rf sputtered aluminum oxide films on silicon

J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y

S O L I D S T A T E

S C I E N C E JULY

1970

RF Sputtered Aluminum Oxide Films on Silicon

C. A. T. Salama* Department of Electrical Engineering, University of Toronto, Toronto, Ontario, Canada

ABSTRACT

The physical and electrical properties of a luminum oxide films deposited on silicon by rf sputter ing from an a lumina target in an argon atmosphere were investigated as a funct ion of sputter ing power density in the range from 0.5 to 3 W / c m 2. The deposition rates ranged from 20 to 80 A/min . The density, index of refraction, and dielectric constant of the films increased while the etch rate decreased with increasing power density. The surface charge at the a luminum oxide-silicon interface was typically larger than 1012 e/cm 2. This charge increased with increasing sput ter ing power densi ty and could be reduced to 7-8 x 1011 e/cm 2 by annealing. The films exhibited t rapping instabilities at room tempera ture but no polarization was observed under bias- temperature stress. The characteristics of composite layers of thermal ly grown silicon dioxide and sputtered a luminum oxide layers on silicon were also in - vestigated and found to exhibit low surface charge densities, no hysteresis, and a "contact potential" as well as charge stored at the interface between the two insulators.

Dielectric films deposited on semiconductor substrates serve many functions in solid state devices. These functions include masking against diffusion of impurit ies dur ing the formation of pn junctions, surface passivation, insulat ion in mul t i layer in tercon- nections, fabrication of capacitor structures, and in- sulated-gate field-effect transistors ( IGFET).

Amorphous silicon dioxide films have been used ex- tensively in silicon device technology and are still the most commonly used dielectrics. However, silicon dioxide is s t ructura l ly porous (as indicated by its low density) and permeable to ionic migrat ion which reduce its effectiveness as a passivating layer. These disadvantages have prompted the invest igat ion of other oxide and ni tr ide dielectrics; among these is a lu- m i n u m oxide.

The interest in a luminum oxide films stems from the following exper imental observations: the ionic mobil i ty of impurit ies (Na +) is very low in these films (1), their radiat ion resistance is high compared to silicon dioxide films (2), and their dielectric constant is double that of silicon dioxide (3). These films are also of interest in double layer (SIO2-A1203) structures where they act as effective junct ion seals (4) and also offer a possible method of controll ing the threshold voltage of IGFET devices (5) due to the presence of a "contact potential" at the silicon dioxide- a luminum oxide interface.

A l u m i n u m oxide films have been deposited on silicon by various methods. Chemical vapor deposition in- volving the hydrolysis of a luminum trichloride (6) or pyrolysis of o rgano-a luminum compounds (7) have been the most commonly used. The films prepared b y these methods exhibit interface charge densities in the range 3-10 x 1011 cm -2 and this charge can be reduced to 1-2 x 1011 cm-2 when a thermal ly grown silicon dioxide in ter layer is used between the silicon

* Elect rochemical Society Act ive Member . Key words : th in film, dielectric, passivation.

and the a luminum oxide (8). Both positive and nega- tive interface charges have been reported in these films (7-11), the charge being strongly process de- pendent. DC reactively sputtered (12-14) a luminum oxide films have resulted in ext remely low surface charge and surface state densities. Plasma anodized (2, 15) a luminum oxide films have also yielded ex- t remely low surface charge densities. More recently, films have also been prepared by reactive evaporation (16), and yielded results s imilar to those obtained with chemically deposited films. Relat ively little has been reported on the deposition of a luminum oxide by rf sput ter ing on silicon or other substrates (17, 18).

The purpose of this study is to investigate the physical and electrical properties of a luminum oxide films deposited on silicon by rf sput ter ing from an a luminum oxide target in an argon atmosphere and to evaluate the sui tabil i ty of these films as passivating layers on silicon. This s tudy also includes an investigation of the characteristics of sputtered a luminum oxide- thermal ly grown silicon, dioxide double layers on silicon.

Experimental The rf sput ter ing apparatus used in this work is

shown schematically in Fig. 1. The dielectric target used was a sintered a luminum oxide (99.97%) disk 12.5 cm in diameter and 1 cm thick. It was metallized on one side and bonded using silver epoxy to a flat water-cooled a luminum disk which formed part of the vacuum chamber wall through a system of vacuum seals and an insulat ing ring. A metal shield sur- rounded the exposed areas of the water-cooled alu- m i n u m disk at a spacing of 0.5 cm in order to prevent sputter ing from this region. The substrate support ing configuration was water-cooled.

