used in CMOS-MEMS Resonators: A Detailed Review

Abstract

In the field of wearables implants use of CMOS-MEMS resonators in sensing applications make a revolutionization due to their miniaturizing capabilities. They are used as frequency deciding elements in an oscillator circuit used in radiofrequency range. The sensed signals are amplified using front end TIAs integrated with the structure itself. It also results in durability of the device used due to low power consumption. The on-chip TIA integration along with CMOS-MEMS structure gives a compact circuit and also helps in amplifying the weak signals sensed by the electrodes of the sensors. The use of LDC helps in converting the analog signals into digital ones. Due to microfabrication techniques involved these MEMS structures are used in various applications including healthcare as sensors, in oscillators for timing, in filters for frequency selection etc. This review work gives an insight about various TIA topologies used in CMOS-MEMS resonators. It also includes a comparative analysis of the various research works giving an insight of the future development.

1. Introduction

Trans-impedance amplifiers plays an important role in biomedical sensors as they are used with photodiodes which converts the incoming optical signals into current signals as shown in figure 1, which ultimately changed into respective voltage signals using the TIA [ 1-2 ]. These amplified voltage signals are fed to either analog to digital converters or any other processor according to the specific application. Biomedical sensors which employ optical fibers have the function of acceptor and converter with features like sensitivity, accuracy, linearity, reproducibility, wide range, precision, quick response time, lower manufacturing cost etc. These optical fibre sensing systems focused on high integration, to reduce the size and consumption of power, eventually moving from discrete components towards on chip implementations [ 3 ].

Mainly wearable blood pressure instruments which are under development are based on PPG (Photo Plethysmo Graphy) or PTT (Pulse Transit Time) techniques, thus require no medical supervision. They are low cost and comfortable to use but needs improvement in terms of accuracy and calibration. The main features of wearables devices include long battery life which is possible due to moderate power consumption along with high reconfigurability to readout signals from different measurement locations and conditions [ 4-5 ]. Recently, LDC (light to digital converter) are used frequently in biomedical optical sensor readout, so as to directly change the light into usable data. LDCs are used to convert light signals from any light source like LED into electrical signals using photodiode. Basically, a LDC has an integrator for sampling, a current to voltage converter and a filter

2022 Copyright for this paper by its authors. together, which is accompanied by a variable comparator and a counter, converting integrator output directly into a digital code [ 6-8 ].

Different design techniques are used in fabricating the TIAs keeping in mind various factors such as size, power consumption, input impedance, bandwidth range, trans-impedance gain, cost etc. The main emphasis is on using microwave components resulting into miniaturization and reduced power consumption which helps in prolonged battery life along with wide dynamic range covering different types of signals. TIAs having fixed trans-impedance gain are not suitable to be used in a wide dynamic range. The dynamic range of a TIA depends on its overload current and sensitivity [ 9 ].

Microwave components like power splitters/combiners, filters, couplers, diplexers, resonators are frequently used now-a-days to replace the conventional practice of connecting individually microwave circuits in RF and sensing applications. This approach results into compact systems with improved performance. Here the size reduction is possible using different ways such as by removing all the discrete matching circuits, by integrating filters with other components, by removing splitting networks and by integrating non-linear or active circuits.

On considering resonators whose type depends upon the system requirement including size, insertion loss, bandwidth achieved, power and cost etc, CMOS-MEMS resonator/oscillator along with TIA results in integrated miniaturized structures which is a prerequisite in case of wearable sensors with low power consumption [ 10 ].

In this paper starting with on-chip trans-impedance amplifiers used in biomedical applications as sensors, section 2 includes CMOS-MEMS oscillators and section 3 gives various front end TIAs used in these with their parameters and section 4 is about recent developments and a comparative analysis of the reported work. The section 5 gives a glimpse of scope of future research work in this field and finally the paper is concluded in section 6.

2. CMOS-MEMS Oscillator

In order to discuss about integrating various components on a single chip, so CMOS-MEMS technology provide the solution in this direction. In this technology, various interfacing circuits such as sensors, actuators, oscillators are fabricated on a single chip. They help in making portable sensing nodes especially in bio-medicals, giving timely information. It is also possible to combine these structures with application specific customized integrated circuits. These combinations are possible using two methods, namely SIP (System in package) and SoC (System on chip). In SIP various mechanical components and MEMS structure are interfaced using chip or wafer level bonding. In SoC both the structures including MEMS and application specific are monolithically integrated on the same substrate by fabricating [ 11 ].

