Ultra-violet laser doping of silicon

Ultra-violet

laser doping of silicon

K.G. IBBS, M.L. LLOYD ln situ generation of boron from triethyl boron has been used in the ultra-violet laser doping of silicon, over a range of dopant concentrations. The quality of the doped material has been investigated using Auger electron analysis and electrical probes, and indicates that high activated dopant concentrations are readily achieved, although the purity of the material is degraded by unwanted alkyl derivatives. KEYWORDS:

lasers, doping, silicon, boron, ultra-violet

J cm2 across the beam were obtained by control of the final focusing lens position subject to a shot-to-shot reproducibility of 10% standard deviation in the output energy.

= 0.1-2.0

Introduction There has been a great deal of interest in recent years, particularly from the large semiconductor companies, in using lasers for directed non-contact processing of materials, such as the laser annealing of ion implant damage in silicon’. More recently, pulsed laser doping of semiconductors has been demonstrated using photochemical and thermal dissociation mechanisms to generate free metal dopant species in situ. Simultaneous laser annealing via a localized shallow melt phase produced at the irradiated site of the substrate promotes diffusion-driven implantation of the metal atoms and subsequent electrical activation in the epitaxially regrown material, with typical thermal cycling times of a few tens of nanoseconds to a microsecond2p3. The advantage of such a system lies, obviously, in the possibility of reducing device processing times by eliminating the separate implantation and annealing steps. Also, isolation of the reaction zone to a region near the irradiated sife on the substrate allows dynamic high resolution ‘writing’ of device structures without the need for masking techniques, as shown by Tsao et a14*‘. We have demonstrated single step doping of silicon with boron-formed from triethyl boron (TEB) - using a 193 nm wavelength ArF laser, over a range of junction depths and dopant concentrations. It is expected that such junction tailoring will permit the formation of multilayer p-n structures by control of various dopant processor vapour pressures and laser energy. Experiment The apparatus is shown schematically in Fig. 1. A spatially filtered rare-gas halide laser with unstable resonator optics (Iamb& Physik EMG 101/70) was focused with a 20 cm lens through a quartz window into a stainless steel cell containing a silicon substrate 25 mm below the window. The substrate was high resistivity, (100) oriented n-type silicon (a 2 x 10” P cm’) wafers. Laser pulses were typically 10 ns FWHM, 50 mJ pulse-’ output energy before spatial filtering, and 10 mJ at the substrate surface, as measured by a Gen-Tee ED200 joulemeter. Variable energy densities over a range of average values The authors are at The General Electric Company pk. Centre, Wembley, UK. Received 27 September 1982.

Hirst Research

0030-3992/83/010035-05/$03.00 OPTICS AND LASER TECHNOLOGY.

FEBRUARY

The measurement of energy density is made complex by the structured nature of the beam even after spatial fdtering, although a finely filtered beam does exhibit a fairly consistent ‘mesa’ shaped energy profile, and, as seen from later results, may produce a consistent melt depth in the substrate across the entire beam area. Because of these problems, and that of measuring the effective beam area, we defined an empirical parameter 6 which is given by the change in final lens position from that required for the onset of optical damage, where 6 = o, and which is determined at the start of each set of data. In a simple geometrical model of lens focusing, the energy density is directly proportional to 6, but near the lens focus, and for structured beams, the proportionality becomes progressively less apparent. For the results quoted below, typical operating conditions required 10 to 25 laser pulses for each implantation area of = 0.5 x 3 mm, depending on lens position, giving an average energy density of = 0.6 J cm2 pulse-’ - a value somewhat lower than that used in conventional visible wavelength annealing due to the increased absorption coefficient of silicon at 193 nm and despite the corresponding increase in reflectivity that tends to reduce the effective energy of the laser pulse. The vacuum cell was normally evacuated to lo4 torr (1.33 x 10e2 Pa) or less to reduce contamination and then backfilled, just prior to firing the laser, with a known vapour pressure of TEB in the range 0.01 to 10.0 torr (1.33 to 133.3 Pa) as measured by an MKS insts. type 222B baratron for control of the vapour-phase dopant precursor numberdensity. Materials characterization was carried out using a VG Escalab system for Auger electron spectra (AES) and AES/ion4ching, for depth profiling of dopant and impurity concentrations. An ASR-lOOC/2 computerized, two-point spreading resistance probe was used for electrical characterization and determining concentration gradients of activated dopant. Routine monitoring of junction depth and dopant activation by less elaborate bevel and stain and surface-spreading-resistance (SSR) was used for maintaining &y-to-&y sample integrity.