A convent ional rf generator operat ing at 13.56 MHz was used as the source of power which was coupled to the target electrode through a tunable ne twork which insured matching between the coaxial t ransmission

913

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Fig. 1. Schematic diagram of rf sputtering apparatus

J. s Soc.: S O L I D S T A T E S C I E N C E

PRESSURE: 5XIO -3 TOrT MAG. INTENSITY: 20 G

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l i n e i m p e d a n c e a n d t h e s p u t t e r i n g s y s t e m load i m - p e d a n c e . T h e p o w e r d e l i v e r e d to t h e d i s c h a r g e w a s m e a s u r e d u s i n g a s t a n d i n g - w a v e p o w e r m e t e r . A p e r m a n e n t m a g n e t i n s i d e t h e v a c u u m s y s t e m w a s u s e d to p r o v i d e a m a g n e t i c f ie ld (20 gauss ) p e r p e n d i c u l a r to t h e d i e l e c t r i c t a r g e t su r face . T h i s m a g n e t i c f ie ld c a u s e d a 20% i n c r e a s e in t h e d e p o s i t i o n r a t e a n d h e l p e d to s t ab i l i ze a n d conf ine t h e g low to t h e space b e t w e e n t h e t w o e l ec t rodes . U l t r a h i g h - p u r i t y a r g o n (99.999%) w a s u s e d as t h e s p u t t e r i n g gas a n d t h e

p r e s s u r e d u r i n g s p u t t e r i n g w a s m o n i t o r e d b y m e a n s of a cold c a t h o d e gauge . T h e s y s t e m w a s e v a c u a t e d b y a 6 in. oil d i f fus ion p u m p e q u i p p e d w i t h a w a t e r - c o o l e d baffle a n d a l i q u i d n i t r o g e n t r ap . A M e i s s n e r t r a p was also f i t ted i n to t h e 18 in. g lass be l l j a r to i m p r o v e t h e u l t i m a t e v a c u u m .

In a t y p i c a l r un , t h e i n i t i a l p r e s s u r e in t h e s y s t e m w a s 4 x 10 -7 Torr . P r i o r to depos i t i on , t h e t a r g e t was c l e a n e d b y a r g o n ion b o m b a r d m e n t ( w i t h t h e s u b - s t r a t e s s h i e l d e d ) a n d t h e s y s t e m w as p u m p e d d o w n a g a i n to 4 x 10 -7 Tor r . A r g o n w a s t h e n r e - i n t r o d u c e d in t h e s y s t e m a n d t h e o x i d e d e p o s i t i o n c a r r i e d ou t u n d e r p r e d e t e r m i n e d o p e r a t i n g cond i t ions . To m i n i - m i z e gas c o n t a m i n a t i o n t h e p u m p i n g s y s t e m w a s no t t h r o t t l e d d u r i n g s p u t t e r i n g a n d l i q u i d n t i r o g e n was u s e d in b o t h t r aps . T h e a r g o n p r e s s u r e w a s k e p t at 5-7~ d u r i n g m o s t of t h e runs . A t t h i s s p u t t e r i n g p r e s - s u r e a 3 cm t a r g e t to s u b s t r a t e d i s t a n c e w as f o u n d to b e c o m p a t i b l e w i t h a s e l f - s u s t a i n e d u n i f o r m d i s - c h a r g e a n d s a t i s f a c t o r y d e p o s i t i o n ra tes .

T h e n o m i n a l s u b s t r a t e t e m p e r a t u r e w a s m e a s u r e d u s i n g a t h i n (0.010 in . ) c h r o m e l - a ] u m e l t h e r m o c o u p l e p r i o r to t u r n i n g on a n d i m m e d i a t e l y a f t e r e x t i n g u i s h - i ng t h e g low d i scha rge . I n m o s t cases, t h e s i l i con s l ices w e r e h e l d on t h e s u b s t r a t e h o l d e r b y m e a n s of a t h i n l a y e r of g a l l i u m w h i c h p r o v i d e d good t h e r m a l c o n t a c t a n d a l l o w e d d e p o s i t i o n at l o w t e m p e r a t u r e s ( ~ 1 0 0 ~ D e p o s i t i o n s w e r e a lso c a r r i e d ou t w i t h t h e s u b s t r a t e s t h e r m a l l y i s o l a t e d f r o m t h e ho l de r . T h e s u b s t r a t e t e m - p e r a t u r e d u r i n g t h e s e r u n s d id no t e x c e e d 200~ a n d r e a c h e d e q u i l i b r i u m w i t h i n a f e w m i n u t e s a f t e r t h e d i s c h a r g e h a d b e e n t u r n e d on (19) .

