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Ion beam modification of the dielectric properties of thin silicon dioxide films
264
Applied Surface Science 48/49 (1991) 264-268 North-Holland
Ion beam modification of the dielectric properties of thin silicon dioxide films S. K a r , A. R a y c h a u d h u r i , A . K . S i n h a Department of Electrical Engineering, Indmn Institute of Technology. Kanpar-208016. India
and S. A s h o k Department of Engineering Science and Mechanics, The Pennsylvania State Unwersity, Unwersity Park, PA 16802. USA Received 20 August 1990; accepted for publication 21 August 1990
The aim of this investigation was to understand the changes brought about in the dielectric properties of thin SiO2 layers after exposure to ion beams. Oxidized silicon wafers were exposed to 16 keV Si ion beams. Subsequently, front and back metallizations were carried out to complete the MOS structures. The C - V measurements revealed many interesting features. One of these was a hysteresis effect in the ion-exposed samples, whose magnitude and sign depended upon the ion dosage. Another interesting observation was the frequency dispersion of the accumulation capacitance at very low frequencies, caused by the formation of a pi junction in the heavily damaged silicon subsurface layer. Further. the oxide dielectric constant was found to increase at high ion dosages.
1. Introduction Ion b e a m processes pervade today's microelecIronic c o m p o n e n t manufacture, and ion beams are also widely used in analysis. The effects of ion b e a m s on semiconductor devices were studied a great deal in the past, a n d the u n d e r s t a n d i n g gained enabled fabrication of defect-free and reliable componems. However, it appears that our knowledge of ion b e a m interaction with semicond u c t o r devices, especially the unpassivated effects of ion b e a m damage on the electronic properties. needs to be deeper to deal with m a n y current a n d future components, in which significant residual d a m a g e m a y b e present. In the case of V L S I / U L S I , it m a y not be possible to anneal out all the ion-beam-induced damage because of several constraints. Secondly, modifications of the
electronic properties by intentional b e a m - i n d u c e d d a m a g e a n d use of a m o r p h i z e d layers are findino increasing applications. Thermally grown silicon dioxide layers are crucial to the f u n c t i o n i n g of the M O S devices. O f all the layers in the M O S devices, these are also the most sensitive to a n d fragile against ion beams. We report in this p a p e r new p h e n o m e n a that we have come across, a n u n d e r s t a n d i n g of which would be useful for device m a n u f a c t u r e a n d fault diagnosis. The aim of this work was to characterize a n d u n d e r s t a n d modifications of the dielectric a n d electronic properties of thin thermal silicon dioxide films u p o n ion b e a m exposure. Most imp o r t a n t a m o n g the modifications that we have observed are the following unusual but interesting p h e n o m e n a : (i) a n ion-dosage-dependent hyster-
0169-4332/91/$03.50 ~;, 19Q1 - Elsevier Science Publishers B.V. (North-Holland)
S. Kar et aL / Ion beam rnod(hcatton of dtelectrw propertws of SIC).,
esis effect; (ii) low-frequency dispersion of the MOS capacitance in accumulation; (iii) enhancement of the oxide dielectric constant at high ion dosages.
2. Experimental To fabricate the MOS structures, p-type single-crystal silicon wafers, of about 10 ~ cm resistivity and (100) orientation, underwent wet chemical cleaning of the surface, following which thermal oxidation was carried out in dry oxygen at 950°C to grow either 115 or 350 ,~ thick silicon dioxide layers. Post-oxidation annealing was carried out in nitrogen at 950°C for 15 min. Subsequently, Si ions of 16 keV energy were implanted into the oxidized Si substrates to reach dosages in the range of 101° to 10 ~5 per cm2. The implantation was carried out at room temperature in a Varian 350D ion implanter. The ion energy was so chosen as to locate the peak of the total target displacements in the middle of the oxide in the case of sample set B with 350 A thick oxides and at the oxide/silicon interface in the case of sample set A with 115 A, thick oxides. Following implantation, AI front (1 mm diameter dots) and Au back contacts were evaporated in an oil-free ionpump,,~d Varian VT-112 UHV system using filament :~ources. Most of the samples were characterized without any post-implantation annealing. The electrical characterization was carried out using the Hewlett Packard 4061S semiconductor/ component test system, equipped with 4192A impedance analyzer, 4140B pA meter/DC voltage source, 7475A graphics plotter, and the 310M controller. Current-voltage, quasi-static capacitance-voltage, and sinusoidal small signal capacitance-voltage, conductance-voltage, and conductance-frequency measurements were made. Static capacitance-voltage ( C - V ) measurements were made using a Keithley 595 meter. Ellipsometric data were obtained using a Rudolph Research A7905 automatic ellipsometer.
