Solid-State Electronics Vol. 34, No. 8, pp. 889-892, 1991 Printed in Great Britain. All rights reserved

0038-l 101/91 $3.00 + 0.00 Copyright 0 1991 Pergamon Press plc

PROPERTIES OF THE FLUORINE-IMPLANTED Si-SiO, SYSTEM

G. S. VIRDI, C. M. S. RAUTHAN, B. C. PATHAK and W. S. KHOKLE Semiconductor Devices Area, Central Electronics Engineering Research Institute, Pilani,

Rajasthan 333 031, India

(Received 25 August 1990; in revised form 31 January 1991)

Abstract-Fluorine was impregnated at the Si-SiO, interface by implanting fluorine ions ( 19F+) into silicon at low energy (20 keV) to a dose in the range 10”-10” ions/cm*. Studies were carried out for two cases: first, silicon was oxidised in dry ox gen after 19F+ implantation and second, silicon was implanted with 19F+ after its oxidation to a 207 1 thick dry thermal oxide. In both the cases, high frequency capacitance-voltage (HF C-V) measurements show reduction in the midgap interface state density (N,) nearly by one order of magnitude. Another important observation was that the incorporation of fluorine reduces the oxidation rate of silicon without deteriorating the interface properties.

1. INTRODUCTION

The development of advanced process technology for nanoscale MOS VLSI/VHSIC devices requires the controlled growth of very thin gate oxides with improved quality of Si-Si02 interfaces. The gate oxide thickness needs to be scaled down with the decreasing trend in device geometries to increase the performance. Also, high quality Si-SiOz interface properties are necessary to ensure the improvement in hot carrier mobility in channel region and leakage current level in p-n junctions.

Addition of halogen (chlorine, fluorine, etc.) compounds into an oxidising medium is employed extensively to improve the interface properties of Si-SiO, system[l-81. Such oxide films show less instability to mobile ion contaminations. They also reduce oxidation-induced stacking faults as well as getter the metallic impurities[7]. A large amount of research work has already been published on the introduction of chlorine atoms/compounds during the oxidation and their behaviour to improve the Si-SiO, interfaces[l-51. Very little is known in the case of fluorine.

Fluorine can be incorporated in SiO, and at Si-Si02 interfaces in a number of ways. Some of these are: HF treatment before oxidation[8], anodic oxidation in a fluorinated electrolyte[4], a CVD pro- cess using fluorine compounds[9, lo], etc. However, incorporation of fluorine in Si-SiO, system by ion implantation is of prime importance.

In our earlier work, we studied the effect of fluorine implantation into boron-implanted silicon[l 11. The present work is a continuation. For the first time, we have investigated the effect of fluorine (introduced into silicon prior to oxidation by ion implantation) on the oxidation rate and interface state density (iv,) in the Si-SiOl system. Similar studies were also carried out by implanting fluorine after oxidation.

2. EXPERIMENTAL DETAILS

P-type, (100) oriented single crystal silicon wafers with 50 mm diameter and 4-6 R cm resistivity were used in the present study. The standard RCA clean- ing process was followed in order to clean the wafers. Wafers were divided into two sets, A and B. The first set, A, was implanted with 19Ff ions. After this, both sets were oxidised at 950°C for 20 min in dry oxygen. A reference wafer was also oxidised at the same time. After oxidation, set B was implanted with 19F+ ions and annealed in a conventional furnace at 950°C for 15 min in dry nitrogen ambient. All the wafers were implanted to a dose in the range 10”-1015 ions/cm2, with 20 keV fluorine ions ( 19F+ ), oriented at 7” from an axis normal to the surface, to reduce channelling. The history of each wafer is given in Table 1.

The oxide thickness on each sample was evaluated on a nanospec AFT thickness measurement system. MOS capacitors were fabricated on these samples for C-V measurements. To complete the MOS capacitor configuration, 1-p thick Al-metal dots of 0.5 mm diameter were defined by evaporating pure aluminum through a metal mask onto the oxide surface. The wafers were metallised on the back side after removing the oxide layer, to ensure back contacts. Finally, wafers were sintered at 450°C for 15 min in dry nitrogen ambient. High frequency capacitance-voltage (HF C-V) characteristics were monitored at 1 MHz by employing a C-Vmeter from MS1 Inc., U.S.A.

3. RESULTS AND DISCUSSION

In the present study, the oxide thickness on each wafer was measured with nanospec, and was also confirmed by calculating it from the asymptotic accumulation capacitance value in HF C-V charac- teristic curves. The oxide thickness measured from

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890 G. s. VIRDI et al.

