1. Theory

Definition: Radical oxidation tools grow SiO $_{2}$ with atomic oxygen and hydroxyl radicals, made either by igniting hydrogen and oxygen at low pressure next to a hot wafer (in-situ steam generation, ISSG) or in a microwave plasma (ISSG patent; plasma-oxidation patent).
Kinetics: Radicals react far more readily than $O_{2}$ or steam, so plasma oxidation has an activation energy of only 0.08 $eV$ against 1.2 $eV$ for thermal oxidation (Azuma).
Low temperature: Plasma oxidation works at 200 to 500 $^{\circ} C$ ; ISSG runs at 950 to 1050 $^{\circ} C$ but needs only seconds (Azuma; ISSG patent; Yoo).
Conformality: Radicals and ion bias oxidize corners and trench sidewalls more evenly than thermal oxidation (plasma-oxidation patent).

2. Working Principle

ISSG: A lamp-heated wafer at 1 to 50 $Torr$ ignites a few per cent of hydrogen in oxygen about 1 cm above its surface (ISSG patent).
Microwave plasma: A slot antenna or surface-wave launcher at 2.45 $GHz$ makes a dense, low electron temperature Ar/ $O_{2}$ / $H_{2}$ plasma (plasma-oxidation patent; Azuma).
Bias: An RF bias on the wafer stage accelerates oxidation and improves corner coverage (plasma-oxidation patent).
Control: Pressure, hydrogen fraction, bias and time set the thickness (ISSG patent; plasma-oxidation patent).

3. Applications

Gate and tunnel oxides: Ultra-thin ISSG oxides below 2.5 $nm$ (Yoo).
STI liners and corner rounding: Uniform oxidation of trench corners (plasma-oxidation patent).
Low-temperature gates: Oxides for thin-film transistors on glass (Azuma).
Non-(100) surfaces: Kr/ $O_{2}$ plasma oxide grows at the same rate on (100), (111) and (110) silicon with low interface states, enabling (110) devices (Kr plasma patent).

In-Situ Steam Generation

Name	Formula	Related Formulae	Variables/Constants
ISSG pressure window	$p_{H_{2}} + p_{O_{2}} \leq 150 Torr$	$1 \leq p \leq 50 Torr, p_{p e ak} \approx 8 to 12 Torr$	$p : p a r t ia l p r ess u r e o f t h e r e a c t in g g a ses; I S S G p a t e n t : co mbin e d p a r t ia l p r ess u r e c a pp e d a t 150 Torr f or s a f e t y, e nhan ce d o x i d a t i o n f r o m 1 t o 50 Torr, p e ak a t ab o u t 8 t o 12 Torr (5 t o 15 Torr in an o t h er e mb o d im e n t), u se f u l minim u m ab o u t 1 t o 3 Torr, c hamb er r a t e d f or a d e t o na t i o n o f 4 a t m or m or e, p r e f er ab l y k e pt b e l o w 2 a t m$ $(m y r e a d in g : l o w p r ess u r e l e t s t h e h y d r o g e n - o x y g e n r e a c t i o n s t a y n e a r t h e h o t w a f er in s t e a d o f f l a s hin g t h r o ug h t h e c hamb er)$
Hydrogen fraction and rate	$\frac{d x _{o}}{d t} \approx 150 \overset{˚}{A} /min (f_{H_{2}} \geq 33 per cent)$	$largest gain for f_{H_{2}} = 1 to 5 per cent$	$f_{H_{2}} : h y d r o g e n f r a c t i o n in o x y g e n; I S S G p a t e n t : s t u d i e d f r o m 2 t o 33 p er ce n t h y d r o g e n, r a t e r i ses m os t b e tw ee n 1 an d 5 p er ce n t an d l e v e l s o f f n e a r 150 \overset{˚}{A} /min ab o v e 33 p er ce n t, m or e t han 3 p er ce n t h y d r o g e n g r o w s f a s t er t han d r y o x i d a t i o n a t a l l p r ess u r es, f l o w ab o v e 5 slm in t o a 2 l i t r e c hamb er$ $(co m p u t e d : 150 \overset{˚}{A} /min i s 2.5 \overset{˚}{A} / s, so 100 \overset{˚}{A} t ak es ab o u t 40 s)$
ISSG thermal cycle	$T = 600 to 115 0^{\circ} C (950 preferred), t_{h o l d} = 30 to 120 s$	$\dot{T} = 10 to 100^{\circ} C/s (50 preferred)$	$I S S G p a t e n t : s t abi l i z a t i o n a t ab o u t 500^{\circ} C, i g ni t i o n t y p i c a l l y a t 600^{\circ} C or ab o v e, r e a c t i o n ab o u t 1 c m ab o v e t h e w a f er an d co n f in e d w i t hin ab o u t 2 in c h es, d escr ib e d a s s u r f a ce c a t a l y se d an d ma ss - t r an s p or t l imi t e d a t l o w p r ess u r e; Y oo : I S S G w a f er a t 1000 t o 1050^{\circ} C, O_{2} / H_{2} f r o m 0.1 t o 10, u l t r a - t hin g a t e o x i d es b e l o w 25 \overset{˚}{A}, s t e am an d O^{*} r a d i c a l s$ $(m y r e a d in g : t h e w a f er i t se l f a c t s a s t h e i g ni t er, so t h e r a d i c a l s u ppl y f o l l o w s w a f er t e m p er a t u r e)$