265
3. Results and discussion Hysteresis in MOS structures can be caused either by ion (such as Na ÷, K +, or H ÷ ) movement from/to the silicon/oxide to/from the metal/ oxide interface through the oxide bulk, or trapping/detrapping of electrons/holes from the silicon bands to defects in the oxide near the silicon/oxide interface. The two mechanisms give rise to hysteresis in opposing directions. In the case of the mobile ion movement, the response time to bias stressing is the sum of the times for detrapping at the interface and drift through the bulk oxide. Hence the response time is controlled by the slower among the two processes. To measure the hysteresis effect, the negative bias stressing was done at - 7 . 0 and the positive bias stressing at +3.0 V, and the 100 kHz C - V characteristics were recorded as functions of stressing time, till the voltage shifts (which were parallel) in the C - V plots tended to saturate. If the voltage shift is negative for positive bias stressing and positive for negative bias stressing, then the hysteresis is caused by the movement of mobile ions through the oxide bulk. On the other hand, if the voltage shift has the same sign as that of the bias stressing, then the event involved is the exchange of electrons/holes between the silicon energy bands and oxide traps close to the oxide/ silicon interface. No hysteresis was detected in the control (unimplanted) sample. At low ion dosages, the hysteresis was found to be dominated by mobile ion movement through the bulk oxide. The magnitude of voltage shift decreased steadily with increasing dosage, till the voltage shift reversed its direction around the dosage of 1013/cm2. Fig. 1 contains the rate of voltage shift as a function of the stress time for samples with dosages below 1013/cm2. Sign of the voltage shifts in these samples indicates the dominance of mobile ion movement through the oxide bulk. It can be seen that the mobile ion response to negative bias stressing was faster than to positive bias stressing. Fig. 2 presents the rate of voltage shift with respect to the stressing time for samples A13 and B13 which had an ion dosage of 10~3/cm2. What is interesting to note here is the simultaneous
266
S. Kar et aL / Ion beam modification of dtelectrw properties o/StO,
--/-I ~
_
0 POSITIVE~AB
-6 0
I
I
I
I
1800 3600 STRESS TIME
(s)
Fig. I. Parallel voltage shift observed in the 100 kHz C - V characteristics of samplesA12, B10,and B12 as functionof the bias stressingtime. occurrence in these samples of two competing mechanisms, namely mobile ion transport through the oxide bulk and trapping/detrapping of carriers from silicon into oxide defects near the silicon/oxide interface. Each of the profiles for sample A13 quite clearly reveals two different response rates for the two mechanisms, and thereby enabling clear observation of both the processes. For example, when sample A13 is stressed at a positive bias, the voltage shift is initially positive and increases very rapidly till it reaches a maximum, after which it begins to decrease at a slower rate, and finally becomes negative. It can be easily inferred from these characteristics that electron trapping is much faster than the detrapping of mobile ions at the oxide/Al interface, and that electron detrapping is much slower than the detrapping of mobile ions at the silicon/oxide interface and their subsequent drift through "he bulk oxide. A careful analysis of our data and its comparison with the literature [1-3] indicates that the mobile ionic species in the oxide are most likely Na + ions, and that in the case of positive bias stressing the mobile ion transport is limited by detrapping at the AI/oxide interface. The normalized voltage shift versus stress time plots, for
positive bias stressing for samples in which mobile ion transport was the dominant hysteresis mechanism, agreed very well with the potential well detrapping model of Pepper and Eccleston [1]. Table 1 contains a summary of some experimental hysteresis data. The large hysteresis at low ionic dosages is caused by the activation of latent sodium species by the ionizing radiation. ]'he decrease in the density of mobile ions with increasing dosage, cf. table 1, indicates their immobilization by defects induced by the lattice damage, which could be non-bridging oxygen created by silicon displacements [4]. The number of displacements per ion was higher in the 350 ,~ thick oxides, as shown by TRIM [5] calculations, hence a comparatively lower mobile ion density in these than in the 115/~ ones, cf. table 1. It is likely that lattice damage, e.g. oxygen displacement sites [4], is also responsible for generating electron traps, and these become noticeable when the lattice damage becomes heavy.. In this context, the large number of total target displacements near the silicon/oxide interface in the thinner oxides, as shown by TRIM calculations, can explain the higher density of electron traps observed in these samples. A limited number of samples were sub-
f-.,
0 POSITIVEBIAS STRESSING 0 NEGATIVEBIAS STRESSING
"1~
~
j
E20C ', I-"o
-40(} B
I i
0
A13 . . . .
