review of atomic MEMS

review of atomic MEMS
review of atomic MEMS

REVIEW PAPER

Review of atomic MEMS:driving technologies and challenges

Haifeng Dong ?Jiancheng Fang ?Binquan Zhou ?

Jie Qin ?Shuangai Wan

Received:8January 2010/Accepted:26April 2010/Published online:16May 2010óSpringer-Verlag 2010

Abstract Atomic MEMS technology is an emerging multidisciplinary subject which bene?ts from different ?elds,including microfabrication,laser technique and atomic physics,etc.This paper gives an overview of atomic MEMS and discusses the challenges faced in the design and manufacture of atomic MEMS devices.

1Introduction

‘‘Atomic MEMS’’is ?rst mentioned in 2007(Eklund and Shkel 2007;Eklund et al.2007;Eklund et al.2008),which includes chip-scale atomic clocks,atomic magnetometers and atomic gyroscopes,etc.These devices produce high-precision frequency signal (Knappe et al.2006;Lutwak et al.2005;Youngner et al.2007),measure sub-pT or even lower magnetic ?eld without cryogenic equipment (Shah et al.2007),and test rotation with high stability through interactions of resonant light with atomic vapor (Kitching et al.2009;Kornack et al.2005;Lust and Youngner 2007;Peng et al.2007).They are enabling technology for high-security communications,ultra-sensitive radar and uncry-ogenic magnetoencephalography,etc.And these devices have applications in geophysical surveying,the detection of unexploded ordinance,earth prediction,space science,nuclear magnetic resonance,health care,telecommunica-tion,navigation,guidance,position and orientation system

and perimeter and remote monitoring,etc.(Hodby et al.2007).

Atomic MEMS is mainly driven by three technologies as illustrated in Fig.1.One is the reliable,small,inex-pensive and tunable diode laser (DL)source,the other is the technique of generating long relaxation dense vapor (LRDV),the third is MEMS which enable the fabrication of atomic vapor cell chip (AVCC)(Budker and Romalis 2007).The applications of these technologies in atomic MEMS are reviewed and discussed here.

Although the high precision of atomic MEMS devices is veri?ed experimentally,there are still a lot of technological challenges in the design and fabrication of atomic devices.We review ?ve such challenges and solutions.Potential solutions for these challenges are also discussed.

2Driving technologies of atomic MEMS

Atomic MEMS devices have the potential to increase the ratio of precision to volume in several orders.The devel-opment of atomic MEMS bene?ts from the development of different ?elds,such as the DLs in laser technology,LRDV in atomic physics and AVCC in MEMS.DL can replace the traditional alkali lamp which has large volume and is energy-sucking.Furthermore,the development of vertical cavity semiconductor laser (VCSEL)makes it feasible for wafer-level fabrication of atomic MEMS devices.The realization of LRDV in millimeter-scale vapor cell makes ultra-high precision atomic MEMS devices feasible.The AVCC enabled by MEMS process uses glass-silicon-glass anodic bonding to generate the chip-scale cavity containing the alkali metal.With the improvement of these technol-ogies,atomic MEMS is spurring more and more invest-ment and attracting research efforts which will greatly

H.Dong (&)áJ.Fang áB.Zhou áJ.Qin áS.Wan

Sino–UK Space Science and Technology Joint Laboratory,School of Instrumentation Science and Opto-electronics Engineering,Beijing University of Aeronautics and Astronautics,New Mainbuilding B603,

Xueyuan Rd.37#,Haidian District,Beijing 100191,China e-mail:shanzhishan@https://www.360docs.net/doc/8613035734.html,

Microsyst Technol (2010)16:1683–1689DOI 10.1007/s00542-010-1089-6

evolve the performance of the devices.The brief intro-ductions of these technologies and their applications in atomic MEMS are as below.2.1Diode laser

Diode lasers include VCSEL and edge emitting lasers (EELs),such as DFB laser and FP laser.The typical center wavelength needed for the atomic MEMS devices are 894.6nm (D1)and 852.3nm (D2)for cesium,795.0nm (D1)and 780.2nm (D2)for rubidium,and 770.1nm (D1)and 766.7nm(D2)for potassium.The D1transition wavelengths has higher performance than D2in the CPT

atomic clock (Lutwak et al.2003;Sta

¨hler et al.2002)and it is also important in the atomic magnetometer and atomic gyroscope for the pumping of the alkali atoms.The sta-bility of the center wavelength should be within the Doppler-broadened atomic linewidth for the life of the devices.Both the linewidth and the power of the light should be optimized because they are related to the noise in atomic clock and polarization in magnetometer and gyroscope.

