EE247B/ME218: Introduction to MEMS Design Lecture 7m1: Lithography, Etching, & Doping CTN 2/6/18

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1 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Dping f Semicnductrs Semicnductr Dping Semicnductrs are nt intrinsically cnductive T make them cnductive, replace silicn atms in the lattice with dpant atms that have valence bands with fewer r mre e - s than the 4 f If mre e - s, then the dpant is a dnr P, As The etra e - is effectively released frm the bnded atms t jin a clud f free e - s, free t mve like e - s in a metal Etra free e - P Dpe P The larger the # f dnr atms, the larger the # f free e - s the higher the cnductivity EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 48 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 49 Dping f Semicnductrs (cnt) Cnductivity Equatin cnductivity electrn mbility q n n q electrn density charge magnitude n an electrn If fewer e - s, then the dpant is an acceptr B B Dpe Lack f an e - = hle = h + When e - s mve int h + s, the h + s effectively mve in the ppsite directin a h + is a mbile (+) charge carrier EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 50 p p hle mbility hle density B hle Diffusin EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/

2 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Diffusin in licn Mvement f dpants within the silicn at high temperatures Three mechanisms (in ) Diffusin in Plysilicn In plysilicn, still get diffusin int the crystals, but get mre and faster diffusin thrugh grain bundaries Result verall faster diffusin than in silicn Substitutinal Diffusin Interstitialcy Diffusin Impurity mves alng Impurity atm vacancies in the lattice replaces a atm in the lattice Substitutes fr a atm in the lattice atm displaced t an interstitial site Interstitial Diffusin Impurity atms jump frm ne interstitial site t anther Get rapid diffusin Hard t cntrl Impurity nt in lattice s nt electrically active EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 59 Fast diffusin thrugh grain bundaries Regular diffusin int crystals In effect, larger surface area allws much faster vlumetric diffusin EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 60 Basic Prcess fr Selective Dping 1 Intrduce dpants (intrduce a fied dse Q f dpants) (i) In implantatin (ii) Predepsitin 2 Drive in dpants t the desired depth High temperature > 900 C in N 2 r N 2 /O 2 Result dpants Predepsitin Furnace-tube system using slid, liquid, r gaseus dpant surces Used t intrduced a cntrlled amunt f dpants Unfrtunately, nt very well cntrlled Dse (Q) range ± 20% Fr ref w/ in implantatin ± 1% (larger range & mre accurate) Eample Brn predepsitin Furnace tube Drive-in EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 61 Gases O 2 + B 2 + carrier gas dibrane (Inert gas eg, N 2 r Ar) O 2, B 2 bat wafer Predepsitin Temp C EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/

3 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Basic Prcedure E Brn Predepsitin O 2 O 2 B B B B B B 1 Depsit B 2 O 3 glass O 2 diffusin barrier (masks ut dpants) 2 B diffuses frm B 2 O 3 E Brn Predepsitin (cnt) Fr better unifrmity, use slid surce Furnace tube wafer Difficult t cntrl dse Q, because it s heavily dependent n partial pressure f B 2 gas flw this is difficult t cntrl itself get nly 10% unifrmity Furnace tube crss-sectin Less B cncentratin Brn/Nitride wafer 2% unifrmity Reactins B 2 + 3O 2 3H 2 O + B 2 O 3 + O 2 O 2 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 63 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 64 General Cmments n Predepsitin Diffusin Mdeling Higher dses nly Q = cm -2 (I/I is ) Dse nt well cntrlled ± 20% (I/I can get ± 1%) Unifrmity is nt gd ± 10% w/ gas surce ± 2% w/ slid surce Ma cnc pssible limited by slid slubility Limited t ~10 20 cm -3 N limit fr I/I yu frce it in here! Fr these reasns, I/I is usually the preferred methd fr intrductin f dpants in transistr devices But I/I is nt necessarily the best chice fr MEMS I/I cannt dpe the underside f a suspended beam I/I yields ne-sided dping intrduces unbalanced stress warping f structures I/I can d physical damage prblem if annealing is nt permitted Thus, predepsitin is ften preferred when dping MEMS EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 65 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/