"ta
~
t 1800
I
I
3600
STRESS TIME Fig. 2. Parallel
(s)
v o l t a g e s h i f t obsel-~ed in the ] 0 0 k H z
C-I/
characteristics of samplesA13 and BI3 as functionof the bias stressingtime.
267
S. Kar et aL / Ion beam modtfication of dtelectrtc properties of SiO,
Table 1 Experimental data on room-temperature hysteresis Sample
Oxide thickness
Dose (cm "" )
Voltage shift
(,~)
(V)
AI2 AI3 BI0 BI2 BI3
115 115 350 350 350
10j2 1013 10 lf~ 10 ~-" 1013
+ 0.00 +0.30 + 0.00 +0.00 +035
- 3.30 -0.65 - 7.30 - 3.30 -0.11
B10/A B13/A
350 350
10 I" 1013
+0.00 - 17.00 +0.10 - 1.80
Mobile charge density (cm-")
Oxide electron trap density
6.3 × 10I-' 1.3 × 10I" 4.2 × 1012 2.0 × l0 ~" 6.5 × 10 I"
0.0 5.9 x lOII 0.0 0.0 2.1 X lOII
9.8 × 10Iz 1.1 × 1012
0.0 6.0 × 10m
(cm 2)
For the voltage shift two values are indicated. The first of these represents the effect of electron trapping/detrapping, while the second represents the effect of mobile oxide species. Samples B10/A and BI3/A are samples B10 and BI3. respectively, after post-metallization annealing in pure H2 at 355°C for 30 rain.
j e c t e d to l o w - t e m p e r a t u r e p o s t - m e t a l l i z a t i o n annealing in h y d r o g e n a m b i e n t , cf. table 1. U p o n annealing, there was a significant d e c r e a s e in the d e n s i t y of electron traps, but a significant increase in the d e n s i t y o f m o b i l e ionic species. This observation s u p p o r t s the p r e c e d i n g inferences. In the case o f regular M O S structures, the device c a p a c i t a n c e in s t r o n g a c c u m u l a t i o n app r o a c h e s a c o n s t a n t value, a n d is e q u a t e o to the o x i d e capacitance. H e n c e the a c c u m u l a t i o n c a p a c i t a n c e is e x p e c t e d to b e free o f f r e q u e n c y d i s p e r sion. H o w e v e r , if the bulk silicon resistivity is high, there will be d i s p e r s i o n at high, b u t n o t at low frequencies. Fig. 3 p r e s e n t s the c a p a c i t a n c e voltage characteristics o f s a m p l e A 13 m e a s u r e d at various frequencies as well as the static one. T h e d i s p e r s i o n in the a c c u m u l a t i o n c a p a c i t a n c e at high frequencies m a y be e x p l a i n e d by the series resistance, due to a high wafer resistivity. But the large d i s p e r s i o n b e t w e e n the 80 H z a n d the static accu-
m u l a t i o n c a p a c i t a n c e s is unusual. This d i s p e r s i o n was f o u n d to increase with the ion dosage, as can b e seen in table 2. In this table, the q u a n t i t y C~.~/C~.~.~ d e n o t e s the ratio o f the 80 H z a c c u m u lation c a p a c i t a n c e to the static one. T h e m o s t likely reason for the a c c u m u l a t i o n c a p a c i t a n c e d i s p e r s i o n is the f o r m a t i o n o f a l a t t i c e - d a m a g e d layer in the silicon subsurface. T h e increase in the series resistance, R , , a n d the c h a n g e in the ellipsometric p a r a m e t e r s 8 a n d ff s u p p o r t this e x p l a n a tion, cf. table 2. In the p r e s e n c e o f such a d a m a g e d layer, the e q u i v a l e n t circuit r e p r e s e n t a t i o n for the M O S s t r u c t u r e takes the f o r m s h o w n in fig. 4. In fig. 4, C,,~ is the o x i d e c a p a c i t a n c e , C~ the surface c a p a c i t a n c e , R, the intrinsic layer elemental resistance, a n d C,p the e l e m e n t a l c a p a c i t a n c e o f the i - S i / p - S i j u n c t i o n . A s the lattice d a m a g e turns heavy, the d o p a n t s b e c o m e d e a c t i v a t e d , a n d the layer t e n d s to b e intrinsic [6]. A t static frequencies, the resistances R , ' s can be neglected, so all
Table 2 Experimental data related to capacitance anomaly Sample A00 AI2 A13 A 14 A15
Dose (cm 2 )
R,
Ca,.,.~
(fl)
(nF/cm 2)
0 10t2 1013 1C~ 1015
76 137 127 595 729
301 306 314 365 473
C~.,.,,/ C ......