Microwave and Surface-Wave Plasma

Name	Formula	Related Formulae	Variables/Constants
Plasma parabolic kinetics	$x_{o}^{2} = B_{p} t, B_{p} = C e^{- E_{a} / k_{B} T}$	$E_{a, pl a s ma} = 0.08 eV, E_{a, t h er ma l} = 1.2 eV$	$B_{p} : pl a s ma p a r ab o l i c r a t e co n s t an t$ $E_{a} : a c t i v a t i o n e n er g y; A z u ma : s u r f a ce - w a v e pl a s ma a t 2.45 GHz, 1200 W, 80 Pa, K r / O_{2} = 97/3 an d p u r e O_{2} g i v in g t h e s am e 0.08 eV, B pl o tt e d f r o m ab o u t 0.1 t o 100 n m^{2} /min, b e l o w 400^{\circ} C$ $(co m p u t e d : f r o m 200 t o 350^{\circ} C t h e pl a s ma B r i ses o n l y 1.6 t im es, a g ain s t ab o u t 1200 t im es f or a 1.2 eV t h er ma l p r ocess)$
Microwave plasma conditions	$T_{e} \leq 1.2 eV, n_{e} = 1 0^{10} to 5 \times 1 0^{12} c m^{- 3}$	$A r : O_{2} : H_{2} = 1200 : 370 : 30 sccm, p = 667 Pa$	$T_{e} : e l ec t r o n t e m p er a t u r e n e a r t h e w a f er$ $n_{e} : pl a s ma d e n s i t y; pl a s ma - o x i d a t i o n p a t e n t : 2.45 GHz mi cr o w a v es, p o w er d e n s i t y 0.41 t o 4.19 W/c m^{2} (1.87 W/c m^{2} in e x am pl es), d e n s i t y u ni f or mi t y w i t hin 5 p er ce n t, w a f er 200 t o 800^{\circ} C (500^{\circ} C in e x am pl es), r e f er e n ce w e t t h er ma l o x i d a t i o n a t 950^{\circ} C$ $(co m p u t e d : o x y g e n 23 p er ce n t an d h y d r o g e n 1.9 p er ce n t o f t h e g a s f l o w)$
Bias-assisted growth	$\overset{r}{ˉ} = \frac{x _{o}}{t} : 0.0089, 0.057, 0.13 nm/s$	$P_{bia s} = 0, 0.15, 0.637 W/c m^{2}$	$\overset{r}{ˉ} : a v er a g e g r o w t h r a t e; pl a s ma - o x i d a t i o n p a t e n t : 6.3 nm in 710 s w i t h o u t bia s, 6.0 nm in 105 s a t 50 W an d 6.0 nm in 45 s a t 200 W bia s o n 200 mm w a f er s, bia s 450 k H z t o 13.56 MHz p r e f er r e d$ $(co m p u t e d : a v er a g e r a t es a s l i s t e d; im pl i e d x^{2} / t o f 3.4, 21 an d 48 n m^{2} /min, in s i d e A z u m a^{'} s pl o tt e d r an g e)$

