8
Laser doping for microelectronics and microtechnology Thierry Sarnet a, * , Gurwan Kerrien a , Nourdin Yaakoubi a , Alain Bosseboeuf a , Elisabeth Dufour-Gergam a , Dominique De ´barre a , Jacques Boulmer a , Kuniyuki Kakushima b , Cyrille Laviron c , Miguel Hernandez d , Julien Venturini d , Tarik Bourouina e a IEF Institut d’Electronique Fondamentale Universite ´ Paris Sud Ba ˆt. 220, 91405 Orsay, France b LIMMS, University of Tokyo, Tokyo 153-8505, Japan c CEA/DRT – LETI/DTS, 17 avenue des Martyrs, 38054 Grenoble, France d SOPRA, 26 rue Pierre Joigneaux, 92270 Bois Colombes, France e ESIEE, Cite ´ Descartes, 2 Bd Blaise Pascal 93162 Noisy-le-Grand Cedex, France Abstract The future CMOS generations for microelectronics will require advanced doping techniques capable to realize ultra-shallow, highly doped junctions with abrupt profiles. Recent experiments have shown the potential capabilities of laser processing of ultra shallow junctions (USJ). According to the International Technology Roadmap for Semiconductors, two laser processes are able to reach the ultimate predictions: laser thermal processing or annealing (LTP or LTA) and gas immersion laser doping (GILD). Both processes are based on the rapid melting/solidification of the substrate. During solidification, the liquid silicon, which contains the dopants, is formed epitaxially from the underlying crystalline silicon. In the case of laser thermal annealing, dopants are implanted before laser processing. GILD skips the ion-implantation step: in this case the dopants are chemisorbed on the Si surface before the laser-shot. The dopants are then incorporated and activated during the laser process. Activation is limited to the liquid layer and this chemisorption/laser-shot cycle can be repeated until the desired concentration is reached. In this paper, we investigate the possibilities and limitations of the GILD technique for two different substrates: silicon bulk and SOI. We also show some laser doping applications for the fabrication of micro and nanoresonators, widely used in the MEMS Industry. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser doping; Ultra shallow junctions; Implantation; Microresonator; Nanoresonator; MEMS; LTP; GILD 1. Introduction The aim of this work is to study a breakthrough technology based on a laser process. A major challenge for the future of CMOS technology concerns the realization of ultra-shallow MOSFET www.elsevier.com/locate/apsusc Applied Surface Science 247 (2005) 537–544 * Corresponding author. Tel.: +33 1 69 15 40 73; fax: +33 1 69 15 40 80. E-mail address: [email protected] (T. Sarnet). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.172

Laser doping for microelectronics and microtechnology

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Page 1: Laser doping for microelectronics and microtechnology

www.elsevier.com/locate/apsusc

Applied Surface Science 247 (2005) 537–544

Laser doping for microelectronics and microtechnology

Thierry Sarnet a,*, Gurwan Kerrien a, Nourdin Yaakoubi a,Alain Bosseboeuf a, Elisabeth Dufour-Gergam a, Dominique Debarre a,

Jacques Boulmer a, Kuniyuki Kakushima b, Cyrille Laviron c,Miguel Hernandez d, Julien Venturini d, Tarik Bourouina e

a IEF Institut d’Electronique Fondamentale Universite Paris Sud Bat. 220, 91405 Orsay, Franceb LIMMS, University of Tokyo, Tokyo 153-8505, Japan

c CEA/DRT – LETI/DTS, 17 avenue des Martyrs, 38054 Grenoble, Franced SOPRA, 26 rue Pierre Joigneaux, 92270 Bois Colombes, France

e ESIEE, Cite Descartes, 2 Bd Blaise Pascal 93162 Noisy-le-Grand Cedex, France

Abstract

The future CMOS generations for microelectronics will require advanced doping techniques capable to realize ultra-shallow,

highly doped junctions with abrupt profiles. Recent experiments have shown the potential capabilities of laser processing of ultra

shallow junctions (USJ). According to the International Technology Roadmap for Semiconductors, two laser processes are able

to reach the ultimate predictions: laser thermal processing or annealing (LTP or LTA) and gas immersion laser doping (GILD).