VCSEL is ?rst proposed by Iga in 1979,and has been studied worldwide (Soda et al.1979).It has numerous advantages over other optical sources,such as wafer testing before packaging,low power consumption,high coupling ef?ciency and fabrication in array con?guration (Levallois et al.2006).Around 2000,technical breakthroughs such as semiconductor distributed Bragg re?ector (DBR)and oxide aperture boost the VCSEL performance up from research to production level (Kevin Nishikata et al.2005).This provides the best light source for the fabrication of chip-scale atomic MEMS devices.VCSELs are soon used in the chip-scale atomic clock (Knappe et al.2004)and later in the chip-scale atomic magnetometer (Peter et al.2004).Jim Tatum’s report in (Tatum 2007)gives a detailed review of the VCSEL’s developments and applications.

Figure 2is the optical spectrum of a VCSEL,EEL and Light Emitting Diode (LED).Usually the side mode sup-pression ratio (SMSR)and linewidth of the single mode VCSEL are enough for the application of chip-scale atomic

devices,and the center wavelength can be adjusted by the temperature and current.Furthermore,VCSEL has less electrical power than the EEL lasers and the out-of-plane beam direction makes the chip-scale integration easier.These advantages make it the most suitable light source for the atomic MEMS devices.But there is still no commercial off-the-shelf (COTS)VCSEL for certain wavelengths,such as the D1of cesium.In this case EEL,such as distributed feedback laser (DFB),may substitute VCSEL.Typical COTS 894nm laser are provided by Nanoplus Inc.of Germany and Modulight Inc.of Finland.2.2Long relaxation dense vapor

Before the realization of LRDV,it is a dilemma for the vapor density design.In one hand,higher density lead to higher output signal,in the other hand,higher density lead to spin-exchange broadening of the linewidth which is linear with the sensitivity of the sensors.Although W.Happer and H.Tang has predicted and analyzed the spin-exchange relaxation free (SERF)regime theoretically in 1970s (Happer and Tam 1977;Happer and Tang 1973),it is till 2002that Romalis et al.observe the phenomenon and employ the regime in their high-precision magnetom-eter (Allred et al.2002).The resolution up to 0.54fT/Hz 1/2is realized in the next year which is better than the super-conducting quantum interference devices (SQUIDs)(Kominis et al.2003).Figure 3shows the linewidth dif-ference among the normal regime,the light-narrowing regime and the SERF regime (Savukov et al.2005).The applications of SERF magnetometer in the NMR detection (Savukov and Romalis 2005)and magnetoencephalogra-phy (Xia et al.2006)have also been reported.

In 2007,the SERF regime is ?rst realized in an AVCC,which is a breakthrough of the atomic MEMS devices.The resolution of chip-scale atomic magnetometer is

enhanced

Fig.2Optical spectrums of VCSEL,LED and EEL (Courtesy of Jim

Tatum)

Fig.1Scheme of three driving technologies (D1–D3)of atomic devices and ?ve challenges (C1–C5)which bridge the atomic MEMS devices with its application

from 5pT to 70fT (Schwindt et al.2007;Shah et al.2007),which demonstrates the ultra-high precision of atomic MEMS devices.

2.3Atomic vapor cell chip

Atomic vapor cell chip (AVCC)is ?rst proposed in 2002(Kitching et al.2002).Coherent population trap (CPT)signal of AVCC is demonstrated later (Liew et al.2004)which is one of the cornerstones of the atomic MEMS technology.It is then used in the atomic MEMS devices such as chip-scale atomic clocks (Knappe et al.2004)and atomic magnetometers (Peter et al.2004;Schwindt et al.2007;Shah et al.2007).The ?rst atomic MEMS gyroscope based on VCSEL and AVCC is proposed in a patent of Honeywell (Lust and Youngner 2007).