Film Quality

Name	Formula	Related Formulae	Variables/Constants
Corner and sidewall coverage	$\frac{d ^{'}}{b ^{'}} \geq 0.9 (target)$	$\frac{d ^{'}}{b ^{'}} = 0.81, 1.20, 1.27; \frac{c ^{'}}{a} = 0.84, 0.92, 0.91$	$d^{'} / b^{'} : cor n er t o f l a t t hi c k n ess r a t i o, c^{'} / a : t r e n c h b o tt o m t o t o p r a t i o; pl a s ma - o x i d a t i o n p a t e n t f or 0, 50 an d 200 W bia s$ $(m y r e a d in g : bia s d r i v es o x i d an t i o n s in t o cor n er s, t u r nin g cor n er t hinnin g in t o t hi c k e nin g)$
Film density	$ρ_{r e l} = \frac{c ^{'}}{c} = 90.0, 95.3, 95.5 per cent$	$TZDB, TDDB \approx thermal oxide$	$ρ_{r e l} : d e n s i t y r a t i o r e l a t i v e t o t h e r e f er e n ce; T Z D B$ $T D D B : z er o - t im e an d t im e - d e p e n d e n t d i e l ec t r i c b r e ak d o w n; pl a s ma - o x i d a t i o n p a t e n t : bia s - a ss i s t e d f i l m s ma t c h t h er ma l o x i d e o n T Z D B an d T D D B$ $(m y r e a d in g : t h e r e mainin g f e w p er ce n t o f d e n s i t y d e f i c i t i s t h e main q u a l i t y g a p t o f u r na ce o x i d e)$
Stoichiometry	$\frac{O}{S i} = 1.98 (200 and 30 0^{\circ} C)$	$\frac{O}{S i} < 1.98 (pure O_{2} plasma)$	$A z u ma : X P S co m p os i t i o n o f ab o u t 4 nm f i l m s f r o m K r / O_{2} pl a s ma, l e ak a g e a t 2 M V / c m n e a r l y co n s t an t f r o m 200 t o 350^{\circ} C; K r * co l l i s i o n s w i t h O_{2} e f f i c i e n tl y p r o d u ce a t o mi c o x y g e n O^{*} (K r pl a s ma p a t e n t)$ $(m y r e a d in g : n e a r - s t o i c hi o m e t r i c o x i d e a t 200^{\circ} C i s w ha t mak es pl a s ma o x i d a t i o n u s ab l e o n g l a ss an d f or l o w - t h er ma l - b u d g e t g a t es)$

Important Questions

Ques: How does in-situ steam generation oxidize silicon?

Hydrogen and oxygen flow into a lamp-heated RTP chamber at low pressure, and the hot wafer ignites them about 1 cm above its surface (ISSG patent). Yoo describes the products as steam and oxygen radicals (O*) that enhance the oxidation rate. The patent caps the combined partial pressure at 150 $Torr$ for safety and finds the strongest enhancement at about 8 to 12 $Torr$ , with the biggest rate gain between 1 and 5 per cent hydrogen. Above 33 per cent hydrogen the rate levels off at about 150 angstrom per minute, so 100 angstrom takes about 40 $s$ (computed). Because the reaction is local to the wafer and limited by mass transport, the gas flow matters: above 5 slm into a 2 litre chamber is specified. Reference: US patent 6,037,273, ‘Method and apparatus for insitu vapor generation’ (continuation-in-part of Ser. No. 08/893,774, filed July 1997; Applied Materials RTP Centura named as equipment) (fetch-tool summary); W. S. Yoo, ‘Method and apparatus for forming an oxide layer’, US patent application 2003/0162372 (filed February 2002) (fetch-tool summary).

Ques: Why is plasma oxidation almost independent of temperature?

Because the oxidant arrives already activated. Azuma’s surface-wave plasma at 2.45 $GHz$ , 1200 W and 80 $Pa$ gives a parabolic rate constant with an activation energy of only 0.08 $eV$ for both Kr/ $O_{2}$ and pure $O_{2}$ plasma, against 1.2 $eV$ for thermal oxidation. From 200 to 350 $^{\circ} C$ the plasma rate rises only 1.6 times, while a 1.2 $eV$ process would rise about 1200 times (computed). The same activation energy for both gases suggests the same oxidizing species, and the remaining small energy probably reflects diffusion of oxygen species through the growing oxide (my reading). Kr dilution gives an O/Si ratio of 1.98 at 200 to 300 $^{\circ} C$ , near stoichiometric, while pure $O_{2}$ plasma gives less; a Kr/ $O_{2}$ plasma patent attributes this to Kr* collisions producing atomic oxygen. Low-temperature, near-stoichiometric growth suits glass substrates (my reading). Reference: K. Azuma, M. Goto, T. Okamoto and Y. Nakata, ‘Low-temperature plasma oxidation using surface wave plasma’, Electrochemical Society meeting abstract 203-0505 (about 2003) (fetch-tool summary); US patent 6,903,393, ‘Semiconductor device fabricated on surface of silicon having <110> direction of crystal plane and its production method’ (USPTO full text; inventors and assignee not shown in the fetched text): Kr/O2 microwave plasma at about 1 Torr and 400 C, Kr* collisions producing atomic oxygen O*, growth rate with no substantial orientation dependence, low interface states on (100), (111) and (110) (fetch-tool summary).

Ques: How does RF bias change microwave plasma oxidation?