Table 1. Experimental details and summary of the results

Dose of Dose of Post- Oxide thickness 20 keV F+ 20 keV F+ implant observed Oxide Interface

ions implanted Oxide ions implanted anneal after F+ Sintering thickness state before the thickness after the implantation at by HF density

Wafer set Sample oxidation (nanospec) oxidation 95& by nanospec 450°C c-v (A) (ions/cm2) (A) (A)

(NJ identification number (ions/cm*) (min) (min) (eV_’ cm-‘)

Set A p, IO” 165.0 - (165.0) 15 170.7 2.34 x IO” PZ 1012 117.5 (117.5) 15 117.9 1.05 x IO” P, 10” 115.3 - (115.3) 15 112.4 8.98 x IO” P4 IO” 107.5 - (107.5) 15 109.9 8.06 x 10’0 p, 10” 97.5 - (97.5) 15 100.6 7.20 x IO”’

Unimplanted P6 - 207.0 (207.0) 15 204.2 4.17 x IO”

Set B P, - 207.0 IO” 15 255.0 15 246.5 1.04 x IO” PS - 207.0 10’2 15 260.3 15 252.2 4.43 x 10’0 P9 - 207.0 IO” 15 263.0 15 255.8 4.50 x 10’0 PI0 207.0 10’4 15 237.0 15 234.7 3.10 x 10’0 P,I 207.0 10’5 15 225.4 15 224. I 2.59 x IO”

both the methods is nearly the same for each wafer and is given in Table 1.

Table 1 also shows experimental details along with the results on oxidation rate and high frequency C-V measurements. It is evident from the table that a fluorine implant prior to oxidation reduces the growth rate of silicon dioxide. The measured thick- ness of the oxide layer was found to be decreasing with the increasing dose of fluorine ions. Figure 1 shows the thickness of oxide layer against the total implanted dose of fluorine ions. The silicon dioxide thickness on unimplanted reference sample was 207 A. For the sample implanted with the highest dose of fluorine ions (10” ions/cm2), the oxide layer thickness was 96.5 A (curve a). This thickness is less than half the thickness of the oxide layer on the unimplanted sample. Thus it can be concluded that the presence of fluorine atoms inhibits the oxidation rate of silicon and is inversely related to fluorine concentration. This behaviour is similar to the oxidation of nitrogen-implanted silicon where the oxidation rate is reduced significantly[l4].

Figure 1 also shows the observed oxide layer thickness with implanted fluorine dose for set B

280

x

240

i~~;~----

--%, _-Lb)

. . ‘%_ ---_x

Ia)

I ,s I I I I I

0 10" 10'2 10'3 10'4 10'5

Dose (F+ions/cm’ 1 Fig. 1. Thickness of the oxide layer vs the implanted dose of igF+ ions. (a) Set A, oxidation carried out after implan- tation. (b) Set B, implantation carried out after oxidation.

(curve b). Different wafers received fluorine doses in the range of 10”-10’5ions/cm2 through an initial oxide layer of 207 8, thickness. It is clearly observed from the figure that there is an increase in the thickness relative to the initial oxide layer thickness. This increase in thickness is noted up to a dose of lOi ions/cm2 followed by a decrease up to lOI ions/ cm2. However, the observed thickness was always greater than the initial thickness of the oxide layer. The variation in oxide thickness may be because of an actual increase in the oxide thickness. Implanted fluorine can get accumulated near the interface during the annealing. The radius of the fluorine atom is very close to the radius of the oxygen atom. It can easily react with silicon and replace the oxygen atom near the interface. The replaced oxygen atom will react with silicon to form oxides of silicon and will subsequently lead to an increase in oxide layer thick- ness. Thickness may also be increased due to for- mation of fluorine complexes with silicon. However, the observed increase in oxide thickness is less at higher doses ( lOi and 10” ions/cm2). At these doses, the higher concentration of fluorine atoms may change the refractive index of the silicon dioxide layer.