Generally,glass-silicon-glass triple layer bonding is used to fabricate AVCC.The dif?culty is the generation of the alkali metal in the micro sealing chamber.There are typically two methods to do this.One is physical method,in which pure alkali metal is injected into the chamber directly.The other is chemical method,in which the alkali metal is generated by chemical reaction.For the physical method,pipette and glove box are often used for the injection of the alkali metal and assembly of the chip stacks (Liew et al.2004).And encapsulated alkali metal in wax micropacket is also a way to put the alkali metal into the chamber.(Radhakrishnan and Lal 2005).For the chemical method,many reactions can be used to produce alkali metal,such as the reaction between barium azide and alkali chloride (Knappe et al.2005),the decomposition of Cs 2CrO 4with reducing agent (St101)under laser heating (Douahi et al.2007),the decomposition of CsN 3under ultraviolet (Liew et al.2007)heating and electrolysis of cesium-enriched glass (Gong et al.2006).

Wafer-level glass-blown process for the fabrication of AVCC is also researched to produce spherical micro cell chip (Eklund et al.2008).After the wafer bonding,the stack is placed inside a furnace with temperature about 850°C.The expansion of the trapped gas causes the high temperature glass to blow into spherical cells.The vapor cell fabricated in this way provides more optical ports,which makes the double beam light scheme easier to be realized than the anodic-bonded AVCC.

Figure 4a,b are the AVCCs fabricated using different methods,respectively.Considering the integrated fabrica-tion object of atomic MEMS sensors,in the future the AVCC will include RF coil,thermal components and even the polarizer and quarter wave plate.Fig.4c is the atomic vapor cell with in-plane RF coil.

3Challenges of atomic MEMS devices

Technical challenges and solutions of atomic MEMS devices are reviewed and discussed as below.These challenges include the thermal management,micro magnetic shielding,high temperature anti-relaxation surface coating,light-?eld scheme design and integrated fabrication.

3.1Thermal management

The challenge of thermal management of atomic MEMS devices comes from two aspects,one is the

temperature

Fig.3Comparison of Zeeman resonance widths in normal regime (high magnetic ?eld and low polarization),the light-narrowing regime (high magnetic ?eld and high polarization),and the SERF regime (very low magnetic ?eld)(Savukov et al.2005)(a –c

)

Fig.4AVCCs of different fabrication processes and structures a Glass-silicon-glass bonding AVCC (Douahi et al.2007)b Wafer level glass blown AVCC fabricated by E.Jesper Eklund et al.(Eklund et al.2008)c Atomic vapor cell with integrated RF coil (a ,b )

control of the diode laser and the other is the heating of the AVCC to realize the SERF regime.

For the typical temperature tuning coef?cient of VCSEL(0.06nm/°C)(Serkland et al.2007;Tatum2007), the laser source temperature variation should be lower than 0.18°C to keep the polarization of the alkali atoms at all times during operation,which corresponds to the typical N2 quenching gas pressure used in atomic devices(B200torr) (Allred et al.2002;Kominis et al.2003;Kornack et al. 2005;Ledbetter et al.2008;Liew et al.2004).The actual operation temperature is dependent on the center wave-length and temperature tuning coef?cient of the laser diode.

To realize the SERF regime,the vapor cell must be heated to above100°C,which increases the power con-sumption and makes the thermal isolation package neces-sary in bio applications.To reduce the power of SERF regime,different methods can be used separately or toge-ther,such as the thermal isolation island structure,local heating and light induced vapor desorption.

Island structure has been fabricated in the chip-scale atomic clocks(CSAC)designed by Draper Laboratory (Mescher et al.2005)and Honeywell(Youngner et al. 2007).Figure5a,b are the SEM pictures which show the serpentine thermal isolation legs and the vapor cavity. Smart thermal conduction such as thermal switch(Laws et al.2008)and variable thermal resistor(Kim et al.2008) are used to regulate the thermal conduction between the thermal isolation island and outer package according to the

chips and ambient temperature variations.