Both processes are based on the rapid melting/solidification of the substrate. During solidification, the liquid silicon, which

contains the dopants, is formed epitaxially from the underlying crystalline silicon. In the case of laser thermal annealing, dopants

are implanted before laser processing. GILD skips the ion-implantation step: in this case the dopants are chemisorbed on the Si

surface before the laser-shot. The dopants are then incorporated and activated during the laser process. Activation is limited to

the liquid layer and this chemisorption/laser-shot cycle can be repeated until the desired concentration is reached. In this paper,

we investigate the possibilities and limitations of the GILD technique for two different substrates: silicon bulk and SOI. We also

show some laser doping applications for the fabrication of micro and nanoresonators, widely used in the MEMS Industry.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Laser doping; Ultra shallow junctions; Implantation; Microresonator; Nanoresonator; MEMS; LTP; GILD

* Corresponding author. Tel.: +33 1 69 15 40 73;

fax: +33 1 69 15 40 80.

E-mail address: [email protected] (T. Sarnet).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.01.172

1. Introduction

The aim of this work is to study a breakthrough

technology based on a laser process. A major

challenge for the future of CMOS technology

concerns the realization of ultra-shallow MOSFET

.

Page 2: Laser doping for microelectronics and microtechnology

T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544538

source and drain. These junctions need to be ultra

shallow, highly doped with abrupt profiles. The ITRS

(Roadmap for Semiconductors) [1] has predicted that

conventional technologies, based on ion implantation

and rapid thermal annealing (RTP) or spike annealing,

will hardly meet these specifications. Recent experi-

ments have shown the potential capabilities of laser

processing of ultra shallow junctions (USJ) for CMOS

technologies. According to the ITRS, two laser

processes are able to reach the ultimate predictions:

laser thermal processing (LTP) and gas laser immersion

doping (GILD) [2–10]. LTP and GILD are both based

on rapid melting/solidification of the substrate. During

solidification, the liquid silicon, which contains the

dopants, is formed epitaxially from the underlying

crystalline silicon. The main difference between the

two laser processes concerns the implantation step. For

LTP, the dopants are implanted on the surface before the

laser processing, using conventional techniques. GILD

does not require the ion-implantation step: in this case

the dopants are directly chemisorbed on the Si surface

just before the laser-shot. The dopants are then

incorporated and activated during the melting/solidi-

fication cycle. The activation is limited to the liquid

layer and this chemisorption/laser-shot cycle can be

repeated until the desired concentration is reached.

ig. 1. SIMS depth profiles of B doped layers realized by GILD

ith 50, 100 and 200 laser-shots at 670 mJ/cm2.

2. Experimental procedure

GILD is performed in a high vacuum chamber

(10�7 mbar) on Si and SOI wafers, using a homo-

genized XeCl excimer laser (308 nm, 30 ns, 200 mJ per

pulse, 1 to 25 Hz). After cleaning and removing the

native oxide the substrate is introduced in the chamber.

The dopant precursor gas (BCl3) is injected and

chemisorbed on the substrate before each laser pulse.

LTP process is performed using a VEL15 SOPRA laser

(308 nm, 200 ns, 0.6 Hz). The samples have been

characterized using four-point probes, SIMS, IR

spectrometry and in situ optical characterization at

675 nm [9].

These laser processes are also carried out through an

oxide mask in order to selectively treat a particular zone

on the wafer. The masks are obtained after a series of

steps involving photolithography (for features with

lateral size>2–3 mm) or electron beam lithography (for

sub-micron features).

3. Experimental results

3.1. Results on Si bulk

The dopant profiles have been measured after

GILD laser implantation by SIMS [11]. Fig. 1 presents

boron concentration profiles obtained with 50, 100 and

200 laser-shots at 670 mJ/cm2. Box-like and abrupt

profiles have been obtained, with high dopant

concentration and very low sheet resistance. For the

same laser energy density, the junction thickness

increases with the number of shots due to the boron

incorporation, which changes the thermodynamic

parameters and the optical absorption at 308 nm.

Thanks to the optical characterization (in situ

reflectivity at 675 nm), we have been able to follow the

junction evolution during the GILD process. Fig. 2a

presents the linear evolution of the junction thickness

as a function of the number of laser-shots for three

different laser energy densities. SIMS data are also

plotted on the same graph and show a good agreement

with the optical diagnostic.

The boron incorporation remains constant during

the process, for the same laser energy density. The

linear evolution of the boron concentration (at/cm2)

versus the number of laser-shots at different energy

densities is shown in Fig. 2b. We have measured with

an IR spectrometer the transmission coefficient of

doped layer. By using the Drude theory for doped layer

index measurement we have been able to find the ratio

of the activated dose over the effective mass for each

sample. We have chosen an effective mass of 0.37 to

find the best fit between the SIMS and FT-IR

F

w

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T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544 539

Fig. 2. Evolution during GILD process of the junction depth (a) and

the boron concentration (number of B atoms/cm2) (b), as a function

of the number of laser-shots for different laser energy densities.