Figure 2 (curves a and b) shows the profile of fluorine ions implanted at 20 keV in Si and Si-Si02 for both cases. The calculations for the ion range and straggling were made for the first 1000 ions using the TRIM 88 software. Curves a and b are implanted ion profiles for pre-implanted (set A, Si only) and post- implanted (set B, silicon with a dry oxide of 207 A) respectively. The corresponding calculated average projected range of the fluorine ions was 486 and 466 A. Similarly, the calculated average straggling of the ions was 206 and 236 A respectively. The energy of the fluorine ions was chosen such that R, and R, - AR, fall inside the silicon below the oxide (for set B) or the expected oxide layer (in case of set A), i.e. deeper than 207 A for both the cases. In the post- implanted case (set B), minimum damage will be done to the oxide layer by the implantation. Our main aim in the present study was to keep fluorine atoms at and near the interfaces. The ion doses were chosen

such that the presence of fluorine does not degrade interface properties. Moreover, in case of @BFz implantation for the fabrication of shallow source/ drain, doses nearly in the order of lOI ions/cm* are required. So, keeping all this in mind, the highest dose of fluorine was chosen one order below the above mentioned dose of 49BFl ions.

Interface properties of the SiSiO, system were evaluated by I-IF C-V measurements. MOS capaci- tors were used as a test vehicle. In Fig. 3, the high frequency C-V characteristics are shown for samples implanted with the highest dose of fluorine ions (10” ions/cm*) along with that of the unimplanted one. Calculations for mid gap interface state density (N,) were made from the C-V curves following the differential method[l2,13]. The variation of interface state density with different doses of fluorine ions is presented in Fig. 4. It is observed from the figure

(a)

Target death t&I

rgF+ implant

2OkeV energy

I

500 1000

Target depth t&

I lxx)

Fig. 2. Distribution of r9F+ ions. (a) Set A, implanted into bare Si. (b) Set B. implanted into Si with 207 A thick oxide.

Properties of the fluorine-implanted Si-SiO, system 891

-3 -2 -1 0 1 2 3

Applied voltage IV1

Fig. 3. Normalised high frequency C-V characteristics of an MOS capacitor fabricated on the samples implanted with 19F+ ions to a dose of 10” ions/cm*. (a) Set A, oxidation carried out after implantation. (b) Set B, implantation carried out after oxidation. (c) Set C, unimplanted reference

sample.

that fluorine-ion implantation into silicon with oxide layer improves the N, value of the Si-Si02 system. A fluorine implant lowers the N, value by nearly one order. The N, value which was 4.17 x 10” eV-’ cm-* for unimplanted sample improved down to less than 4.5 x 10” eV-’ cm-* for samples implanted with a fluorine dose greater than lOI* ions/cm*. An im- provement in N, value was also observed for all the samples implanted before the oxidation of silicon (set A). The N, value came down to 7.2 x lOlo eV-’ cm-* for the highest dose of lOI ions/cm*.

The improvement in the N, value may be attributed to the accumulation of, and complex formation by, the fluorine atoms at and near the Si-SiO, interfaces. This is expected due to the post- implant oxidation in set A and post-implant anneal-

‘x-x

\ x--x 2x10’0

t,, ’ I I I I

0 10” 10’2 10’3 10’4 10’3

Dora lionr/cm2)

Fig. 4. Midpp interface states density vs total implanted dose of 19F+ ions. (a) Set A, oxidation carried out after implantation. (b) Set B, implantation carried out after

oxidation.

3SE W-F

892 G. S. Vtrw~ et al.

ing in set B. It has been observed that the oxidation of silicon with a prior treatment in HF leads to the pile-up of fluorine at and near the Si-SiOr inter- face[8]. A similar behaviour is expected when the oxidation of fluorine implanted silicon is carried out (set A). In the second case, where the oxide layer was grown before the implantation, the same may happen due to post-implant annealing. Annealing promotes the diffusion and redistribution of the fluorine atoms[lS]. Thus, for both the cases, fluorine accumulates at and near the interfaces and is expected to form bonds with dangling silicon bonds. However, the slight improvement in the N, value in the pre- implanted samples may be due to the loss of fluorine by outdiffusion during the oxidation. Thus there may not be a sufficient number of fluorine atoms to saturate the dangling bonds.

4. CONCLUSION

The effect of low energy (20 keV) fluorine implantation before and after dry oxidation was studied for SiSiO, system. Fluorine plays an import- ant role. The presence of fluorine reduces the inter- face state density (N,). Oxidation rate is also reduced by implanting fluorine prior to dry oxidation, with- out deteriorating the interfaces. This may be useful for growing thin gate oxide layers with a high degree of control over the process parameters. Further studies to understand the various other aspects are in progress.

Acknowledgement-The authors acknowledge the help rendered by Mr Satish Kumar during the metallisation.

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