2004-Apr-21 JaeYeong Kim

1. Chap. 9 Shallow Trench Isolation (STI) 2004-Apr-21 JaeYeong Kim

2. STI Process Flow • Pad Oxidation • Thermally grown oxide (typically 10~20nm) on bare silicon • To prevent stresses by pad nitride • If pad nitride deposited directly on bare silicon, then nitride stress can cause defect in silicon substrate. • Relatively lower stress than LOCOS, lower thickness than LOCOS • Pad Nitride Deposition • Role of pad nitride • Hard mask for etching of trench • CMP polish-stop-layer for planarization after trench fill by CVD oxide Pad Nitride Pad Oxide Silicon Substrate Silicon Substrate

3. STI Process Flow • STI Photo • STI field area will be open • Nitride and pad oxide etch • Anisotropically etched using plasma etching process • Trench Etch (Silicon) • Using plasma etch • Goals of etch process • Correct depth : without etch stop layer  time etch • Desired sidewall slope : more vertical • With rounded bottom corner • Smooth sidewall and minimum Si damage • Photo resist can be stripped after or before PR Silicon Substrate

4. STI Process Flow • Undercut of pad oxide with HF dip • To make top corner rounding during liner oxidation • Liner Oxidation • Role of liner oxide • Remove residual damage caused by plasma etch process • Passivate Si surface of trench • provide stable interface between Si and trench fill oxide for higher threshold voltage of parasitic field transistor, lower leakage current • Top corner rounding of active region • After trench etch, top corner has sharp corner. •  Making rounded top corner by liner oxidation Under Cut Liner Oxide

5. STI Process Flow • Gap fill of trench by CVD oxide deposition • Without void • LPCVD TEOS, APCVD, HDP CVD Oxide • Densification • To make less field loss during wet etch and cleaning step • Reverse active photo and RIE etch Liner Oxide

6. STI Process Flow • STI CMP • Nitride strip • HF + hot phosphoric acid • HF step is to remove oxynitride layer grown during liner oxidation • Using wet process for nitride removal • Inexpensive and high selectivity than dry etch • Need to remove backside nitride deposited during pad nitride deposition • Using remained pad oxide as buffer oxide for well implant • Or strip remained pad oxide and grow sacrificial oxide as buffer oxide Liner Oxide

7. Kooi Effect • Trench oxide is densified in O2 ambient, then O2 react with residual H2 in trench oxide or nitride film and form H20. • H2O react with Si3N4 to form NH3. • NH3 diffuse through oxide and react with Si substrate. • Form silicon nitride spot or white ribbon. • gate oxide thinning problem on nitrided region • GOI problem  Kooi Effect : To prohibit Kooi effect, growing sacrificial oxide after removal of pad oxide Si3N4 + H2O  SiO2 + NH3 Si+NH3 Si3N4 + H2 Si

8. 1.E+00 1.E-02 0 1.E-04 1.1 1.E-06 Id [A] 2.2 1.E-08 3.3 1.E-10 4.4 1.E-12 1.E-14 -0.5 0.5 1.5 2.5 3.5 Vg [V] 1.E+00 1.E-02 0 1.E-04 1.1 1.E-06 Id [A] 2.2 1.E-08 3.3 1.E-10 4.4 1.E-12 1.E-14 -0.5 0.5 1.5 2.5 3.5 Vg [V] Top Corner Rounding • Sharp top corner • Enhance electric field at corner  lowering threshold voltage • Degrade turn-off characteristics : large off current • Parasitic double “hump” characteristics in IDS-VGS curves • To prohibit of hump characteristics, top corner rounding is needed. • Technique for top corner rounding • Adding HCl gas to dry oxidation • High temperature dry oxidation 1050~110C • Mini LOCOS process • H2 annealing after trench etch With corner rounding Without corner rounding

9. Trench Fill Process • Criteria of CVD material for trench fill • Void-free • comparable etch rate with thermal oxide • Film stress, shrinkage and composition after high temperature anneal • CMP polish rate relative to nitride and uniformity • Isolation characteristics of N+/PW and P+/NW diode • HDP CVD • Good gap-filling capability by dep & etch process • Deposition with silane gas and etch with Ar ions • High density plasma density with low pressure to obtain directionality for good gap filling 45 degree facets

10. Inverse Narrow Width Effect • In case of LOCOS isolation, threshold voltage is increased as active width become narrower.  Narrow width effect (NWE) • For STI Isolation, threshold voltage is decreased as active width become narrower.  Inverse Narrow effect (INWE) • Decrement can be controlled by amount of field recess at active edge.