Silicon used in this work was in the form of polished circular slices (Monsanto) 2.5 cm in diameter prepared by the Czochralski method. The dislocation count was specified to be less than 103 cm-2. The slices used were n-type phosphorus doped, (Iii) orientation of approximately i0 and 0.01 ohm-cm resistivities. The high-resistivity slices were used for surface studies and the low-resistivity ones were used for investiga- tions of the dielectric and optical properties of the aluminum oxide films. Prior to aluminum oxide deposition or silicon dioxide growth the polished silicon wafers were degreased, boiled in nitric acid, dipped in hydrofluoric acid, and then rinsed in deionized water. The silicon dioxide growth was carried out at II00~ in dry oxygen in a horizontal resistively heated furnace provided with a quartz tube and mullite liner. The silicon dioxide was then annealed at the same

R/ATE OF ~ OSITION

/ ETCH RATE o / (P ETCH)

20

MJ

I0

i

o I ' ~ ' POWER DENSITY W/cm 2

Fig. 2. Effect of power density on deposition and etch rotes

t e m p e r a t u r e in n i t r o g e n for 30 min . B o t h t h e o x y g e n a n d n i t r o g e n gases w e r e o b t a i n e d f r o m l i q u i d sources .

A f t e r d e p o s i t i o n t h e a l u m i n u m o x i d e f i lms w e r e a n - n e a l e d in n i t r o g e n fo r 30 m i n a t 300~ u n l e s s o t h e r - wise n o t e d : T h e h e a t t r e a t m e n t s w e r e r e s t r i c t e d to a t e m p e r a t u r e b e l o w 320~ b e c a u s e of t h e pos s ib l e oc- c u r r e n c e of a n e w p h a s e of a l u m i n u m o x i d e (~ ') a b o v e t h i s t e m p e r a t u r e (20) .

T h e t h i c k n e s s of t h e f i lms was m e a s u r e d u s i n g a T a l y s t e p ( T a y l o r - H o b s o n ) c a p a b l e of • a c c u r a c y . T h e s t r u c t u r e of t h e f i lms w a s e x a m i n e d b y e l e c t r o n mic roscopy . T h e d e n s i t y w a s d e t e r m i n e d b y a d i f fe r - e n t i a l w e i g h i n g t e c h n i q u e u s i n g a m i c r o b a l a n c e . T h e i n d e x of r e f r a c t i o n w a s o b t a i n e d f r o m m e a s u r e m e n t s on t h e i n t e r f e r e n c e p a t t e r n i n t h e v i s i b l e u.v. r a n g e b y m e a n s of a P e r k i n - E l m e r 450 s p e c t r o p h o t o m e t e r f i t ted w i t h a 350 s p e c u l a r r e f l e c t a n c e a t t a c h m e n t . K n o w i n g t h e f i lm t h i c k n e s s f r o m T a l y s t e p m e a s u r e - m e n t s , i t was poss ib l e to e s t i m a t e t h e i n d e x of r e - f r a c t i o n as a f u n c t i o n of w a v e l e n g t h (21) . T h e i n f r a - r ed s p e c t r a w e r e o b t a i n e d u s i n g a P e r k i n - E l m e r 621 s p e c t r o p h o t o m e t e r f i t ted w i t h t w o m i c r o s p e c u l a r r e - f l ec tance a t t a c h m e n t s .

Most of t h e e l e c t r i c a l e v a l u a t i o n s i n v o l v e d M I S ( m e t a l - i n s u l a t o r - s e m i c o n d u c t o r ) s t r u c t u r e s . T h e s e w e r e p r e p a r e d b y e v a p o r a t i n g a l u m i n u m (99:999%) field p la tes , 1000A th ick , 500g in d i a m e t e r o v e r t h e d i - e l ec t r i c t h r o u g h a m a s k . A l u m i n u m w a s also e v a p o - r a t e d on t h e b a r e b a c k - s i d e of t h e s i l i con fo r con tac t . T h e e v a p o r a t i o n s w e r e c a r r i e d ou t f r o m a r e s i s t i v e l y h e a t e d t u n g s t e n f i l a m e n t in a v a c u u m of 10-~ Tor r . E l e c t r i c a l m e a s u r e m e n t s w e r e t h e n c a r r i e d o u t as d e - s c r i b e d l a t e r in t h e p a p e r .

