HDL Simulation of a Thermal Accelerometer for System Design

Transcription

1 DL Smulaton of a Themal Acceleomete fo System Desgn Aboubaca Chaeho, Lauent Latoe, Olve Leman, Fédéck Mally, Pascal Nouet To cte ths veson: Aboubaca Chaeho, Lauent Latoe, Olve Leman, Fédéck Mally, Pascal Nouet. DL Smulaton of a Themal Acceleomete fo System Desgn. XX EUROSENSORS, Sep 006, Götebog (Sweden), 006. <lmm00070> AL Id: lmm Submtted on 9 Sep 006 AL s a multdscplnay open access achve fo the depost and dssemnaton of scentfc eseach documents, whethe they ae publshed o not. The documents may come fom teachng and eseach nsttutons n Fance o aboad, o fom publc o pvate eseach centes. L achve ouvete pludscplnae AL, est destnée au dépôt et à la dffuson de documents scentfques de nveau echeche, publés ou non, émanant des établssements d ensegnement et de echeche fanças ou étanges, des laboatoes publcs ou pvés.

2 DL SIMULATION OF A TERMAL ACCELEROMETER FOR SYSTEM DESIGN O. Leman, A. Chaeho, F. Mally, L. Latoe* and P. Nouet Laboatoe d Infomatque de Robotque et de Mcoélectonque de Montpelle UMR 5506 CNRS / Unvesty Montpelle II, 6 ue Ada, 3439, Montpelle Fance *Coespondng autho: Lauent Latoe, Tel: , emal: latoe@lmm.f Abstact: Ths study concens the ntegaton of convectve acceleometes on CMOS. The desgn appoach s based on an DL modelng of the sensng cell. The development of a monolthc system s then llustated usng multdoman smulatons wthn the mcoelectonc desgn envonment. Fo llustaton, the senso equvalent nput nose (.e. the esoluton) s then computed fo vaous eadout ccuty confguatons. Keywods: MEMS, acceleomete, DL modelng INTRODUCTION Convectve acceleometes ae qute dffeent fom the sesmcmassbased equvalents. The wokng pncple eles on the heatng of a small volume of gas, a at atmosphec pessue n ou case, whch s dstubed by an appled acceleaton. As a esult, a tempeatue gadent appeas n the gas volume, whch can be measued by means of tempeatue sensos. Such stuctues have been nvestgated fo decades and epoted on slcon [ 4]. Ths wok addesses the mplementaton of ths sensng pncple on standad CMOS technology. Ths monolthc appoach allows batch poducton and damatc costs eductons ove hybd sensos whch use MEMS and sgnal condtonng on seveal dedcated des. The am s to povde mcoelectoncs engnees wth MEMS IPs, smla n use to analog cells comng fom desgn kt lbaes, whch can be taloed to the desed dmensons, ntegated among the sgnal condtonng electoncs, and fnally smulated wthn the desgn envonment. Ou modelng appoach eles on the use of heat tansfe fundamentals and empcal paametes, povded by pevous FEM smulatons and chaactezatons of physcal test vehcles [5, 6]. Those complementay methods have led to a compact model, eady fo DL ntegaton and system optmzaton. then maxmum at the heate locaton and mnmum at the cavtes boundaes. In absence of acceleaton, the detectos measue the same tempeatue fo symmety eason. As a esult of an acceleaton, the tempeatue gadent defoms and a tempeatue dffeence can be measued acoss the detectos. Because the esstos ae tempeatuedependent, the sgnal s conveted nto a voltage by means of a Wheatstone bdge. eate Sgnal condtonng (a) Detecto s Sensng decton DEVICE UNDER STUDY The acceleomete s composed of thee suspended bdges, composed of the CMOS backend layes (oxde, polyslcon, alumnum, and ntde). Polyslcon s used to mplement esstos, fo both eatng (Rh) and sensng (Rd, Rd ) puposes (fgue a). The heate R s suppled by an electcal cuent n ode to ceate a tempeatue gadent n the top and bottom cavtes (fgue b): the tempeatue s (b) Fgue. Pctue of the senso (a) and coss secton of the senso wth dmenson paametes (b).

