM resonator is obtained under certain bias conditions (e.g.: sub-threshold operation, low drain voltage) and could be of great interest for low power applications. Further, the multi-gate configuration allows to use the MEM resonator to broaden the tuning range of the signal gain and gives direct control of the output signal phase (0 and π, for positive/negative bias voltages; additionally the phase depends on the mode shape of the resonator).
In one embodiment, an active MEM resonator with signal gain in an open loop configuration is proposed based on the gain provided by the integrated FET. This is interesting for channel selective filtering in RF communications, with low signal levels.
In another embodiment, a mixing filtering technique is proposed making use of either one (two tone signal (LO+RF)) or multiple electrodes (single signal on every electrode, may include drain electrode) to generate the difference or sum of the two applied frequencies. The mechanical response of the resonator directly filters the IF signal.
In one embodiment an oscillator is proposed based on the gain provided by the active MEM resonator. Conventional oscillators use a dedicated amplifier, to compensate for the loss in the resonator, to sustain the oscillation. For active MEM resonators the gain provided by the external amplifier is no longer needed, simplifying therefore the circuit design and reducing the cost.
In a further embodiment, a resonant sensor is proposed based on the active MEM resonator. The current modulation of the active MEM resonator is offering a high robustness to noise and the surface treatment and passivation (for example SiO2) of the active MEM resonator provides electrical isolation and the possibility to add functionalization for bio-sensing applications, SiO2 surface passivation is a standard of FET technology and allows a thermal compensation of the silicon material properties. Surface functionalization is used for resonant sensors: the surface becomes sensitive to one specific particle, which can then be detected.
A simplified three dimensional drawing is shown in
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).
In the latter case, a strong acoustic impedance miss-match decreases the amount of energy radiating from the channel into the gate region.
The simple structure of
As illustrated in
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
(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 c) The etch mask is removed.
(step d) A mask for implantation is formed and different regions of the active MEM resonator are implanted to form 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 g) the following contact opening and metallization steps.
(step h) The active MEM resonator is released from the protection material.
A frequency response of an active MEM resonator with a signal gain of approx. +3 dB on a 50Ω input is shown in
The presence of gain in the current invention is of importance and allows several new architectures and applications. Possible architectures include active filers (
A possible layout of tuning fork filter based on an active MEM resonator is shown in
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
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:
(i) RF and LO on the same gate(s),
(ii) RF on the gate(s), LO on the vibrating body,
(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.
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 (
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.