Results and Discussion F o r r f s p u t t e r i n g of i n s u l a t i n g f i lms on s e m i c o n -

d u c t o r s u b s t r a t e s , t h e p o w e r d e n s i t y affects t h e r a t e of depos i t ion , t h e e t c h r a t e , t h e dens i ty , t h e d i e l ec - t r i c c o n s t a n t , t h e c o n d u c t i v i t y , a n d t h e s u r f a c e c h a r - a c t e r i s t i c s of t h e s e m i c o n d u c t o r - i n s u l a t o r i n t e r f a c e (22) . S o m e of t h e s e effects w i l l b e c o n s i d e r e d in t h e f o l l o w i n g sec t ions .

Physical Properties T h e r a t e of d e p o s i t i o n of a l u m i n u m o x i d e f i lms as a

f u n c t i o n of p o w e r d e n s i t y is s h o w n in Fig. 2. T h e d e p - os i t ion r a t e is a l i n e a r f u n c t i o n of p o w e r dens i ty . T h e f i lm t h i c k n e s s w a s u n i f o r m w i t h i n 5% o v e r t h e w h o l e a r e a of a n y one s l ice a n d f i lms cou ld be d e p o s i t e d o v e r s ix s l ices s i m u l t a n e o u s l y w i t h a t h i c k n e s s u n i f o r m i t y of b e t t e r t h a n 10%. No c r a c k s w e r e o b s e r v e d in f i lms r a n g i n g up to 5000A in t h i c k n e s s .

The etch rate is a sensitive indicator of the struc- tural properties of the film. The P-etch~ rate for alu-

z A n n e a l i n g at such low t e m p e r a t u r e s was f o u n d to h a v e n e g l i g i - ble effect on the dens i f i ca t ion of t he layers .

15 pa r t s 49% HF; 10 pa r t s 70% HNO~; 300 pa r t s H~O.



Vol. 117, No. 7 RF SPUTTERED ALUMINUM OXIDE FILMS

minum oxide films is shown in Fig. 2 and is seen to increase drast ical ly at low power densities, indicating porous films. This fact was also confirmed by measurements of the density of the films as a function of power density. The low power depositions resul ted in lower film densities as shown in Fig. 3. The density ranged f rom 3.5 g / c m 3 at a power density of 1 W / c m 2 to about 3.8 g / c m 3 at 2.5 W / c m 2.

The films examined by low-angle electron diffraction were found to be amorphous independent ly of the deposition power densities used in this investigation. The index of refract ion of the films was found to increase wi th increasing power density ranging f rom 1.55 to 1.65 at 0.5~. A typical inf rared reflectance spec- t rum for an a luminum oxide film deposited on low- resis t ivi ty silicon is shows in Fig. 4. It exhibits a broad peak around 15~, in general agreement wi th the data repor ted for a -a luminum oxide (23). No trace of water could be detected in the films f rom these inf rared measurements . Rela t ive ly minor variat ions were observed in the physical propert ies of annealed films deposited on substrates held by gal l ium on the holder and those deposited on substrates the rmal ly isolated from the holder.

Electrical Properties Metal-aluminum oxide-silicon (MAS) s t ruc tures . -

Dielectric constant.--The dielectric constant of a luminum oxide films sputtered under the conditions described above were obtained from capacitance measurements of 1 kHz using a GR 1650B bridge. The values obtained ranged f rom 7 at a power density of 1 W / c m 2 to 8.5 at a power density of 2.5 W / c m 2, Values of 8.55 for sput tered a luminum oxide films have been reported by Pra t t (18). The low dielectric constant observed at low power density is probably related to the low film density (24). The dissipation factor for these films ranged f rom 0.01 to 0.004.

3.9

5 , 8

3Y

:>-

3.6

3.5

/- i I i

o I ' 2

POWER DENSITY W/cm 2

Fig. 3. Film density as a function of deposition power density

WAVE LENGTH (MICRONS) 8 9 I 0 12 15 2 0 30 i = t , , = =

f

! !