It speeds growth and improves corner coverage. In the plasma-oxidation patent, a 5 $Torr$ Ar/ $O_{2}$ / $H_{2}$ plasma at 500 $^{\circ} C$ grew 6.3 $nm$ in 710 $s$ with no bias, 6.0 $nm$ in 105 $s$ with 50 W and 6.0 $nm$ in 45 $s$ with 200 W, average rates of 0.0089, 0.057 and 0.13 $nm/s$ (computed), a 15-fold increase. The implied $x^{2} / t$ values of 3.4 to 48 $n m^{2} /min$ lie within the range Azuma plots for plasma oxidation (computed). The corner-to-flat thickness ratio went from 0.81 to 1.20 and 1.27 against a target of at least 0.9, and relative density from 90.0 to 95.5 per cent. Bias accelerates oxygen ions onto the surface, and the electron temperature stays below 1.2 $eV$ , which limits damage (my reading; patent). Reference: US patent 7,910,495, ‘Plasma oxidizing method, plasma processing apparatus, and storage medium’ (USPTO full text; assignee not shown in the fetched text) (fetch-tool summary); K. Azuma, M. Goto, T. Okamoto and Y. Nakata, ‘Low-temperature plasma oxidation using surface wave plasma’, Electrochemical Society meeting abstract 203-0505 (about 2003) (fetch-tool summary).

Ques: What hydrogen-to-oxygen ratios do the steam sources use?

The sources quote different safe ratios. The ISSG patent lists external torch systems as limited to $H_{2} : O_{2}$ from 0.5:1 to 1.8:1, while the RTP patent caps its torch at 2 with a safety ratio of 1.67 (Rapid Thermal Oxidation System page). ISSG avoids the torch altogether and runs at hydrogen fractions from a few per cent up to hydrogen-rich mixtures, controlled by keeping the partial pressure at or below 150 $Torr$ rather than by the ratio. For an external torch, 1.67 is more likely the safe working ratio, since it lies inside both patents’ limits and leaves a margin below the cap (my reading). The furnace page now uses the same patent value. Reference: US patent 6,037,273, ‘Method and apparatus for insitu vapor generation’ (continuation-in-part of Ser. No. 08/893,774, filed July 1997; Applied Materials RTP Centura named as equipment) (fetch-tool summary); Rapid Thermal Oxidation System page of this vault; Thermal Oxidation Furnace page of this vault.

Ques: When is radical oxidation chosen over thermal oxidation?

When thin oxides must be uniform over corners, grown at low thermal budget, or grown on materials that oxidize poorly. ISSG grows ultra-thin gate oxides below 25 angstrom at 1000 to 1050 $^{\circ} C$ in seconds (Yoo), while microwave plasma oxidation works at 400 to 500 $^{\circ} C$ and matches thermal oxide on TZDB and TDDB (plasma-oxidation patent). The corner ratio above 0.9 matters for STI corners and FinFET fins, where thermal oxidation thins at convex corners (my reading). A Kr/ $O_{2}$ microwave plasma patent also reports no substantial orientation dependence of growth rate, unlike the 1.68 orientation ratio of thermal oxidation (Kr plasma patent; Thermal Oxidation Furnace page). Thick field oxides remain furnace work, since ISSG levels off at about 150 angstrom per minute and plasma rates are lower still (ISSG patent; Thermal Oxidation Furnace page). Reference: W. S. Yoo, ‘Method and apparatus for forming an oxide layer’, US patent application 2003/0162372 (filed February 2002) (fetch-tool summary); US patent 7,910,495, ‘Plasma oxidizing method, plasma processing apparatus, and storage medium’ (USPTO full text; assignee not shown in the fetched text) (fetch-tool summary); US patent 6,037,273, ‘Method and apparatus for insitu vapor generation’ (continuation-in-part of Ser. No. 08/893,774, filed July 1997; Applied Materials RTP Centura named as equipment) (fetch-tool summary); US patent 6,903,393, ‘Semiconductor device fabricated on surface of silicon having <110> direction of crystal plane and its production method’ (USPTO full text; inventors and assignee not shown in the fetched text): Kr/O2 microwave plasma at about 1 Torr and 400 C, Kr* collisions producing atomic oxygen O*, growth rate with no substantial orientation dependence, low interface states on (100), (111) and (110) (fetch-tool summary); Thermal Oxidation Furnace page of this vault.

References

“Method and apparatus for insitu vapor generation,” US Patent 6,037,273. Available: https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6037273
W. S. Yoo, “Method and apparatus for forming an oxide layer,” US Patent Application 2003/0162372, 2003. Available: https://patents.justia.com/patent/20030162372
“Semiconductor device fabricated on surface of silicon having <110> direction of crystal plane and its production method,” US Patent 6,903,393. Available: https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6903393
K. Azuma, M. Goto, T. Okamoto and Y. Nakata, “Low-temperature plasma oxidation using surface wave plasma,” Electrochemical Society Meeting Abstracts, MA 203, abstract 0505. Available: https://www.electrochem.org/dl/ma/203/pdfs/0505.pdf
“Plasma oxidizing method, plasma processing apparatus, and storage medium,” US Patent 7,910,495. Available: https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/7910495
Rapid Thermal Oxidation System page of this vault.
Thermal Oxidation Furnace page of this vault.

Quartz 5

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Plasma and Radical Oxidation Tool