0 1983 Butterworth & Co (Publishers) Ltd 1983

35

taminants unless strict cleanliness is observed in prior handling. This surface layer is etched away in a few seconds, as is clear from the falling carbon and oxygen signals, exposing the real surface indicated. Continuing the etch further into the substrate, significant levels of carbon and boron persist to a depth of some 0.61 pm. This is interpreted in terms of the dissociation products formed from TEB. Apart from free boron there will inevitably be hydrocarbons, formed by recombination, which will diffuse into the melted surface. It is not yet clear how the hydrocarbons are affected by the high temperatures found in the silicon melt-phase, or how their presence affects the structure of the regrown material, but it is unlikely to contribute to improved crystal ‘quality. Work is currently in progress to determine the degree of defect ftirmation in laser implanted silicon due to the presence of organic species.

Spatially filtered beam

JJ \ L

I I I

Quartz 1

window

r

MO vopour

-

1

Buffer

gas

Spreading resistance and beveliing

Fig. 1

Schematic

A convenient way of highlighting the p and n areas of a doped region in silicon involves shallow angle bevelling (< 1”) and copper staining, as shown in Fig. 3. The copper has stained only the n-type high resistivity starting material, leaving a trough of unstained, doped substrate at the laser illuminated site. In this case, 25 laser pulses were used, with

of doping cell

Results All of the results here refer to the implantation of silicon with boron formed from TEB vapour. The absorption spectra of a number of metal alkyl species have been characterized, all of which exhibit the well documented short-wavelength absorption continuum representing photodissociation to yield free metal”-8. The choice of materials used in doping largely depends on the position of the laser wavelength with respect to the continuum absorption edge, described by a vapour-phase absorption coefficient LX&). For TEB the absorption occurs below = 245 nm, and at 193 nm has a value of 0.0153 cm-’ torfl corresponding to only 4% absorption of the laser beam at 1.Otorr over a 25 mm path length. As the absorption is slight we make the assumption that the laser energy at the silicon surface is constant for TEB pressures of less than 1 ton.

Auger electron spectra

4.6

---------

,

g ._ E E : s 0

\

0

569;

t

Auger electron spectra (AES) was used with argon ion etching of the substrate surface to obtain species concentration as a function of depth. As AES is inherently a rather insensitive technique for bulk concentration measurements, quantitative spectra of boron are not available for concentrations below a few percent - about the solid solubility limit of boron in silicon”of = 6 x 1020 cc3 - particularly in the case of AES/etching depth-profdes where the major boron peak overlaps that of the argon etchant, introducing the constant background signal shown by the broken line in Fig. 2. This figure represents a laser implant using 25 laser pulses and a TEB vapour pressure of 1 .Otorr, showing concentrations of boron, carbon and oxygen as a function of etch time, which in turn is proportional to depth, given a constant etch rate. The overall depth of 0.74 cun corresponds to an etch time of 55 15 s, giving an etch rate of 7.5 s nni’ . The silicon signal has been omitted from Fig. 2 for clarity, leaving significant surface levels of only boron, carbon and oxygen.

Background Ar signal

2 i 0

Etch time (s.)

Surface Fig. 2 Auger electron spectra (AES) depth profiles of boron, carbon and oxygen. The silicon surface, after removal of surface contamination, is clearly shown

High levels of carbon and oxygen are often a characteristic feature of AES due to surface-adsorbed atmospheric con-

Fig. 3 Bevelled and stained junction. 0.5 mm and depth * 0.5 Icrn

36

OPTICS AND LASER TECHNOLOGY.

