In order to assess the survivability of MEMS  device s over nume rous cycles in vacu um, we set  up a te st of the larg e sliding do or device. The   device  comp rises a f ringing field  driven comb  drive coupled to a fo rce-di stance multip lier,7 and  then to 40x4 46 µm sliding  door. T his device was  built u sing the Sa ndia National  Lab oratory   SUMMiT  process  [http://mems.sandia.gov/scripts/index.asp] and  made u se of several sta ndard pa rts from thei r  design catalo g. 

We ran th e door over a  range of m otion of ~2 0 µm  using a simple si nusoidal drive r. While thi s  waveform is not optimized for th e mech anical  structure, fre quen cies up to 10 kHz were po ssible  without ab normal motion s. Becau se we ran the   structure o utside  of it s resona nce, t he p eak  driving voltag e wa s relati vely high (~1 00V). The  test took pla ce over several wee ks of continuo us  vacuum  exposure with several sh ort intervals of  oscillation during our set up process.  We also  exposed thi s device to over-voltag e conditions on  several occa sions and h ad to mecha nically retu rn  the structu re to its nomi nal positio n using on e of  the microprobes. Thu s, this process constituted a  severe ove r-test of th e structure compared to th e  sort of h andling that  wou ld be  done on space  flight parts. 

Once the driving voltages and freq uency had   been  worked  out m anuall y, we b egan the lo ng  duratio n testi ng und er th e com puter control. A  total of >28 7 hours total runni ng time wa s  collected at 1 0 kHz, prod ucing >1010 cycle s on the  entire m echanism. Th roughout this tim e, the blur  image was periodically m onitore d, and  no obviou s  change i n the motion was ob served. At the end o f the test, sl ow (1-10 Hz) cyclin g of t he do or was  perfo rmed; th is motion did not indicate a ny chan ge co mpared to the pre -test mo tion.  Figure 4.  Required op erating v oltage, dev ice Qfactor, and resona nt frequency  plotted v ersus partial pr essure o f air in the micro probe s tation. All six osci llator geom etries show the s ame basic res ponse, with ful l vacuum-t ype motions not being experien ced until the pressu re is below ~13 Pa (0.1 Torr).  

Discus sion 

We h ave con structed th e first va cuum microprobe te st facility intend ed spe cificall y for the dev elopme nt of  MEMS devices for in -space appli cation s. This extre mely flexible and hig hly integrate d facilit y allows u s to  optimize the developm ent of MEMS for the rigo rs of the space environ ment.  Further, initi al in-vacuum  testing has shown extrem ely promi sing results, indicating that ME MS will likely be suitable for num erous  appli cation s in variou s space sen sors a nd co ntrol sy stem s. 

In addition to indicating th at MEMS are suitable for space applications, these re sults sugg est that many  MEMS devices used in  sealed packa ges for terre strial applicatio ns might benefit from va cuum packagin g  instea d. Vacuum pa ckagi ng will sig nificantly enha nce the Q of various  oscillators, improving thei r  perfo rman ce for appli cations such as RF filters and precision resonant freq uency devices. Comp arison of  perfo rman ce in air and  that in vacuum  indicates that the Q of the devi ce was ~1000 tim es greater in  vacuum.  While some i mproveme nt in the motion  was exp ected, this ve ry large  differen ce was not  anticip ated. In addition, th e results sho wn in this study indicate that the damping process obse rved at   variou s partial pre ssures does not a ppear to be d ue to sim ple gas-dynami c damping alone. Rathe r, a  more compli cated and inte resting set of phy sical me chani sms a ppears to be at work.  183