C.87 0.72 0.72 0.68 0.56
~
#,
(deg)
(deg)
148.3 146.1 147.1 149.5 148.6
11.50 11.63 12.00 13.46 20.92
268
S. Kar et aL ,I Ion beam modtfication of dwlectrtc properties of SIO,
MOS capacitance, smaller than Cox, in accumulation. One can see from table 2 that the static accumulalion capacitance, C~..... becomes much larger at dosages of 1014/cm2 and higher. These values are much larger than that calculated from the standard dielectric constant of silicon dioxide. The increase in the oxide dielectric constant at high dosages is likely to be due to space-charge effects. At very high ion dosages, target displacements in the oxide may give rise to silicon-rich regions, which can be expected to have much lower band gaps than silicon dioxide, and thereby create traps for electrons and holes. Slow charging/discharging of these traps can lead to a higher dielectric constant for the oxide at static frequencies.
OOSE : 1x1013cm2 I SAMPLE NO.A~
I 8~-tz
1MHz
0 I I 1 I I I I I I ~ I I I/ -10 -5 0 APPLIED BIAS (V)
5
Fig. 3. Capacitance-voltage characteristics of sample AI 3 measured at various frequencies.
the capacitances C~p's ate in parallel, and the sum capacitance, EC, p, is large as the back-contact area is orders of magnitude larger than the gate area. In accumulation, C~ also is large, so the total MOS capacitance approaches C,,~. The time constant R,C,p may be of the order of I Hz. So, at low sinusoidal frequencies, the ip junction capacitance in series with C~ may be small. Th!s will give an GATE METAL
T
Cox OXIDE
T
4. Conclusions In ion-beam-exposed samples, hysteresis can be caused by both mobile ion transport and electron/hole trapping. The mobile ions are most likely sodium activated by the ionizing radiation. With increasing dosage, these ions get immobilized in traps generated by ion-beam-induced atomic displacements. Electron/hole trapping becomes significant after the ion dosage exceeds 1 0 ' / c m 2. These traps are also generated by lattice damage. The dispersion of the accumulation capacitance at low frequencies is related to the formation of a damaged layer in the silicon subsurface. This leads to the formation of a pi junction in series with the MOS capacitor. At high dosages, the increase in the oxide dielectric constant is related to space-charge effects in the oxide.
References m
~-sitieo, BACK CONTACT
I'~ ""RS
Y
Fig. 4. Equivalent circuit representation for the MOS structure with a damaged silicon subsurface region.
[1] J.P. Stagg, Appl. Phys. Lett. 31 (1977) 532. [2] M. Pepper and W. Eccleston, Phys. Status Solidi 12 (1972) 199. [3] M. Nemeth-Sallary,R. Szabo. I.C. Szep and P. Tutto, Thin Solid Films 70 (1980) 37. [4] M. Offenburg and P. Balk, Appl. Surf. Sci. 30 (1987) 265. [5] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping Range of Ions in Solids (Pergamon, New York, 1985). 16] W.R. Fahrner, C.P. Schneider and E F. Gorey, Phys. Status Solidi 95 (1986) 343.