4 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Diffusin Mdeling (cnt) Diffusin Mdeling (Predepsitin) EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 67 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 68 Diffusin Mdeling (Limited Surce) Diffusin Mdeling (Limited Surce) EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 69 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/

5 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Tw-Step Diffusin Tw step diffusin prcedure Step 1 predepsitin (ie, cnstant surce diffusin) Step 2 drive-in diffusin (ie, limited surce diffusin) Fr prcesses where there is bth a predepsitin and a drive-in diffusin, the final prfile type (ie, cmplementary errr functin r Gaussian) is determined by which has the much greater Dt prduct (Dt) predep» (Dt) drive-in impurity prfile is cmplementary errr functin (Dt) drive-in» (Dt) predep impurity prfile is Gaussian (which is usually the case) EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 71 Successive Diffusins Fr actual prcesses, the junctin/diffusin frmatin is nly ne f many high temperature steps, each f which cntributes t the final junctin prfile Typical verall prcess 1 Selective dping Implant effective (Dt) 1 = (R p ) 2 /2 (Gaussian) Drive-in/activatin D 2 t 2 2 Other high temperature steps (eg, idatin, reflw, depsitin) D 3 t 3, D 4 t 4, Each has their wn Dt prduct 3 Then, t find the final prfile, use Dt Diti tt in the Gaussian distributin epressin EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 72 i The Diffusin Cefficient Diffusin Cefficient Graphs E A D D ep kt (as usual, an Arrhenius relatinship) Substitutinal & Interstitialcy Diffusers Interstitial Diffusers Nte the much higher diffusin ceffs than fr substitutinal EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 73 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/

6 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Metallurgical Junctin Depth, Epressins fr = pint at which diffused impurity prfile intersects the backgrund cncentratin, Lg[N()] N O eg, p-type Gaussian eg, n-type N O - Lg[N()- ] Net impurity cnc p-type regin Assuming a Gaussian dpant prfile (the mst cmmn case) N 2 Dt 2 j j, t N ep Fr a cmplementary errr functin prfile j 2 N Dt ln N B = distance f/ surface n-type regin = distance f/ surface N Dt 2 j j, t Nerfc j 2 Dterfc 1 N N B EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 75 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 76 Sheet Resistance # Squares Frm Nn-Unifrm Traces Sheet resistance prvides a simple way t determine the resistance f a given cnductive trace by merely cunting the number f effective squares Definitin What if the trace is nn-unifrm? (eg, a crner, cntains a cntact, etc) EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 77 EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/

7 EE247B/ME218 Intrductin t MEMS Design Lecture 7m1 Lithgraphy, Etching, & Dping Sheet Resistance f a Diffused Junctin Irvin s Curves (fr n-type diffusin) Fr diffused layers Sheet resistance R s j Effective resistivity 1 1 j d Majrity carrier mbility d This epressin neglects depletin f carriers near the junctin, thus, this gives a slightly lwer value f resistance than actual Abve epressin was evaluated by Irvin and is pltted in Irvin s curves n net few slides Illuminates the dependence f R s n, N (the surface cncentratin), and (the substrate backgrund cnc) EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 79 qn [etrinsic material] Net impurity cncentratin Eample p-type Given = cm -3 N = cm -3 (n-type Gaussian) = 277 m Can determine these given knwn predep and drive cnditins Determine the R s EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/09 80 Irvin s Curves (fr p-type diffusin) Eample n-type Given = cm -3 N = cm -3 (p-type Gaussian) = 277 m Can determine these given knwn predep and drive cnditins Determine the R s EE C245 Intrductin t MEMS Design LecM 4 C Nguyen 8/20/