1500 I000 500

W A V E NUMBER (cm "=)

Fig. 4. Infrared spectrum of aluminum oxide films

915

SAMPLE #27 Si: n-IOnCm A I 2 0 3 : 2 1 0 0 A =

N ~,NI~

I I m ~ _ _

-20 -15 -Io -s o V G (volts}

C/C A

3.8

'0.6

0.4

.0.2

Fig. 5. Capacitance-voltage plots for a MAS device showing the effect of annealing. (CA ~ aluminum oxide capacitance per unit area.)

Breakdown lfeld.--The breakdown field of a luminum oxide films was measured by the method described by Worthing (25). A general correlat ion be tween power density and breakdown field was not possible and the measured breakdown values ranged fom 2 to 8 x l0 s V/cm. These values agree wi th the ones repor ted by Pra t t (18).

Surface charge.--The charge per unit area QsA stored in the a luminum oxide was de te rmined f rom the ca- paci tance-vol tage characterist ics (C-V) of the MAS structure. This method involves the determinat ion of the flat-band vol tage V~B of the exper imenta l C-V curve. The charge density QSA is then related to VFB by the expression3:

VFB=--QsA[Xa]@d~MS-- [1] ~a

where r is the meta l -semiconductor work function difference (26), and Xa, ea are, respectively, the th ick- ness and dielectr ic constant of the a luminum oxide layer. The capacitance measurements were carried out at 1 MHz using a Boonton 71A capacitance meter .

As ment ioned in previous sections, h igher deposition power densities result in denser, h igher qual i ty films, however , a ma jo r de t r imenta l effect in using high power densities is the deter iorat ion of the silicon surface properties. The induced charge was measured as a function of deposition power density pr ior to annealing of the films. The charge induced was observed to increase wi th sput ter ing power density in agreement wi th the observat ions of Hu and Gregor (24). When the power density was below 1 W / c m 2 the induced charge was about 1012 e / c m 2. At power densities of 2.5 W / c m 2 and above the surface charge was i013 e / c m 2 or larger. In all cases the effective charge stored in the oxide was positive. Anneal ing at 300~ for 30 min in N2 prior to electrode deposition was found to reduce this charge in most cases to about 7-8 x 1011 e / c m 2. For init ial (prior to anneal ing) charge densities higher than 10 TM e / c m 2 N2 anneal ing was found to have l i t t le or no effect and the charge stored in the oxide could not be significantly reduced. F igure 5 shows the C-V characterist ics of a MAS sample pr ior to and af ter N~ annealing. The increase in induced charge with increasing deposition power density can be a t t r ibuted to an increase in the charge stored in the oxide due to s t ructural differences be tween oxides deposited at various power densities and /o r to bombardment induced surface damage during sput ter ing (24). Since the surface charge after anneal ing was approximate ly

3 T h e c h a r g e QsA w a s a s s u m e d to be a t t h e s i l i c o n - a l u m i n u m o x - ide i n t e r f a c e .



916 J. EZectrochem. Soc.: SOLID STATE SCIENCE J u l y 1970

constant for power densities below 2.5 W / c m 2, the re - duction in surface charge can be due to anneal ing of radiat ion damage at the s i l icon-a luminum oxide in te r - face. However , at h igher power densities the surface charge is more l ikely to be associated wi th s t ructura l changes in the oxide or permanent surface damage which could not be annealed.

The distr ibution of oxide charge was studied as a function of distance in the oxide. The oxide charge, af ter annealing, was found to be located predominant ly at or near the s i l icon-a luminum oxide interface. This observat ion was the resul t of a series of e tch ing-anneal ing experiments . A luminum oxide layers, 2000A thick, were th inned by etching to produce oxide th ick- nesses ranging f rom 500 to 2000A in several steps. The oxides were then annealed as described previously and the surface charge measured. In all cases the effective surface charge densities were of the order of 7-8 x 10 H e / c m 2 independent of oxide thickness. Similar results have been observed in sput tered silicon dioxide films on silicon (27).

A fur ther observat ion which can be made regarding the C-V plots of Fig. 5 is the difference in the shape of annealed and unannealed samples. The slow transi- t ion in the C-V curve for the lat ter is probably related to a high surface state densi ty which can be reduced by annealing.