Applied Surface Science 48/49 (1991) 264-268 North-Holland
Ion beam modification of the dielectric properties of thin silicon dioxide films S. K a r , A. R a y c h a u d h u r i , A . K . S i n h a Department of Electrical Engineering, Indmn Institute of Technology. Kanpar-208016. India
and S. A s h o k Department of Engineering Science and Mechanics, The Pennsylvania State Unwersity, Unwersity Park, PA 16802. USA Received 20 August 1990; accepted for publication 21 August 1990
The aim of this investigation was to understand the changes brought about in the dielectric properties of thin SiO2 layers after exposure to ion beams. Oxidized silicon wafers were exposed to 16 keV Si ion beams. Subsequently, front and back metallizations were carried out to complete the MOS structures. The C - V measurements revealed many interesting features. One of these was a hysteresis effect in the ion-exposed samples, whose magnitude and sign depended upon the ion dosage. Another interesting observation was the frequency dispersion of the accumulation capacitance at very low frequencies, caused by the formation of a pi junction in the heavily damaged silicon subsurface layer. Further. the oxide dielectric constant was found to increase at high ion dosages.
1. Introduction Ion b e a m processes pervade today's microelecIronic c o m p o n e n t manufacture, and ion beams are also widely used in analysis. The effects of ion b e a m s on semiconductor devices were studied a great deal in the past, a n d the u n d e r s t a n d i n g gained enabled fabrication of defect-free and reliable componems. However, it appears that our knowledge of ion b e a m interaction with semicond u c t o r devices, especially the unpassivated effects of ion b e a m damage on the electronic properties. needs to be deeper to deal with m a n y current a n d future components, in which significant residual d a m a g e m a y b e present. In the case of V L S I / U L S I , it m a y not be possible to anneal out all the ion-beam-induced damage because of several constraints. Secondly, modifications of the
electronic properties by intentional b e a m - i n d u c e d d a m a g e a n d use of a m o r p h i z e d layers are findino increasing applications. Thermally grown silicon dioxide layers are crucial to the f u n c t i o n i n g of the M O S devices. O f all the layers in the M O S devices, these are also the most sensitive to a n d fragile against ion beams. We report in this p a p e r new p h e n o m e n a that we have come across, a n u n d e r s t a n d i n g of which would be useful for device m a n u f a c t u r e a n d fault diagnosis. The aim of this work was to characterize a n d u n d e r s t a n d modifications of the dielectric a n d electronic properties of thin thermal silicon dioxide films u p o n ion b e a m exposure. Most imp o r t a n t a m o n g the modifications that we have observed are the following unusual but interesting p h e n o m e n a : (i) a n ion-dosage-dependent hyster-
0169-4332/91/$03.50 ~;, 19Q1 - Elsevier Science Publishers B.V. (North-Holland)
S. Kar et aL / Ion beam rnod(hcatton of dtelectrw propertws of SIC).,
esis effect; (ii) low-frequency dispersion of the MOS capacitance in accumulation; (iii) enhancement of the oxide dielectric constant at high ion dosages.
2. Experimental To fabricate the MOS structures, p-type single-crystal silicon wafers, of about 10 ~ cm resistivity and (100) orientation, underwent wet chemical cleaning of the surface, following which thermal oxidation was carried out in dry oxygen at 950°C to grow either 115 or 350 ,~ thick silicon dioxide layers. Post-oxidation annealing was carried out in nitrogen at 950°C for 15 min. Subsequently, Si ions of 16 keV energy were implanted into the oxidized Si substrates to reach dosages in the range of 101° to 10 ~5 per cm2. The implantation was carried out at room temperature in a Varian 350D ion implanter. The ion energy was so chosen as to locate the peak of the total target displacements in the middle of the oxide in the case of sample set B with 350 A thick oxides and at the oxide/silicon interface in the case of sample set A with 115 A, thick oxides. Following implantation, AI front (1 mm diameter dots) and Au back contacts were evaporated in an oil-free ionpump,,~d Varian VT-112 UHV system using filament :~ources. Most of the samples were characterized without any post-implantation annealing. The electrical characterization was carried out using the Hewlett Packard 4061S semiconductor/ component test system, equipped with 4192A impedance analyzer, 4140B pA meter/DC voltage source, 7475A graphics plotter, and the 310M controller. Current-voltage, quasi-static capacitance-voltage, and sinusoidal small signal capacitance-voltage, conductance-voltage, and conductance-frequency measurements were made. Static capacitance-voltage ( C - V ) measurements were made using a Keithley 595 meter. Ellipsometric data were obtained using a Rudolph Research A7905 automatic ellipsometer.