Local heating is a technique that has been used extensively in the MEMS fabrication,the group of Liwei Lin at Berkeley University does a lot of researches on the local heating and bonding processes for the pack-aging of MEMS devices(Cheng et al.2002;Cheng et al. 2000;Kim et al.2002;Su and Lin2001).Local heating by laser is also used in the atomic cavity fabrication in which the dispenser temperature is heated up to800°C locally(Douahi2007).It is also a way to increase the vapor density with no increasing of the whole tempera-ture of the vapor cavity.

Light induced vapor desorption(LIAD)is a potential method to decrease the power consumption and realize the SERF regime in room temperature.The dynamics of LIAD with rubidium and silane-coated cells has been reported by Mariotti et al.(1994).The effect has also been reported in paraf?n-coated cells in which the density increases by a factor of four(Alexandrov and Balabas2002).T. Karaulanov et al.extend the spectral range of the desorbing light into the ultraviolet and investigate the LIAD effect with potassium,rubidium,cesium and sodium.The density increasing factor is close to ten under closed stem (Karaulanov et al.2009).3.2Micro magnetic shield technology

To many atomic MEMS devices,such as chip-scale atomic clock and atomic gyroscope,magnetic?eld?uctuations and strays can cause shifts of the atomic transition fre-quencies and thus the instability of the sensing output. Furthermore,for those atomic MEMS devices working under the SERF regime,low magnetic?eld is needed. Micro magnetic shield is necessary to separate the devices from the external magnetic?eld.

The magnetic shielding factor S is de?ned as the ratio of the magnetic?eld outside the shielded volume to the?eld inside the shielded volume.In case that the magnetic per-meability is much larger than one and the shell thickness is much smaller than the shield diameter.The spherical shell shielding factor can be simpli?ed as(Donley et al.2007). S?1t

4

3

lát

D

e1TWhere l is the magnetic permeability,t is the shell thickness,D is the shield diameter.

From Eq.(1),we can get two deductions.One is that for the certain permeability and shell thickness,the smaller shield diameter can achieve larger S.The other is that

for Fig.5SEM pictures of the serpentine thermal isolation structure,a is the frontside and b is the backside(Courtesy of D.W.Younger)

certain volume,multi-layer shielding can achieve much larger S than increasing the shell thickness.

Figure 6shows the multi-layer micro magnetic shields with shielding factor up to 69106for a three layer shields.(Donley et al.2007)In comparison,the macro magnetic shields can only get a shielding factor of 106for ?ve layer shields.(Allred et al.2002).

The magnetic shield fabricated on-chip has not been experimented so far as the authors know,which should be a better choice because multi-layer is easier to be fabricated using deposition technology.

3.3High temperature anti-relaxation surface coating Recently SERF regime atomic sensors are realized in tra-ditional glass vapor cell (Kominis et al.2003)and chip-scale vapor cell (Shah et al.2007),respectively.The elimination of self exchange relaxation under SERF regime increases the contribution of diffusion to the relaxation.The scale effect of relaxation due to diffusion is propor-tional to l 2,where l is the radius of the vapor cell.(Allred et al.2002)So relaxation due to diffusion becomes more important when the devices are going to be scaled down.There are two methods to decrease the relaxation rate due to diffusion,one is the buffer gas,and the other is the anti-relaxation coating.Buffer gas scheme is easier to be realized and can be used to measure the gradient of the magnetic ?eld.Anti-relaxation coating has other advanta-ges such as larger optical rotation signals,lower laser power requirements,and the avoidance of linewidth broadening caused by the spin destruction collisions with buffer gas atoms and the magnetic gradient.(Seltzer 2008)Unfortunately,the commonly-used and well-documented paraf?n coating cannot work when the temperature is above 80°C.Octadecyltrichlorosilane (OTS)is also reported as an anti-relaxation coating,compared to the 10,000times bounce before unpolarization of paraf?n coating,the OTS is less than 2,100(Seltzer et al.2008).There is also report on chip scale vapor cell fabrication with octadecyltrimethoxysilane (ODS)coating,which is

used in a chip-scale atomic clock.(Hasegawa et al.2008).A paraf?n-quality coating which resists high tempera-ture can bene?t high-precision atomic MEMS devices greatly.