For generation of desired frequency, CMOS-MEMS Oscillator/Resonator is used in closed loop with TIA in series. In contrast to high value of dynamic impedance present in crystal oscillators due to parallel resonance present in them, these CMOS-MEMS Oscillators works on series resonance with TIA. They have three stages as shown in figure 2, first stage is of current to voltage converter, second is of phase compensator and last is of amplifier [ 12 ]. MEMS structures are classified in three categories based on the materials used. They are back end of line metal layers, polysilicon layers and stack of metal layers and silicon dioxide. Different etching methods are used for their fabrication [ 13 ].

In order to get the desired frequency, the TIA gain is kept higher than the impedance of the oscillator. So, to achieve the gain bandwidth product the third stage of voltage amplification is added. Here the TIA used depends on the frequency of resonator.

3. Front END TIAs

Various front end TIAs used with such resonators are explained in subsequent paragraphs.

3.1. TIA with buffer and resistor as load

It is the simplest one as shown in figure 3, but there is a trade-off between gain bandwidth products here. To overcome its drawback, feedback is added. The negative feedback added here results in stability and increase in bandwidth range in the verge of gain reduction [ 14 ]. Its parameters are given by:

3.2. TIA with Negative Feedback

3.3. TIA as current amplifier

Here FETs are used as shown in figure 5 which gives lesser input impedance values, improving gain further but on the sake of noise performance Here transistors used as feedback resistor to minimize the output ripple and omit the extra drawn current. Bandwidth of TIA increases by decreasing the photodetector capacitance (~1pF). In order to boost the voltage swing and match the output impedance to drive the photo-receiver output [ 16 ].

• • • • • • •

1 1

where gm1 is the trans-conductance 4 ϒ( 1 + 3)+ 4 (ϒ 2 + 1

2) 12 where ϒ denotes

3.4. RGC (Regulated Cascode) TIA

It utilizes local feedback technique as shown in figure 6 and gives gain on wider bandwidth scale but has high value of input inferred noise [ 17 ]. It has less input impedance and high output impedance. The local negative feedback within the RGC TIA (M3 &

M4) boosts the transconductance of M1 by the value of the voltage gain of the common-source amplifier (M3 & M4). Input referred noise current 4 + 4 ϒ 3 + 4 ϒ( 2+ 4)ω2 2

3.5. TIA with capacitive feedback

In this topology input current is amplified and then is converted to voltage by R2 resistor has lesser input inferred noise and higher gain. But they are used only for low frequencies shown on figure 7 [ 18 ]. 1 ( 1 + ) 2 1 (1+ 21) Input referred noise current (

3.6. TIA with integrator and differentiator

The first stage is an integrator biased with a pseudo resistor RB giving a phase shift of 90°. The total feedback loop phase to 0°/180°. In this topology due to high impedance value given by RB, the capacitance ratio (C2 /C1) is reduced to increase gain and reduce noise shown in figure 8 [ 19 ].

3.7. Differential TIA

( 12)2 + ∆2 ω2( 1 + ) 4 2

This topology is used due to its complementary results of the two different stage which tolerates interface to the following main amplifier stage as shown in figure 9. The differential configuration through the photo-receiver decreases the effect of bond wire inductance of ground and supply voltage by reducing concurrent switching currents. This trans-impedance amplifier is selected mutually with source having shunt feedback resistance and a resistive load. Its main use is to change the input current to an output voltage with a further rise in the gain of the amplifier [ 20 ].

3.8. Switched capacitor feedback TIA

The switched capacitor based trans-impedance amplifier shown in figure 10 has high accuracy, high linearity, CMOS integrated structure and is mostly used for Lab-on-chip type systems. They also offer desirable features such as low power and low noise [ 21 ]. Gain ≈

where T is the clock period. n is 1,2,3….

TIA output voltage at the completion of each T is ( ) = 1 ∫ 3+ ( −1) 1 2+ ( −1) ( ) where

3.9. TIA with T-resistor feedback network

This topology overcomes the trade-off between gain and bandwidth which is a disadvantage in resistive feedback topology. It is possible to increase the bandwidth range here even after smaller values of Rf as shown in Fig. 11 but with an increased value of input inferred noise [ 22 ].