3 In ode to compae expemental esults wth FEM smulaton and to buld an DL model of the senso, two pototype vesons (A and B) have been desgned, fabcated and chaactezed. Table summazes the physcal paametes. A B 0 µm 0 µm 570 µm 340 µm D 00 µm 5 µm Rh 70 Ω 350 Ω Rd, Rd 50 kω 50 kω h 300 µm h mm Table. Physcal paametes of the two pototypes DL MODEL OF TE SENSOR The DL model of the senso s epesented on the fgue and some detals ae gven n [7]. Secton models the aveage tempeatue of the heatng bdge: T = T T = R P A th whee T and T A (K) ae espectvely the aveage heate tempeatue and ambent tempeatue, R th (K.W ) s the equvalent themal esstance of the beam and P (W) s the joule powe dsspated n the heate esstance. Note that the value of the heate esstance also nceases lnealy wth ts tempeatue. The second one (secton 3) expesses convectve tansfe fom whch depends the output sgnal. The tempeatue pofle establshed n secton defoms, poducng a tempeatue dffeence acoss the detectos T D. Ths tempeatue dffeence has been found to be popotonal to the Gashof numbe [6]: T D ( Ts Ta) 3 S. G aβρ l = = S. τ p µ τ p () wth S (K) a fttng coeffcent (extacted fom FEM analyss and expemental esults) whch epesents the system senstvty, a (m.s ) the acceleaton, ρ (kg.m 3 ) the gas densty, β (K ) the gas coeffcent of expanson, µ (kg.m.s ) the gas vscosty, l (m) a lnea dmenson elated to the cavty volume and geomety and τ (s) the tme constant of the flud. Fnally, secton 4 mplements detectos tansfe functon takng nto account the tempeatue senstvty of the polyslcon (TCR) and the themal tme constant of the bdges (τ D ): Rd = Rd nom TCR T ± T (3), CM D τ D p Then, detectos vaatons ae conveted by a Wheatstone bdge and a themal nose spectal densty s also added: V n, f ) 4kTD, ²( = Rd (4) SYSTEM LEVEL DESIGN Fgue. Functonal bloc epesentaton of the model. eat tansfe n the a, fom the heate to the detectos, s then sepaated nto two functonal blocs. The fst one (secton ) descbes the conducton heat tansfe, whch was shown to be esponsble of the common mode tempeatue on detectos. Ths model s based on a cylndcal geomety appoxmaton and the common mode tempeatue at a dstance fom the heate s uled by: γ ( T T ) ( T T ) A A T T T γ = ln T γ ln () Ths DL model s used to develop onchp ccuty n ode to ase oveall system pefomance. The whte nose level at the output of the Wheatstone bdge s computed fst and consdeed as a nose floo efeence (ntnsc nose). The smulaton takes nto account the common mode tempeatue of the detectos. Thee eadout ntefaces have been compaed usng multdoman smulatons wthn Cadence. The fst nteface s a smple nstumentaton amplfe ( amplfe ) of gan 000 (see fgue 3a) whch was mplemented on ou fst pototype [56]. Fgue 4 shows that the nput nose spectal densty s then at least one ode of magntude geate than the ntnsc nose n the senso bandwdth (fom DC to 0z). Ths s due to /f nose of the CMOS tansstos. Assumng a lowpass flteng of the output voltage fom DC to 00z, the RMS nose voltage would be.33 µv RMS. Takng nto account the senstvty of ou fst pototype (375 µv/g wthout amplfcaton [5, 6]), the equvalent nose acceleaton would be 6 mg. Fo the second case (amplfe ) a dedcated lownose peamplfe of gan 0 has been customzed

4 and placed ght afte the Wheatstone bdge (see fgue 3b). The CMOS tansstos of the dffeental pa wee dmensoned (W = 400µm, L = 6µm) to educe the /f nose and the nstumentaton amplfe gan was set to 00. Fgue 4 shows that the nose densty s then educed n the senso bandwdth and 00z lowpass flteng would lead to a nose voltage of 549 nv RMS and to a esoluton mpovement of the senso by a facto 4. and 0 kz). It shows that a bandpass flte has to be used to mpove the senso esoluton. e e OP0B k.5k Radj.5k k OP0B 3 a 0k s OP0B 0k Vdd/ Ibas = 0.5mA 400/6 400/6 e e s s 0k 0k 3 b Fgue 5. Electonc achtectue based on a choppestablzed amplfe wth a tackandhold sgnal demodulato Nose Spectal Densty (nv/ z) Fgue 3. Amplfe Schematc Resstos Amplfe Resstos Amplfe Resstos only Bas modulaton & flteng 0,00 0,0 0, Fequency (kz) Fgue 4. Equvalent nput nose spectal densty. Amplfe f M (kz) e nose (nv.z / ) Table. Equvalent nput nose spectal densty fo dffeent bas modulaton fequences. A modulaton of the Wheatstone bdge bas at hgh fequency (.e. befoe the amplfe cutoff fequency) can mpove the senso esoluton by educng the output nose close to the ntnsc nose level of the senso (6 nv.z / ). Table gves the aveage RMS nose spectal densty aound dffeent modulaton fequences f M (, 5 Anothe soluton s to use a choppestablzed amplfe wth a tackandhold sgnal demodulato. Electonc achtectue s pesented on fgue 5. As a symmetcal squae wave basng s used, the senso senstvty s now twce hghe. A hghpass flte (f c =.5kz) s used to elmnate the flcke nose of the onchp amplfe and to each the ntnsc nose of the esstos. Consdeng an nput whte nose spectal densty V n = 6nV.z / fom DC to the amplfe cutoff fequency (f c ), the output nose voltage V out of the tackandhold demodulato s gven by [8, 9]: V V out n f < fck / d h n= snc ( nd) ( d) ( h) (5) whee, f ck s the clock fequency of the demodulato, d s the duty cycle of the tack opeaton and h s the closest ntege to the f c /f ck ato. Wth f ck = 0kz, f c = 00kz, and d=0.4, ths achtectue should pemt to each an output nose only twce hghe than the ntnsc nose of the senso. Obvously, the ntnsc nose of the senso and ts esoluton would also be mpoved by educng the nomnal esstance of the detectos. EXPERIMENTAL RESULTS Ths secton pesents expemental esults obtaned wth the two pototypes. To educe the supply nose densty and 50z paastc sgnals, a battey s used as the powe supply. The heate powe s adjusted to obtan the same heate tempeatue on both sensos. Fst measuements have been made usng 5V (dc) to bas the Wheatstone bdge and to