3.4Laser beam scheme for the atomic MEMS devices There are typically two laser beam schemes for the atomic devices,one is the single-beam scheme (Kitching 2007;Schwindt et al.2007;Shah et al.2007)and the other is the orthogonal double-beam scheme (Allred et al.2002;Bud-ker et al.2000;Kominis et al.2003;Ledbetter et al.2008).In the single-beam scheme,the amplitude of laser is measured and inputted to the lock-in ampli?er.In the double-beam scheme,the rotation of the probe beam can be measured through differential photo detector where the laser amplitude ?uctuation is eliminated.

When the atomic MEMS devices are concerned,the single-beam scheme is much easier to be fabricated than the orthogonal double-beam scheme.John Kitching group of NIST uses the single-beam scheme and Mx technique to achieve the sensitivity of 5pT/Hz 1/2and 70fT/Hz 1/2,respectively,in a chip-scale atomic magnetometer with MEMS components (Schwindt et al.2007;Shah et al.2007).Honeywell company applies a patent of an atomic MEMS gyroscope with a double-beam scheme,in which the \111[surface of the silicon etched by KOH is used to re?ect the light from out-of-plane to in-plane direction (Lust and Youngner 2007).And the re?ectivity of the 54.74°surface can be improved through multi-layer coat-ing (Perez et al.2008a ,b ,2009).NIST also applies a patent based on a diverging beam of light that passes through an alkali atom vapor cell.This patent utilizes the distribution of the beam propagation to realize longitudinal optical pumping of the atomic system and detection of the trans-verse atomic polarization simultaneously.It can be used both in a magnetometer and in a gyroscope (Kitching et al.2009).Xing Song et al.design a double-beam micro atomic scheme in which the orthogonal beams are inclined and intersect in the atomic cavity.(Song et al.2008)This design simpli?es the fabrication by using the same AVCC with that of the single-beam scheme.3.5Integrated fabrication

Most of the prototypes of the current atomic MEMS devices are assembled after dicing (Lust and Youngner 2007;Lutwak et al.2005;Schwindt et al.2007),but they have the potential to be fabricated in the wafer-level mode which can reduce the manufacturing costs and increase the reliability.

Considering the components and 3D topology of the atomic devices,bonding is a suitable integrated

fabrication

Fig.6Micro magnetic shields developed by E.A.Donley et al.(Donley et al.2007)

method.The bonding of the MEMS-fabricated vapor cell with the polarizer,wave plate,neutral density ?lter,diode laser chip and the photodiode will be the focus of the next step to the ?nal integrated fabrication after the MEMS-fabricated vapor cell with RF and thermal components are realized.Fig.7is the wafer-level conceptional design for atomic MEMS devices (Knappe et al.2006).All compo-nents are fabricated on individual wafers.The wafers are bonded and diced afterwards into individual sensor dies (Knappe et al.2006).Just like the multilayer gas turbine of MIT (Savoulides et al.2008),this is a similar daunting task for the microfabrication researchers.And the challenge is to bond multilayer with different material and structures.

4Conclusions

The development of diode laser (DL),long relaxation dense vapor (LRDV)and atomic vapor cell chip (AVCC)makes the atomic MEMS an exciting ?eld which can achieve very high measurement precision in a batch-fab-ricated small volume.This paper reviews the driving technology and challenges of this new ?eld.Some possible solutions for these challenges are also mentioned and discussed.

Acknowledgments This work is supported by the grant of Key Programs of National Science Foundation of China under Grant No.60736025and Major Programs of China National Space Adminis-tration under Grant No.D2120060013.The authors would like to thank Prof.Lei Guo,Prof.Gang Liu,Prof.Wei Sheng,Dr.Ye Hong and all the staffs in the Novel Inertial Instrument and Navigation System Laboratory of BUAA for their support and bene?cial discussion about the paper.

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MEMS设计、仿真软件的综合比较

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