The doped area width is

FEBRUARY

1983

a TEB vapour pressure of 1.Otorr and 6 = 2 mm. After implantation, the substrate is covered with an oxide layer to clearly delineate the starting point of the bevel slope so that quantitative depth measurements can be realized. In this example the substrate has been consistently doped to a depth of 0.5 cun over the entire OS mm width of the sample. To identify a junction, the bevel and stain technique requires only that p and n areas exist. It gives no indication of the relative number-density or degree of electrical activation of the dopant, serving only as a measure of junction depth and therefore as a method of determining homogeneity of laser energy-density over the beam area by fluctuations in the depth of the stain interface. To determine dopant number density as a function of depth we employed an ASR-lOOC/2 two-point spreading resistance probe to step down a shallow bevel, angle tan-’ 0.0029, in steps of 5 pm, giving a depth resolution of 14.5 run. Typical results of electrical profiling are shown in Fig. 4. The measured spreading resistance varies from 45 G! at the surface to over lo5 S2for the undoped material below the melt layer at a depth of 0.37 pm, corresponding to the calculated numberdensity gradient shown. The peak surface dopant concentration of 2 x 10” cm3 represents a high level of doping. This profile, exhibiting a relatively constant concentration throughout the doped region, is a characteristic of multiple-

0.1 mm x

x

X

x

ArF

\

/ *LX

9x_*-x-x

/*

Positionon sample

. . .. . .. . . . . . . . . .._/ 20

7

19

6

I8

5

16

3

I5

2

14

I

0

Fig. 5 Surface spreading resistance across the width of doped samples made by XeF and ArF laser implantation

N

Oopant ccncentrction ,

...

pulse laser processing where the integrated melt duration of the substrate is long compared with the liquid-phase diffusion rate of the doped species in the substrate. At the crystal melt interface the diffusion rate drops rapidly, ensuring a sharp dopant profile and therefore a well defined junction edge. In practice the rare-gas halide laser used has a rather poorly reproducible energy output per pulse so that the melt depth achieved in each pulse of a sequence of 10 pulses is not constant. The effect of varying melt depths ensures that near the final junction edge the integrated melt duration corresponds to fewer pulses than that at the surface. Considerable sharpening of the junction edges can be expected for more consistent pulse energies.

Resistance,R

\_:-*

. ..- ..-

. .-* . .

: :-

. . . . . .. . . . . I

I

I

I

04

0.2

0.3

0.4

Depth (ym) Fig. 4 Dopant profiling by spreading resistance. The resistance value changes from 45 52 to over lo6 Sz corresponding to calculated dopant concantrations of 2 X 10zo cme3 to less than 1013 cmW3 over a depth of 0.37 pm

OPTICS AND LASER TECHNOLOGY.

FEBRUARY

1983

Assuming a completely constant distribution of dopant through the melt, it is a relatively simple matter to relate the surface spreading resistance (SSR) to dopant concentration, given a known junction depth. Using only the quickly determined SSR it is possible to analyse a larger number of samples; and because the technique is essentially nondestructive, the entire surface may be repeatedly sampled and mapped. An example of SSR across a laser doped sample is shown in Fig. 5. The lower curve of Fig. 5 corresponds to similar conditions met in Figs 3 and 4. The SSR probe has different characteristics from those of the ASR probe, and the lowest resistance value seen may be 10 C!, or slightly below for heavily doped silicon. Using known calibrations, the lowest SSR value found in this sample represents a dopant concentration of 1.6 x 10” cm3 , slightly underestimating the value found from Fig. 4, though the variation is within our reproducibility errors. The second curve in Fig. 5 represents the variation of SSR across a sample implanted using an XeF laser source of wavelength 351 nm in place of the 193 nm ArF laser, with

