428 Nuclear Instruments and Methods in Physics Research B21 (1987) 428-430 North-Holland, Amsterdam

A N E T C H - S T U D Y O F I O N I M P L A N T E D S i O 2 F I L M S

R a d u M A R I N E S C U

Microelectronica-Bucharest, Str. Erou lancu Nicolae 34B, 72996 Bucharest, Romania

For masking ion implantation in IC technology SiO 2 films are mainly used. Since ion implantation causes important structural changes in the SiO 2 masking film, the investigation of these changes provides information on the implantation process itself. The work presents an experimental study performed on thermal-grown and CVD SiO 2 samples, which were implanted with doses between 5 x 1012 and 5 x 1015 ions/cm 2, different ion species (HB+, 3ip+ and 75As+) being used. The experimental study was focussed on the dependence of the etch-rate of SiO 2 films in diluted HF versus the ion implantation parameters (ion type and dose). The results demonstrate the importance of the ion implantation effects especially on the thermal-grown SiO 2 films (variations of 700% in the etch-rate were obtained after ion implantation). The work also presents a direct application of this effect in the control of the beam adjustment.

1. Experimental procedure

Monocrystallme (111), 3-5 fl cm silicon wafers were used. The oxidation was performed by the following methods: (i) thermal oxidation at 1100°C in wet 02; (ii) atmospheric pressure chemical vapour deposition (APCVI)) of the SiO 2 film at 400°C from the reaction SiH4/O2; (i.i.i) APCVD and densification of the SiO 2 film at 1000°C in wet 02. The processing times of all involvedprocesses were chosen so that a final thickness of 8400 A to be obtained on all the samples. The wafers were subsequently implanted with riB+/100 keV, 31p+/100 keV or 75As+/150 keV; in each case the dose was varied between 5 × 1012 and 5 × 1015 ions/era 2. Then the films were etched in a 1:20 solution of HF (50%) in D.I. water, which providedtypical etch-rates of 2.5 A / s in thermal SiO 2 and 17 A / s in as-deposited APCVD SiO 2. The etch temperature was 20°C. The implanted films were etched until the etch rate de- creased to the values corresponding to the onimplanted samples. Therefore the total etch times shown in table 1 were used.

The etch process was interrupted at determined peri- ods of time (20-60 s for as-deposited APCVD films and 30-60 s for the other film types) and the film thickness was measured by ellipsometry and reflectometry in order to determine the variation of the etch rate versus the depth in the etched SiO 2 film.

2. Results

The dependence of the medium etch rate (calculated by the division of the total removed thickness to the total etch times presented in table 1) versus the ira-

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planted dose is displayed in fig. 1. These results demon- strate that the thermal-gown and the APCVD densified SiO 2 films present similar sensibilities to the ion im- plantation. These film types are much more sensitive than the as-deposited APCVD films. The results dis- played in fig. 1 also show that the minimum dose from which the etch rate (and therefore the oxide structure) begin to be affected does not depend on the SiO 2 film type; for the studied ion/energy pairs this dose has the following values: 2 × 1014 ions/cm 2 for XlB+/100 keV, 2 × 10 is ions/era 2 for 31p+/100 keY and 5 x 1012 ions /cm 2 for 75As+/150 keV. Low etch rates (under 2 A / s ) were found for the first 100 ,~, SiO 2 layer im- planted with high doses (over 1015 ions/era 2) of liB+ and 3Xp+. This effect is more important in the case of the as-deposited APCVD SiO 2 films and explains the decrease of the medium etch rate which can be noticed in fig. la. The dependence of the etch rate versus the depth in the etched thermal-gown SiO 2 film for differ- ent doses is presented in fig. 2. These results show that the etch rate increases with the ion mass for constant implantation dose. This effect is stronger for doses up to 5 × 1014 ions/era 2. One can also observe a good agree- ment between the range distribution limit of implanted ions (given by the theoretical models [1]) and the depth up to which the etch rate is influenced by the implanta- tion process.

3. Application: beam-shape visualization

A direct application of the presented effect is the beam-shape visualization as well as the fast detection of major scanning defaults. For beam-shape visualization, unscanned beam currents of 20-30/~A, exposure times

R. Marinescu / Study of ion implanted SiO., 429

Table 1 The total etch times (in seconds) used for different ion/energy pairs and SiO 2 film types

Ion/energy pair "B+/100 keV nP+/100keV 7SAs+/150 keV

SiO2 film type Thermal-grown 550 400 230 As-deposited APCVD 300 300 230 APCVD densified 550 400 230.

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Fig. 2. The dependence of the etch rate versus the depth in the etched thermal-grown SiO 2 film for different doses.

VI. DOSIMETRY

430 R. Marinescu / Study of ion implanted SiO_,

of 15 s to a 100 keV-phosphorus beam and etching times of 200-300 s were found stltlicient for causing a 1000 ,A variation between the SiO 2 thickness on the implanted zone and the SiO 2 thickness on the rest of the sample. The substrate can be used again after a film removal and a reoxidation, this fact representing an important advantage in comparison with other methods [2,3].

The author thanks E. Muntiu and F. Burlescu for the assistance with the preparation of the thermal oxidized wafers. Useful discussions with C. G'ingu, C. Cobianu and C. Pavelescu are also acknowledged.

References

4. Conclusions

The above results show the structural differences between SiO 2 film types as well as the changes induced by ion implantation in the SiO 2 structure. The results demonstrate the capability of the method based on this effect in the fast and direct diagnosis of beam defaults.

[1] H. Ryssel and I. Ruge, Ionenimplantation (Geest & Portig K.-G., Leipzig, 1978) p. 319-340.

[2] P.L.F. Hemment, in: Ion Implantation Techniques, eds., H. Ryssel and H. Glawischnig, (Springer, Berlin, Heidelberg, New York, 1982) p. 222-223.

[3] J.D. Hoepfner, in: Ion Implantation: Equipment and Tech- niques, eds., H. Ryssel and H. Glawischnig, (Springer, Berlin, Heidelberg, New York, Tokyo, 1983) p. 327-329.