3.10. Inverter TIA

This topology consists of a PMOS and NMOS as shown in figure 12 with a pseudo-feedback resistor Rf. The transistor works in subthreshold region providing high gain bandwidth product with less power consumption. Its performance in terms of noise is also better [ 23 ]. • Input referred noise current )]

4 [(2 (1 − )2 + ()2 )2 + 1 + (2(1− )2)2 ( + 4 + 2 ( 1

∆ 2 + ω2( + )2) 4 + 2 ( 1 ∆ 2 + ω2( + )2)

RGC (Regulated cascode) TIA TIA capacitive feedback

with

TIA with integrator and differentiator Inverter TIA

(1 + 2) 1 2 1 −( + ) 1

4. Recent Developments

The author in [ 21 ] reported a dual probe STM (Scanning Tunnelling Microscopy) system fabricated using CMOS-MEMS technology with a switched capacitor TIA on single chip for each

1 1 + 2) 2 probe. The tunnelling current is sensed and amplified by the TIA. Here imaging throughput of the STM is increased by multi-channel imaging and the TIA has low power consumption and lesser noise.

The author in [ 23 ] designed a dual frequency oscillator made of seesaw shaped tungsten resonator using BEOL (back ed of line) CMOS technology and ultra-compact TIA core. It was operated with reduced voltage supply and its phase noise at two different operating frequencies are noted and finally a comparative analysis with existing topologies were given.

The author in [ 24 ] proposed a back end of line embedded CMOS-MEMS resonator with a double ended tuning fork working in non-linear region and designed using standard 0.35µm CMOS process along with TIA with integrator and differentiator to find their figure of merit. The TIA has high gain and very less noise.

The author in [ 25 ] proposed a Lame mode MEMS resonator with TIA in fully differential configuration showing the performance of oscillator in terms of noise and stability. The TIA bandwidth can be configured to achieve minimum input referred noise current and frequency stability.

The author in [ 26 ] designed RGC-TIA for a MEMS oscillator to minimize its loading effect and also reducing the input parasitic capacitance of the TIA. A wine glass disk resonator is proposed using silicon carbide substrate operating in bulk mode. Here the RGC-TIA acts like a buffer and is so designed to minimize the input referred noise.

The author in [ 27 ] proposed a low power RGC-TIA and Cherry-Hooper wide band amplifier for VHF micromechanical systems. A linear RLC model is interfaced with this TIA for showing oscillations. The TIA gain and bandwidth are chosen to compensate the loss of resonator.

The author in [ 28 ] presented a tuneable gain front-end TIA integrated with CMOS-MEMS resonators with low power consumption and lesser noise. The system is analysed both in open loop and closed loop configuration. Here different resistances are used to achieve extremely low input current noise.

The author in [ 29 ] presented a monolithic MEMS oscillator with ultra-low power consumption fabricated with TIA on same die to get a higher Q value giving a comparative analysis with quartz crystal oscillators. Here capacitive TIA compensate the high insertion loss of the resonator keeping zero phase delay at operating frequency.

The author in [ 30 ] proposed a CMOS-MEMS oscillator with integrated TIA and analysed with piezoelectric crystal oscillators with ultra-low power specifications. Here optimization of input capacitance and thermal noise voltage of the amplifier is done by integrating the TIA with resonator.

The author in [ 31 ] presented a micromechanical active low pixel array with BEOL CMOS-MEMS structures and a power scalable TIA and analysed to obtain better phase noise for improving the frequency stability, the phase feedback noise suppression technique. This proposed TIA occupies less chip area in comparison to ID-TIA.

CMOS

Technolo gy 0.35 µm 0.35µm

Topology

Capacitive

feedback

Switched

capacitor Power consumptio n 930 µW 3.2Mw

5. Scope of Future Work

CMOS-MEMS resonators are designed and integrated using TIA for the purpose of size reduction, lower power consumption and fabrication on a single chip. NEMS (Nano electromechanical systems) are the mechanical structures in Nano-metres range and BIOTRONICS are the miniaturized hardware circuits or technologies involved in the life sciences these are some of the fields in which research is going on and still have large scope for future exploration along with silicon less CMOS-MEMS structures for a cheaper fabrication [ 32, 33 ].

6. Conclusion

In this paper various front-end TIA topologies are discussed which are used with CMOS-MEMS resonators using on- chip integration. These integrated structures occupy minimum chip area for fabrication and have longer life due to reduced power consumption. The use of TIA helps in amplifying the current signals obtained from the transducer by converting the light signals of the photodiode sensors. The selection of the TIA topology used directly depends on the phase noise and nonlinearities involved. The value of input inferred noise gives the phase noise and there is a trade-off between noise and bandwidth achieved. The deciding factor is the resonator frequency with which the TIA is used. Among all the topologies discussed above, the resistive feedback topology is the simplest one in configuration and gives the best performance in terms of low noise, better gain, and higher bandwidth trade-off. 7. References

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