Fig. 4. Evolution of sheet resistance vs. laser energy density for

different laser-shots on SOI 200/1000 nm and Si substrates.

measurements. The evolution of boron concentration

incorporated by shots on silicon substrate versus the

laser energy density is shown in Fig. 3.

3.2. Results on SOI

Additional GILD experiments have been carried

out on SOI (200/1000) substrate. Due to the 1000 nm

Fig. 3. Boron dose incorporation (number of B atoms/shots/cm2),

comparison on silicon bulk and SOI 200/1000 substrates.

of silicon oxide the heat is confined to the 200 nm of

the c-Si layer, decreasing the melting threshold to

260 mJ/cm2. An amorphization threshold also

appears above 610 mJ/cm2 because the liquid phase

reaches the oxide layer. Sheet resistances evaluated

from FT-IR (effective mass of 0.37) are in good

agreement with those measured by the four-point

probe. We have observed that the junction depth

increases with the number of laser-shots, like in the

GILD experiment on c-Si. In Fig. 3, we compare the

evolution of the boron concentration incorporated by

shots as measured by FT-IR versus the laser energy

density on SOI and c-Si substrates. For SOI, the

boron dose is much higher than the one on silicon

bulk and the boron incorporation can reach up to 30%

of the total chemisorbed sites on the silicon surface.

We explain this fact by the important difference

during the heating of the substrate before melting.

The boron incorporation efficiency increases with the

temperature gradient before melting. As a conse-

quence, the measured sheet resistance on SOI

substrate is lower than the one on Si substrate, but

the evolution of sheet resistance with the laser-shots

is the same. Fig. 4 shows the evolution of the sheet

resistance versus the laser energy density for

different laser-shots on SOI 200/1000 nm and Si

substrate.

4. ITRS comparison

According to the ITRS predictions, no conventional

techniques for USJ formation can meet the CMOS

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T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544540

sub-0.1 mm specifications, mainly because of the

limitation due to channel implantation effects and

annealing dopant diffusion.

Laser processing can solve these technological

problems. Thanks to optical characterization of laser

processing, we have been able to determine the

evolution of junction thickness and boron incorpora-

tion as a function of the laser energy density on c-Si

bulk [9]. Therefore, we can extrapolate a theoretical

limit of GILD process for USJ.

In Fig. 5, we present these limitations and

characteristics of junctions made by GILD as

measured by four-point probes and SIMS. We

compare the universal Rs/X j tradeoff of implanted

and thermally annealed boron with recent conven-

tional techniques [2], and with laser thermal proces-

sing (LTP) studies from our previous work and from

Felch et al. [12].

The measurements show that GILD and LTP meet

the ITRS 2002 specifications for the 45 nm technology

node and laser processing is more efficient than any

other usual technique. In the case of LTP, transient

enhanced diffusion (TED) does not occur but the

implanted profile determines the doping profile.

Because GILD skips the implantation step, problems

coming from this implantation step can be avoided.

Junctions can reach depths as thin as 6 nm, and

because the activation rate is very high, the sheet

resistance can decrease down to 400 V/& with only

10 laser-shots and with an energy density just above

the melting threshold. The corresponding abruptness

is then close to 2 nm/decade.

Fig. 5. Sheet resistance vs. junction depth according to the ITRS

2002 specifications. Comparison of GILD results with conventional

techniques and laser annealing.

5. Application of laser doping for MEMS

5.1. Introduction

Recently, there has been a great deal of interest in

micromechanical resonators for a wide range of appli-

cations including wireless communications, thanks to

its superior characteristics, such as high Q-factor,

temperature stability, and aging properties. Moreover,

the integration compatibility with CMOS circuits

enables smaller chip size and low-cost mass production.

However, fabricating small structures of low mass and

high frequency becomes challenging. We have used the

potentiality of gas immersion laser doping (GILD) to

fabricate such micro silicon bridges [13].

Generally, boron-doped silicon has residual tensile

stresses, since the boron atom is smaller than silicon: the

stress largely depends on the concentration of boron.

Huang and Najafi have reported tensile stresses ranging

from 13 to 140 MPa with boron concentration of

9 � 1019 cm�3, depending on subsequent annealing

processes [14]. In our case the dopant concentration is

very high, which means that we have the possibility to

increase the resonant frequency for the same bridge

dimension.