37

otherwise similar conditions. TEB does not have a measurable absorption at 351 nm, confirming that there is some degree of wavelength dependence on the efficiency of doping. However, the change in SSR to a low value of 100 S’Zin the case of XeF doping still produces a significant dopant concentration of 1.5 x 1019 cm’ by the, presumably, thermal dissociation of TEB. This is discussed in more detail in a later section. An implanted boron concentration of w 1020 cm3 is rather high for many applications, particularly if multiple p-n layers are required. By using a variable vapour pressure of TEB we have demonstrated a variation in SSR of almost two orders of magnitude, as shown in Fig. 6, maintaining otherwise constant conditions, so that the SSR is a direct measure of relative dopant concentration. In this example a single silicon slice was irradiated in five separate areas withlOlaserpulseseachandaval~S=2mmusinga fured TEB pressure for each area. The SSR is not, theoretically, linearly related to dopant concentration, except over a limited regime in the centre of Fig. 6, because of the effects of the concentration dependent carrier mobility that are accentuated at high dopant concentrations. Also, from experiments on clean samples, it is clear that some degree of surface absorption of TEB on silicon distorts the data at the low pressure end of the graph where the vapour pressure is not a true reflection of the number density of TEB near the substrate surface. We have not yet determined the full extent of adsorption, but it appears to be negligible above about 0.1 torr and completely dominates the number of dopant species below about 0.01 torr for the case of TEB.

lo4 ti

x

Discussion Using the vapour phase absorption o,(X) and introducing the substrate absorption coefficient oai(A), one can interpret a range of laser materials processing effects in terms of the relative magnitudes of o,(x) and oai (x), of which laser photochemical doping is one limiting case. In general, we recognize three non-trivial limiting cases that are currently being assessed by this group, and elsewhere, specifically for the fabrication of semiconductor materials and devices. In the limiting case of small o&X), that is, a transparent vapour, and large asi( several groups have demonstrated laser assisted chemical vapour deposition (LCVD) of a number of metals and compounds’0-‘3. This requires that oat(X), the laser energy density and relaxation rates in the substrate are sufficient to raise the substrate temperature to a point where chemical vapour deposition (CM)) reactions occur. Metal deposition from the alkyls by LCVD is exactly analogous to metal-organic CVD” of, for example, GaAs films, where electrical heating of the substrate is replaced by laser heating, and has the advantage that LCVD film growth uses a directed source and may be correspondingly localized. Clearly, in the case of high laser powers, the substrate surface may be locally melted allowing diffusion of the CVD products into the melt phase as shown by Turner et al in the Alexandrite laser formation of Si solar cells by phosphorous doping from a PHs vapour16. In the limiting case of large (Y,(A) and either small oai (X) or low energy density, vapour-phase dissociation forms products which may condense on nearby surfaces that remain at low temperature. This effect was demonstrated by the ultra-violet photodeposition of lead, from tetraethyl lead, onto silica as early as 196S1’, and has more recently been exploited by Ehrlich et al in the low temperature laser photochemical deposition (LPD) of a number of metals for conduction tracks4 and photolithographic mask repair r* for possible use in silicon device technology. Surprisingly perhaps, LPD can be used for high resolution deposition, partially overcoming the spreading effects of vapour phase diffusion by the process of prenucleation”. The final case, that of large or,,(A) and osi (h) leads to the photochemical generation of dissociation products as well as high surface temperatures, and it is in this regime that much of the silicon doping has been achieved20*21.This is the most complex case to model for the metal alkyl precursors as they are both thermally and photochemically unstable, leading to competition from the case I and case II mechanisms in the formation of metal products. In the absence of relative dissociation rates it is unclear which of the competing mechanisms is responsible for providing the majority of metal species for a particular I and surface temperature; though it is clear from our results involving changing the wavelength that the overall efficiency of doping is increased by excitation of the vapour. The increase may arise as a result of enhanced boron formation from a specific volume near the substrate surface by photolysis, or from the formation of boron throughout the irradiated volume of the cell between window and substrate producing ‘fallout’ of boron at the doping site between laser pulses, but not contributing to single pulse doping.