Trapping.--Hysteresis is present in the C-V curves shown in Fig. 5. The sense of the hysteresis (f lat-band voltage more negat ive af ter the m a x i m u m negat ive ex- cursion) indicates charge interchange across the aluminum oxide-si l icon interface. Hole emission f rom the silicon to the insulator or electron emission from traps in the insulator to the silicon could account for the observed sense of the hysteresis (28). An increase in hysteresis was observed when the b ias-vol tage swing about f lat-band was increased. A displacement of the C-V curves was also observed at room tempera tu re af ter application of a stress field exceeding a threshold of 1-2 x 106 V/cm. The direction of this displacement was opposite to that caused by polarization or ion migrat ion and was t empera tu re independent and de- pendent on previous bias history. Figure 6 shows the effect of stress vol tage on the f lat-band vol tage of a MAS capacitor. Similar results have been observed on silicon ni t r ide (28), (29), and vapor-deposi ted silicon oxide (30) on silicon. This instabil i ty is postulated to be associated with deep t rapping centers in the dielectr ic coupled with charge t ransfer across the interface region between the dielectr ic and the semiconductor. The t rapping instabil i ty and hysteresis could be caused by the presence of a ve ry thin silicon dioxide layer (~20A) which is usual ly present on silicon.

Polarization and ion migration.--Polarization and ion migrat ion are character ized by a displacement of the C-V curves in a direction opposite to the polar i ty of the applied bias when the MAS sample is subjected

SAMPLE #12 Si: n-lO ~~cm AI203:900 A e 4.12

d

VFB

40

-12-

STRESS VOLTAGE (volts)

o

>o

Fig. 6. Flat-band voltage as a function of stress voltage for MAS device.

to b ias - tempera ture stress. The displacement is a function of applied bias and temperature . Annea led a luminum oxide layers were subjected to b ias - tempera ture stress for 10 rain at 250~ The field applied during these exper iments was smaller than the threshold field requi red for t rapping ( typical ly 1 x 106 V / c m ) . The resul t ing vol tage shifts were typical ly less than 1V. Bias - tempera ture stress exper iments wi th control led Na + contamination indicated that Na + did not migra te appreciably through a 1000A layer of a luminum oxide in agreement with the results of Tung and Caffrey (1). The above exper iments indicate that polarizat ion and ion migrat ion in the sput tered films are negligible.

M e t a l - a l u m i n u m ox i de - s i l i c on d i ox i de - s i l i con (MAOS) structures.--The electr ical characterist ics of a luminum oxide-si l icon structures discussed above indicate that in spite of the absence of polarization and ion migration, the sput tered a luminum oxide films are not ent i re ly satisfactory for passivation purposes be- cause of the t rapping instabil i ty associated wi th the insula tor-semiconductor interface. Since the rmal ly grown silicon dioxide on silicon does not exhibi t charge instability, the propert ies of double layers of SiO2 + A1203 were investigated.

Characterizat ion of the insulator charge in MAOS structures was per formed by C-V techniques. The analysis in this case is sl ightly complicated by the possible presence of a charge Qii at the silicon dioxide- a luminum oxide interface and a "contact potential" r at the same interface (4, 31). Taking these factors into account the f lat-band vol tage for the double layer is given by

-Jr - Q i i -~- ~bii -~- ~bMs [2] 5o

where Qss is the fixed posi t ive surface charge density at the Si-SiO2 interface and Xo, eo are, respectively, the thickness and dielectric constant of the silicon dioxide layer. Distr ibuted charge in the a luminum oxide and silicon dioxide has been neglected in the above equation (32).

In order to invest igate the val id i ty of Eq. [2] and to eva lua te the magni tude of the various terms in that equation, four sets of samples having I000A of the r - mal ly grown silicon dioxide with over layers of aluminum oxide ranging in thickness f rom 700 to 3000A were prepared. The a luminum oxide layers were deposited at different power densities ( ranging f rom 1 to 2 W / c m 2) for each set of samples. The double layers were then annealed at 300~ for 30 rain in N2 prior to electrode deposition. F igure 7 shows the C-V characteristics of one of the MAOS test samples. The C-V characterist ics of the double layer do not exhibi t hysteresis. Trapping is also absent in the double layer

SAMPllE #32

+ AI20 3

-0.8

- 0.6

-0.4

-0.2

I I I I !

-8 -6 - 4 -2 0 2 V G (volts)

Fig. 7. Capacitance-voltage plot for a MAOS device. (CAo double-layer dielectric capacitance per unit area.)