265
3. Results and discussion Hysteresis in MOS structures can be caused either by ion (such as Na ÷, K +, or H ÷ ) movement from/to the silicon/oxide to/from the metal/ oxide interface through the oxide bulk, or trapping/detrapping of electrons/holes from the silicon bands to defects in the oxide near the silicon/oxide interface. The two mechanisms give rise to hysteresis in opposing directions. In the case of the mobile ion movement, the response time to bias stressing is the sum of the times for detrapping at the interface and drift through the bulk oxide. Hence the response time is controlled by the slower among the two processes. To measure the hysteresis effect, the negative bias stressing was done at - 7 . 0 and the positive bias stressing at +3.0 V, and the 100 kHz C - V characteristics were recorded as functions of stressing time, till the voltage shifts (which were parallel) in the C - V plots tended to saturate. If the voltage shift is negative for positive bias stressing and positive for negative bias stressing, then the hysteresis is caused by the movement of mobile ions through the oxide bulk. On the other hand, if the voltage shift has the same sign as that of the bias stressing, then the event involved is the exchange of electrons/holes between the silicon energy bands and oxide traps close to the oxide/ silicon interface. No hysteresis was detected in the control (unimplanted) sample. At low ion dosages, the hysteresis was found to be dominated by mobile ion movement through the bulk oxide. The magnitude of voltage shift decreased steadily with increasing dosage, till the voltage shift reversed its direction around the dosage of 1013/cm2. Fig. 1 contains the rate of voltage shift as a function of the stress time for samples with dosages below 1013/cm2. Sign of the voltage shifts in these samples indicates the dominance of mobile ion movement through the oxide bulk. It can be seen that the mobile ion response to negative bias stressing was faster than to positive bias stressing. Fig. 2 presents the rate of voltage shift with respect to the stressing time for samples A13 and B13 which had an ion dosage of 10~3/cm2. What is interesting to note here is the simultaneous
266
S. Kar et aL / Ion beam modification of dtelectrw properties o/StO,
--/-I ~
_
0 POSITIVE~AB
-6 0
I
I
I
I
1800 3600 STRESS TIME
(s)
Fig. I. Parallel voltage shift observed in the 100 kHz C - V characteristics of samplesA12, B10,and B12 as functionof the bias stressingtime. occurrence in these samples of two competing mechanisms, namely mobile ion transport through the oxide bulk and trapping/detrapping of carriers from silicon into oxide defects near the silicon/oxide interface. Each of the profiles for sample A13 quite clearly reveals two different response rates for the two mechanisms, and thereby enabling clear observation of both the processes. For example, when sample A13 is stressed at a positive bias, the voltage shift is initially positive and increases very rapidly till it reaches a maximum, after which it begins to decrease at a slower rate, and finally becomes negative. It can be easily inferred from these characteristics that electron trapping is much faster than the detrapping of mobile ions at the oxide/Al interface, and that electron detrapping is much slower than the detrapping of mobile ions at the silicon/oxide interface and their subsequent drift through "he bulk oxide. A careful analysis of our data and its comparison with the literature [1-3] indicates that the mobile ionic species in the oxide are most likely Na + ions, and that in the case of positive bias stressing the mobile ion transport is limited by detrapping at the AI/oxide interface. The normalized voltage shift versus stress time plots, for
positive bias stressing for samples in which mobile ion transport was the dominant hysteresis mechanism, agreed very well with the potential well detrapping model of Pepper and Eccleston [1]. Table 1 contains a summary of some experimental hysteresis data. The large hysteresis at low ionic dosages is caused by the activation of latent sodium species by the ionizing radiation. ]'he decrease in the density of mobile ions with increasing dosage, cf. table 1, indicates their immobilization by defects induced by the lattice damage, which could be non-bridging oxygen created by silicon displacements [4]. The number of displacements per ion was higher in the 350 ,~ thick oxides, as shown by TRIM [5] calculations, hence a comparatively lower mobile ion density in these than in the 115/~ ones, cf. table 1. It is likely that lattice damage, e.g. oxygen displacement sites [4], is also responsible for generating electron traps, and these become noticeable when the lattice damage becomes heavy.. In this context, the large number of total target displacements near the silicon/oxide interface in the thinner oxides, as shown by TRIM calculations, can explain the higher density of electron traps observed in these samples. A limited number of samples were sub-
f-.,
0 POSITIVEBIAS STRESSING 0 NEGATIVEBIAS STRESSING
"1~
~
j
E20C ', I-"o
-40(} B
I i
0
A13 . . . .
