q p P : Si : : : : Si : : : Si : Si : Si : : Extra free e - Dope Here, energy to get 1/26/10 Lecture Topics: Silicon

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1 EE 143 Microfbriction Technology Lecture 3 Mterils II 1/26/10 Reding portions throughout Jeger Lecture Topics licon licon Dioxide licon Nitride Aluminum & Other Metls In this course, the focus will be on silicon microfbriction, but the methods used re lso relevnt to other mteril systems, e.g., gllium rsenide, InP, etc. Mterils in licon-bsed Microfbriction Four bsic mterils () licon (single crystl; polycrystlline) (b) licon Dioxide (c) licon Nitride (d) Aluminumm & Other Metls Mterils for dvnced processes Tungsten, silicides, polyimide Need to consider three min items for ech mteril Electricl chrcteristics Chemicl chrcteristics (i.e., stbility) Usge in IC s () licon () (i) ngle Crystl licon () Electricl semiconductor Cn chnge its conductivity by introducing impurities, clled dopnts Semiconductors re not very conductive To mke them conductive, replce silicon toms in the lttice with dopnt toms tht hve vlence bnds with fewer or more e - s thn the 4 of If more e - s, then the dopnt is donor P, As The extr e - is effectively relesed from the bonded toms to join cloud of free e - s, free to move like e - s in metl Extr free e -.. P. Dope P. The lrger the # of donor toms, the lrger the # of free e - s the higher the conductivity Here, energy (e.g., het tht rises temperture) cn give n e - in the vlence bnd sufficient energy to get to the conduction bnd This puts n e - in the conduction bnd nd h + in the vlence bnd, both of whichh cn conduct Doping bsiclly dds n tom with one e - tht is only loosely held to the tom; so very little energy (e.g., from n electric field) is needed to strip it from the tom nd let it wnder the conduction bnd If it hs fewer e - s thn silicon, then the dopnt is n cceptor B Lck of n e - = hole = h + When e - s move into h + s, the h + s effectively move in the opposite direction h + is mobile (+) chrge crrier Conductivity Eqution chrge mgnitude on n electron conductivity σ = qμ n n + qμ p electron mobility electron density q p hole mobility hole density If fewer e - s, then dopnt is n cceptor B Lck of n e - = hole = h + When e - s move into h + s, the h + s effectively move in the opposite direction h + is mobile (+) chrge crrier Nomenclture n- = lightly doped (1014 to 1017) n = modertely doped (1017 to 1019) n+ = hevily doped (>1019) Chemicl rther inert, but cn be etched in KOH, HNO3/HF Ltter rection forms oxide first, then etches the oxide with HF Usge in IC s ctive devices (obviously), but lso locl interconnect, resistors 1

2 EE 143 Micro E ofbriction Technology T L Lecture 3 M terils II Plne nomenclture e Miller indic ces z z (1110) plne e y y (110 0) plne ] to th his vector licon Wfe er single-cry ystl silicon [110] x coordinte (1,1,0) defines vector [1111] ] (111) plne coordin te (1,1,1), (111)) plne to resulting vector licon crysttllogrphy licon hss bsic dim mond structurre two merge ed fce-centtered cubic (F FCC) crystl structures s In FCC, toms re ssu umed to touch long fce digonls Plness on silicon wfer Verry pure 1 in 1010 impurity level 2

3 EE 143 Microfbriction Technology Lecture 3 Mterils II Very low defect density 1-3/cm 2 Different sizes over the yers (50mm) (75mm) (100mm) (125mm) (150mm) (200mm) (300mm) (450mm) Why go to lrger wfers? 1. Qulity goes up (will explin lter) 2. Yield gets better (reduces cost/chip) Of course, the clenliness of the clen room lso gretly influences the yield Clss 100 no more thn 100 prticles lrger thn 0.5 μm in one cubic meter In ddition, you cn get more chips on lrger wfer, since you wste less spce long the edges Doping concentrtion/resistivity Type either p or n Concentrtion cm m -3 Fltness Very importnt for lithogrphy, where focusing is needed For 8 wfer, fltness must be better thn ±200nm Polishing technology criticl for wfer production Wfer production Generlly use the Czochrlski method, invented by Jn Czochrlski in 1916 while investigting the crystlliztion rtes of metls Finished crystl cn be up to 2 meters long Must control temperture nd velocity fields during crystl growth to minimize defects Wfer orienttion (ii) Polycrystlline silicon (poly) Bsiclly, grins of silicon Electricl Result conducts when doped, like single crystl silicon, but now hs grin boundries tht impede 3

4 EE 143 Microfbriction Technology Lecture 3 Mterils II conduction t low to moderte doping levels (nd increse conduction t extremely high degenerte doping levels) Chemicl sme s single crystl silicon Usge in IC s gte mteril in MOSFET s, locl interconnect Doping concentrtion/resistivity Also used quite hevily in MEMS s structurl mteril Deposition Most common technique chemicl vpor deposition (CVD) This lmost binry resistnce dependence on doping is firly unique to polysilicon nd quite useful why does it hppen? If you pply n E-field, then If low doping, e - s cn t get b the grin boundries; they get trpped in the trps If high doping, e - s cn now get by the grin boundries (b) licon Dioxide (O 2 ) Amorphous structure (this is NOT qurtz) Electricl insultor Resistivity Ω cm Compre to Al s 10-5 Ω cm Chemicl firly inert, but etches in HF Usge in IC s gte dielectric in MOSFET, scrificil lyer in MEMS Grin size strong function of temperture Depends on the bility of dtoms to run round nd find the lowest energy stte loctions leds to lrge grin size At 610 o C, grin size is 200nm or so At 585 o C, the comes down morphous, but then cn be crystllized into very fine grins by nneling Cn be deposited or grown thermlly (c) licon Nitride ( 3 N 4 ) Electricl insultor 4

5 EE 143 Microfbriction Technology Lecture 3 Mterils II Chemicl firly inert, but etches in hot phosphoric cid Usge in IC s insultor, s in metl-to-metl insultion in MOSFET s nd interconnect insultion in MEMS Usully deposited vi chemicl vpor deposition (CVD) Rectnts H 4 + NH 3 (d) Aluminum (Al) Electricl conductor; 20x10-6 Ω cm Chemicl ctive; cn be etched in mny cids Usge in IC s interconnect; gte mteril (metl gte technology before 1974, nd fter 2007) Deposited vi therml evportion, e-bem evportion, or sputtering Melting point ~550 o C Structure In IC s, the polycrystlline form is normlly seen, with grins on the order of 100nm Why is Al preferred mong mny other metls? Wht do we wnt in metl? It must conduct. It must stick to O 2. Al does both of these Au, Ag, nd other metls cn be used, but they require lyer between them nd the O 2 to help them stick (need n dhesion lyer)