Voi. 117, No. 7 RF SPUTTERED ALUMINUM OXIDE FILMS 917

0 4--" 0

0 >

.--I ' ' - - 6

Si: n- IO ncm Si02: I000 A ~

~ , , K ~ j Q s e : 4XlO" e/cm ~

Qii : l X l O " e/cm z

~ 1 volts

! I I , l ! 0 2000 4000

AI203 THICKNESS (A e)

Fig. 8. Flat-band voltage as a function of aluminum oxide thickness in MAOS devices.

even for stress voltages up to the destructive breakdown of the dielectric layer. Similar results have been observed in Si3N4-SiO2 double layers (29). Figure 8 shows a plot of the f lat-band voltage of the double layer as a function of the a luminum oxide thickness for one set of samples. The a luminum oxide films in this case were deposited at 1.5 W/cm 2.

The constant parameters in Eq. [2] were estimated by curve fitting and found to be

Qss ~ 4 • 1011 e/cm 2 (positive)

Qii ~-~ 1 • 1011 e /cm 2 (positive)

r ~ 1.1V

It should be noted that the intercept of the curve with the f lat-band voltage axis for zero a luminum oxide thickness is not the value of ~bii. This intercept depends on the values of Qss, Xo, Co, and CMS (--0.35V in this case). The magni tude of Qss in the annealed MAOS samples was found to be approximately the same as the one observed in MOS samples when the silicon dioxide was subjected to the same annealing. Wu and Formigoni (27) reported similar results on double layers of silicon dioxide sputtered on thermal ly oxi- dized silicon. The charge Qii was found to be positive for all the samples investigated and increased with in - creasing power density ranging from 1 x 1011 to 4 x 1011 e /cm 2. The charge Qii may be associated with charged traps at the interface. The density of these traps in - creases wi th increasing power density thus causing an increase in the stored charge. The origin of ~bii is not completely understood (33), and it was found to range from 1 to 1.2V for the samples investigated. Both Qii and eli have considerable effect on the threshold of IGFET's and the control of these parameters by deposition or anneal ing processes 4 may be required before the advantages of the double- layer s tructure can be ful ly realized.

Conclusions Good qual i ty a luminurh oxide films can be pre-

pared by rf sputter ing of an a lumina target in an argon atmosphere. The rate of deposition of the films is in the range 20-80 A / m i n for power densities ranging from 0.5 to 3 W/cm 2. The films were found to be amorphous and the etch rate, the density, the index of refraction, as wel l as the dielectric constant, were found to increase with increasing power density. In general, high power sput ter ing is preferred however, in this case surface charge becomes a problem. The init ial surface charge after deposition was found to be larger than 1012 e /cm 2. After anneal ing this charge could be reduced to 7-8 x 1012 e /cm 2. The films de-

F or d o u b l e l ayers of a l u m i n u m o x i d e - s i l i c o n d i o x i d e on s i l icon, N i g h et al. (4) h a v e r e p o r t e d a p o s i t i v e r (of a p p r o x i m a t e l y t he same v a l u e as the one r e p o r t e d here) b u t no Q l l ; N i s h i m a t s u et aL (10) h a v e r e p o r t e d t he p re sence of b o t h a n e g a t i v e Q t i a n d a d is - t r i b u t e d c h a r g e i n the A12Oa f i lms b u t no r K a l t e r e t al, (31) h a v e r e p o r t e d a Pos i t i ve ~bli b u t no t Q*l.

posited directly on silicon exhibited t rapping ins ta- bilities at room temperature, bu t no polarization was observed under b ias - tempera ture stress. Double layers of thermal ly grown silicon dioxide and sputtered a lu- m i n u m oxide exhibited low-surface charge densities, no t rapping instabilities, and a "contact potential" as well as charge stored at the interface between the two dielectrics. Both the interface charge and the contact potent ial were found to be process dependant and both will have to be controlled if full advantage is to be taken of characteristics of the double- layer structure.

Acknowledgments This work was supported by the Defence Research

Board of Canada (Grant 5566-34) and by the National Research Council of Canada (Grant A-4408). The author wishes to thank Mrs. S. Tre t t in for assistance in the sample preparations, Dr. J. G. Simmons for use of the Talystep, and Dr. S. Zukotynski for use of the spectrophotometers.

Manuscript submit ted Nov. 5, 1969; revised manuscript received Jan. 29, 1970. This was Paper 96 pre- sented, in part, at the Detroit Meeting of the Society, Oct. 5-9, 1969.

Any discussion of this paper will appear in a Dis- cussion Section to be published in the June 1971 J O U R N A L .

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rf sputtered aluminum oxide films on silicon

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