5 supply the onchp amplfe (amplfe ) and an extenal addtonal amplfe. As the senso bandwdth s about 5z, the output sgnal s lowpass flteed at 00z to emove the hgh fequency nose. Table 3 pesents the esults obtaned wth both sensos. These esults ae pesented n tems of equvalent nput nose voltage and nput senstvty. It takes nto account the electonc gan (amplfe extenal). The nose voltages equal 3.54µV RMS and 3.85µV RMS fo the sensos A and B espectvely, vey close to the.33µv RMS calculated by smulaton. Snce the senstvty of the senso A s hghe, ts nose equvalent acceleaton (NEA) s lowe. Senso A Senso B P heate (mw) 7 0 T heate ( C) Wheatstone bdge bas Senstvty (µv.g ) e nose (nv RMS.z / ) DC 0kz squae wave DC 0kz squae wave NEA (mg.z / ) Table 3. Expemental esults The choppestablzed amplfe wth the tackandhold sgnal demodulato (fgue 5) has been also tested. As expected, senstvtes ae twce hghe on both sensos. Nose voltage and nose equvalent acceleaton have been also mpoved. CONCLUSION Ths pape addesses the desgn of CMOS convectve acceleometes. An DL model s poposed fo a theebdges senso. Ths model s used fo system level smulaton n Cadence. As an llustaton of system level smulatons, thee eadout ccuty confguatons ae compaed, fom the smple nstument amplfe to a complete synchonous detecton scheme. It s shown that usng a bas modulaton and subsequent flteng, the nose of the electoncs can be neglected compaed to the ntnsc nose of the senso tself. Ths esult s confmed by expemental esults obtaned fom two pototypes. REFERENCES. A.M. Leung, J. Jones, E. Czyzewska, J. Chen, B. Woods, Mcomachned acceleomete based on convecton heat tansfe, poc. IEEE MEMS 98, edelbeg, Gemany, 59 Jan. 998, pp V. Mlanovc, E. Bowen, M. E. Zaghloul, N.. Tea, J. S. Suehle, B. Payne, and M. Gatan, Mcomachned convectve acceleometes n standad ntegated ccuts technology, Appl. Phys. Lett. 76 (4) (000), pp F. Mally, A. Gan, A. Matnez, R. Bonnot, P. TempleBoye and A. Boye, Mcomachned themal acceleomete, Sens. Actuatos A 03 (3) (003), pp X.B. Luo, Y.J. Yang, F. Zheng, Z.X. L and Z.Y. Guo, An optmzed mcomachned convectve acceleomete wth no poof mass, J. Mchomech. Mcoeng. (00), pp A. Chaeho, L. Latoe, F. Mally, P. Nouet, Expemental and fnteelement study of convectve acceleomete on CMOS, poc. EUROSENSORS 05, Bacelona, Span, Sept. 4, A. Chaeho, F. Mally, L. Latoe, P. Nouet, Expemental and fnteelement study of convectve acceleomete on CMOS, extended pape of Euosensos 05, to be publshed n Sens. Actuatos. 7. O. Leman, A. Chaeho, F. Mally, L. Latoe, P. Nouet, Modelng of a CMOS Convectve Acceleomete fo DL Integaton, poc. ESSDERC 006, Monteux, Swtzeland, Sept. 8, J.. Fsche, Nose souces and calculaton technques fo swtched capacto fltes, IEEE Jounal of Soldstate Ccuts, Vol. SC7, no. 4 (98), pp A Bllot, G. Moneal, Choppestablzed amplfes wth a tackandhold sgnal demodulato, techncal pape STP 99, Allego McoSystems.