5.2. Fabrication

A single polished (1 0 0) wafer (n-type 50 V cm)

was cleaned by acid chemicals and was thermally

oxidized to form a 100 nm mask layer of SiO2. This

mask thickness was chosen to limit the absorption of

laser light during the subsequent laser irradiation

(Fig. 6).

We used both photolithography and electron beam

lithography to pattern the SiO2 mask in order to open a

bridge shape along the h1 0 0i direction. Boron doping

was done by GILD using a homogenized XeCl

excimer laser (l = 308 nm, 30 ns, 700 mJ/cm2, 2 Hz)

and BCl3 as a precursor gas into a high vacuum

chamber. The thickness of the p-layer is tuned within

the 10–200 nm range with a boron concentration up to

3 � 1021 at/cm3 [3,9], by using different energy

densities and numbers of shots. The wafer was then

annealed at 600 8C for 1 h in order to measure the

residual stresses. The sheet resistance has been

measured using four-point probes, before and after

annealing (Fig. 7).

Page 5: Laser doping for microelectronics and microtechnology

T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544 541

Fig. 6. Reflection coefficient R of the substrate at l = 308 nm as a

function of the silicon mask thickness.

Fig. 8. Description of the process: (a) (1 0 0) Si substrate cleaning

and native oxide removal, (b) oxide layer formation (100 nm), (c)

oxide patterning (photolithography or e-beam lithography), (d) laser

doping, (e) oxide mask removal by HF and (f) TMAH selective

etching of undoped silicon to release the doped bridge.

After the heat treatment, the junction is still very

electrically activated (R < 100 V/sq.). After removing

the SiO2 mask with HF, an anisotropic etching of the

undoped silicon was performed with tetra methyl

ammonium hydroxide (TMAH) to selectively release

the doped bridges. The GILD process and the

anisotropic etching are summarized in Fig. 8.

5.3. Results and measurements

A typical SEM image of a laser-doped bridge (4 mm

in width and 50 mm in length) is shown in Fig. 9. The

thickness of the bridges were measured on cross-

Fig. 7. Evolution of the square sheet resistance before and after vs.

number of laser-shots.

sectional views using a high resolution FEG-SEM, as

shown in Fig. 10. The resonance was obtained by laser

illumination (l = 780 nm) modulated by network

analyzer. The vibration was measured using a laser

Doppler vibrometer (l = 632.5 nm). All these char-

acterizations were done under vacuum (5 � 10�3 Pa)

[13].

Fig. 11 shows a typical spectrum of a 100 mm

bridge, with a resonance at 2.77 MHz and a Q-factor

of 1835. This frequency is 25 times higher than that of

stress-free single crystalline Si bridge with the same

dimensions. This resonant frequency tends to a

Fig. 9. SEM micrograph of a GILD boron-doped single-crystalline

Si bridge. Dimensions of the bridge (length � width � thickness):

50 mm � 4 mm � 120 nm.

Page 6: Laser doping for microelectronics and microtechnology

T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544542

Fig. 10. SEM–FEG micrograph of a GILD boron-doped single-

crystalline Si bridge. Thickness of the bridge: 90 nm.ig. 12. Evolution of the resonant frequency as a function of the

umber of laser-shots. The fstressed/fstress-free ratio is shown on the right.

ig. 13. AFM image of a laser treated zone showing a roughness of

.5 nm RMS. Sample: pre-amorphized Si (BF25 keV, 2e 15 at/cm2.

aser irradiation conditions: 2.2 J/cm2, 308 nm, 200 ns, single shot.

maximum for 100 laser-shots and slightly decreases

above this value (Fig. 12). We also compared in

Fig. 12 this frequency to the stress-free bridge, which

has a resonant frequency of 100 kHz (100 mm bridge)

and 400 kHz (50 mm bridge): the laser doping largely

increases the resonant frequency (up to 40 times

higher).

The RMS roughness of the laser treated areas has

been analyzed with an AFM. In most cases the

roughness after the laser treatment is maintained at

very good levels, usually lower than 5 A RMS

(Fig. 13). However we have observed a slight increase

in this roughness when working with pre-amorphized

silicon wafers (Fig. 14). In that particular case we have

Fig. 11. Output spectra of a 100 mm bridge at 1st mode measured in

a vacuum chamber. The resonant frequency of 2.77 MHz is more

than 20 times higher than that of stress-free bridge (100 kHz). The

Q-factor is 1835.

ig. 14. Evolution of the roughness and melted depth as a function

f the laser energy density. Laser: 308 nm, 20 ns. Sample: pre-

morphized Si wafer (Ge+ 10 keV, B+ implanted 0.5 keV) processed

ith LTP. Zone 1: below melting threshold, no change in roughness;

one 2: partial melting of the pre-amorphized layer, increased

oughness, zone 3: total melting of the pre-amorphized layer, low

oughness.