IO

1.0

0.1

0.01

Fig. 6 Surface spreading resistance of doped areas against vapour pressure of TEB during doping

The latter mechanism would be expected to produce boron over an extended area of the substrate, and preliminary results on Auger element mapping suggest that this is indeed the case, reducing the possible resolution of device

38

OPTICS AND LASER TECHNOLOGY.

Vapour pressure (tom)

FEBRUARY

1983

structures. High resolution, maskless writing requires that the precursor be confined to a layer very close to the substrate surface, as in surface adsorption. Surface adsorption does locate the precursor species and prevents diffusion from the reaction site, as in the case of low temperature prenucleated ffirn growth; but for one step doping it also means that the unwanted dissociation products are .similarly ideally located to diffuse into the melt zone.

3 4 5 6 7 8

demonstrated controlled single-step laser implantation and doping of boron in silicon by the use of variable laser wavelength and vapour pressures of dopant precursor. It is expected that developments in laser beam profiling will similarly lead to further control of junction depth so that complete tailoring of multilayer devices can be realized. There is clearly a great deal of work yet to be done on the mechanisms of deposition and doping, and in particular on the development of new materials with variable surface adsorption characteristics and nondeleterious dissociation components. We have

2

Peescy,P.S.,Wrmpker, W.B. Appl Phys Left, 40 (1982) 768-770 Dew&, T.F., Fan, LCC., Ehrtich, D.J., Turner, G.W., x ILL...,Gale, R$‘. Appl Phys Lett, 40 (1982)

OPTICS AND LASER TECHNOLOGY.

12 13 14 15 16 17 18 19

References 1

9 10 11

FEBRUARY

1983

20 21

Deutsch.T.P.,Ehslkh.D.J.,Oaeood,RM., Lian,Z.L. Appl PI& kit. 36 (1980) 847&g. Tsm. J.Y.. Ehrllch. D.J.. Silversmith. D.J..Mount8in. R.W. IEEE Eie&on D&ice L&f, 3 (198231641166 ’ Ehrkh, DL, C&aod, RM., Deutsch,T.F. Appl Phys Let& 36 (1980)916-918 ‘i’hompncm, It, Lirmett,J.&K Roy Sot, London,156(1936) 108-130 Tamir, M., Halawe, U., Levine, R.D. Chem Phys Let& 25 (1974) 38-42 Jonah, C, Chmwh, P., Ben&n, R. J C%ernPhys, 55 (1971) 1903-1907 TNmbose,F.A. Bell @sf Tech J, 39 (1960)205 Allen, S.D.JApplPhys, 52 (1981)6501-6505 Sole&i, R, Boyes,P.K., Mehen,J.E.,Collh, G.J.Appl Phys km, 38 (1981)572-574 Johnson,W.E., Scldie,L.A. Appl Phys Lett, 40 (1982) 798-801 Bauerk,D., highs, P., Leyendecker, G., No& H., W-s, D. Ap@Phys Let& 40 (1982) 819-821 Bayer,P.K.,Roche,G.A., Ritchie,WM., CdBns,G.J. AppZ PlrysLett, 40 (1982)716-719 Met, KM., Simpson,W.I. JElecfrochemSot, 116 (1969) 1725 Turner,G.B.,Temnt, D., Potkwk,G. Appl J%ys Lett, 39 (1981)967-969 Rigby, Faradw Sot, 65 (1969)2421-2429’ - -. LJ. Trclns Ehrkh, D.J.,Oqood, dM., Siivemmith, DJ., Deutsch,T.F. IEEE Electron Device Lett. 1 (1980) 101-103 ISMkh, D.J., Dqood, Rti., D&taci, T.F. Appl Phys Lett, 38 (1981)946-948 Deutach, T.F., Fan,J.CC., Tumes,G.W., Chpmen, RL, Ehrl&h,DJ.,Oqood,RM.ApplPhysLctr, 38(1981) 144-146 Deutach,T.F., Ehskh, D.J.,Bathmm,D.D., Shemmith,D.J., ol%ood,RM. ApplPhys Left, 39 (1981)825-827

39