"ta
~
t 1800
I
I
3600
STRESS TIME Fig. 2. Parallel
(s)
v o l t a g e s h i f t obsel-~ed in the ] 0 0 k H z
C-I/
characteristics of samplesA13 and BI3 as functionof the bias stressingtime.
267
S. Kar et aL / Ion beam modtfication of dtelectrtc properties of SiO,
Table 1 Experimental data on room-temperature hysteresis Sample
Oxide thickness
Dose (cm "" )
Voltage shift
(,~)
(V)
AI2 AI3 BI0 BI2 BI3
115 115 350 350 350
10j2 1013 10 lf~ 10 ~-" 1013
+ 0.00 +0.30 + 0.00 +0.00 +035
- 3.30 -0.65 - 7.30 - 3.30 -0.11
B10/A B13/A
350 350
10 I" 1013
+0.00 - 17.00 +0.10 - 1.80
Mobile charge density (cm-")
Oxide electron trap density
6.3 × 10I-' 1.3 × 10I" 4.2 × 1012 2.0 × l0 ~" 6.5 × 10 I"
0.0 5.9 x lOII 0.0 0.0 2.1 X lOII
9.8 × 10Iz 1.1 × 1012
0.0 6.0 × 10m
(cm 2)
For the voltage shift two values are indicated. The first of these represents the effect of electron trapping/detrapping, while the second represents the effect of mobile oxide species. Samples B10/A and BI3/A are samples B10 and BI3. respectively, after post-metallization annealing in pure H2 at 355°C for 30 rain.
j e c t e d to l o w - t e m p e r a t u r e p o s t - m e t a l l i z a t i o n annealing in h y d r o g e n a m b i e n t , cf. table 1. U p o n annealing, there was a significant d e c r e a s e in the d e n s i t y of electron traps, but a significant increase in the d e n s i t y o f m o b i l e ionic species. This observation s u p p o r t s the p r e c e d i n g inferences. In the case o f regular M O S structures, the device c a p a c i t a n c e in s t r o n g a c c u m u l a t i o n app r o a c h e s a c o n s t a n t value, a n d is e q u a t e o to the o x i d e capacitance. H e n c e the a c c u m u l a t i o n c a p a c i t a n c e is e x p e c t e d to b e free o f f r e q u e n c y d i s p e r sion. H o w e v e r , if the bulk silicon resistivity is high, there will be d i s p e r s i o n at high, b u t n o t at low frequencies. Fig. 3 p r e s e n t s the c a p a c i t a n c e voltage characteristics o f s a m p l e A 13 m e a s u r e d at various frequencies as well as the static one. T h e d i s p e r s i o n in the a c c u m u l a t i o n c a p a c i t a n c e at high frequencies m a y be e x p l a i n e d by the series resistance, due to a high wafer resistivity. But the large d i s p e r s i o n b e t w e e n the 80 H z a n d the static accu-
m u l a t i o n c a p a c i t a n c e s is unusual. This d i s p e r s i o n was f o u n d to increase with the ion dosage, as can b e seen in table 2. In this table, the q u a n t i t y C~.~/C~.~.~ d e n o t e s the ratio o f the 80 H z a c c u m u lation c a p a c i t a n c e to the static one. T h e m o s t likely reason for the a c c u m u l a t i o n c a p a c i t a n c e d i s p e r s i o n is the f o r m a t i o n o f a l a t t i c e - d a m a g e d layer in the silicon subsurface. T h e increase in the series resistance, R , , a n d the c h a n g e in the ellipsometric p a r a m e t e r s 8 a n d ff s u p p o r t this e x p l a n a tion, cf. table 2. In the p r e s e n c e o f such a d a m a g e d layer, the e q u i v a l e n t circuit r e p r e s e n t a t i o n for the M O S s t r u c t u r e takes the f o r m s h o w n in fig. 4. In fig. 4, C,,~ is the o x i d e c a p a c i t a n c e , C~ the surface c a p a c i t a n c e , R, the intrinsic layer elemental resistance, a n d C,p the e l e m e n t a l c a p a c i t a n c e o f the i - S i / p - S i j u n c t i o n . A s the lattice d a m a g e turns heavy, the d o p a n t s b e c o m e d e a c t i v a t e d , a n d the layer t e n d s to b e intrinsic [6]. A t static frequencies, the resistances R , ' s can be neglected, so all
Table 2 Experimental data related to capacitance anomaly Sample A00 AI2 A13 A 14 A15
Dose (cm 2 )
R,
Ca,.,.~
(fl)
(nF/cm 2)
0 10t2 1013 1C~ 1015
76 137 127 595 729
301 306 314 365 473
C~.,.,,/ C ......