F

n

F

0

L

F

o

a

w

z

r

r

Page 7: Laser doping for microelectronics and microtechnology

T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544 543

Fig. 16. Measured resonant frequencies and fitting to calculations.

The thicknesses of the bridges are depicted. The stress-free silicon

bridge with a thickness of 120 nm is also shown.

seen that it is necessary to melt the entire pre-

amorphized layer in order to obtain a good recon-

struction of the surface, and, consequently, a low

roughness (<5 A RMS).

5.4. Discussion

The design of the resonator is based on a vertical

resonating bridge, where the motion of the bridge is

perpendicular to the substrate (Fig. 15). If we apply a

stress (s) to this structure the resonant frequency tends

to increase. This dependence is generally described by

the following equation:

fRstressed¼ 1:028

t

L2

ffiffiffiffiE

r

r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 þ 12L2

ap2Et2s

r

where k and m are the spring constant and its mass, E

the Young’s modulus, r the mass density and s the

applied in-plane tensile stress.

We have estimated the residual stress by fitting the

previous equation to the experimental resonant

frequencies. The highest stress value obtained was

190 MPa at 100 laser-shots; these values are repre-

sented in Fig. 16.

By increasing the number of shots, the resonant

frequency tends to decrease while the thickness still

increases. This can be explained by a decrease in the

crystallinity and mechanical properties of the Si

bridge when using excess laser exposure (over 100

shots). From our previous reports [3,9], the boron

concentration at more than 200 laser-shots exceeds the

solubility limit of boron in silicon at room tempera-

ture: we anticipated that some defects of this super-

Fig. 15. Schematic layout showing the resonance frequency

increase due to in-plane tensile stresses on a laser-doped bridge.

doped silicon or maybe inactive boron incorporation

[15] might have slightly decreased the resonance.

Also, one might suggest that the annealing (1 h at

600 8C) tends to release the stress and decrease the

resonant frequencies. Additional laser-doped bridges

have already been realized, without annealing, with

length between 1 and 500 mm, which corresponds to a

wide resonant frequency range that should cover

numerous MEMS applications (estimated range

1 MHz to 1 GHz). These unique features suggest

several MEMS applications, currently under investi-

gation (e.g. RF filter, mass detection sensors, etc.).

Smaller features have also been realized by

combining electron-beam lithography and GILD. In

that case we have spin-coated a thin PMMA layer on

top of a pre-oxidized Si wafer. The mask was directly

written in the PMMA layer using a high resolution

FEG–SEM and, after development, has been trans-

ferred to the oxide layer. The wafer has been laser-

doped through this mask. This process is still under

development and should eventually allow us to make

very promising nanoresonators and nanostructures.

Fig. 17 shows a SEM photograph of two micro bridges

that were obtained using GILD laser doping through a

mask that was designed by electron beam lithography:

the width of the bridges is 900 and 800 nm, with a

thickness below 100 nm and a typical RMS roughness

below 10 A.

Page 8: Laser doping for microelectronics and microtechnology

T. Sarnet et al. / Applied Surface Science 247 (2005) 537–544544

Fig. 17. SEM micrograph of two micro bridges obtained using

GILD laser doping through a mask designed by electron beam

lithography. Dimensions of the bridges: 9 mm � 900 nm (up) and

8 mm � 800 nm (down). Thicknesses of the bridges: <100 nm,

typical RMS roughness <10 A.

6. Conclusions

In this work, we present the GILD process which

could be an alternative for implantation and rapid

annealing in the making of ultra shallow junctions. By

studying GILD on Si and SOI 200/1000 nm substrates,

we have seen that the number of boron atoms

incorporated by shots depends on laser energy density

and temperature gradient. Thanks to optical char-

acterizations, we have been able to estimate the

theoretical limit curve for the Rs/X j on Si bulk.

Therefore, we have been able to determine the laser

energy density and the number of laser-shots to make

an ultra thin junction with excellent activation rate,

which can meet the final ITRS specifications. Finally

we have realized micro and nanoresonators for MEMS

applications using both laser doping and photolitho-

graphy or electron beam lithography.

Acknowledgements

This work has been supported by the French

Ministry of Research and Technology in the frame-

work of the RMNT DOLAMI project. The authors

would like to thank all DOLAMI participants for their

contribution to this work.

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