C.87 0.72 0.72 0.68 0.56
~
#,
(deg)
(deg)
148.3 146.1 147.1 149.5 148.6
11.50 11.63 12.00 13.46 20.92
268
S. Kar et aL ,I Ion beam modtfication of dwlectrtc properties of SIO,
MOS capacitance, smaller than Cox, in accumulation. One can see from table 2 that the static accumulalion capacitance, C~..... becomes much larger at dosages of 1014/cm2 and higher. These values are much larger than that calculated from the standard dielectric constant of silicon dioxide. The increase in the oxide dielectric constant at high dosages is likely to be due to space-charge effects. At very high ion dosages, target displacements in the oxide may give rise to silicon-rich regions, which can be expected to have much lower band gaps than silicon dioxide, and thereby create traps for electrons and holes. Slow charging/discharging of these traps can lead to a higher dielectric constant for the oxide at static frequencies.
OOSE : 1x1013cm2 I SAMPLE NO.A~
I 8~-tz
1MHz
0 I I 1 I I I I I I ~ I I I/ -10 -5 0 APPLIED BIAS (V)
5
Fig. 3. Capacitance-voltage characteristics of sample AI 3 measured at various frequencies.
the capacitances C~p's ate in parallel, and the sum capacitance, EC, p, is large as the back-contact area is orders of magnitude larger than the gate area. In accumulation, C~ also is large, so the total MOS capacitance approaches C,,~. The time constant R,C,p may be of the order of I Hz. So, at low sinusoidal frequencies, the ip junction capacitance in series with C~ may be small. Th!s will give an GATE METAL
T
Cox OXIDE
T
4. Conclusions In ion-beam-exposed samples, hysteresis can be caused by both mobile ion transport and electron/hole trapping. The mobile ions are most likely sodium activated by the ionizing radiation. With increasing dosage, these ions get immobilized in traps generated by ion-beam-induced atomic displacements. Electron/hole trapping becomes significant after the ion dosage exceeds 1 0 ' / c m 2. These traps are also generated by lattice damage. The dispersion of the accumulation capacitance at low frequencies is related to the formation of a damaged layer in the silicon subsurface. This leads to the formation of a pi junction in series with the MOS capacitor. At high dosages, the increase in the oxide dielectric constant is related to space-charge effects in the oxide.
References m
~-sitieo, BACK CONTACT
I'~ ""RS
Y
Fig. 4. Equivalent circuit representation for the MOS structure with a damaged silicon subsurface region.
[1] J.P. Stagg, Appl. Phys. Lett. 31 (1977) 532. [2] M. Pepper and W. Eccleston, Phys. Status Solidi 12 (1972) 199. [3] M. Nemeth-Sallary,R. Szabo. I.C. Szep and P. Tutto, Thin Solid Films 70 (1980) 37. [4] M. Offenburg and P. Balk, Appl. Surf. Sci. 30 (1987) 265. [5] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping Range of Ions in Solids (Pergamon, New York, 1985). 16] W.R. Fahrner, C.P. Schneider and E F. Gorey, Phys. Status Solidi 95 (1986) 343.