APPLICATIONS OF TRANSIENT ANNEALING TO SOLAR CELL PROCESSING

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APPLICATIONS OF TRANSIENT ANNEALING TO SOLAR CELL PROCESSING

G. Bentini

To cite this version:

G. Bentini. APPLICATIONS OF TRANSIENT ANNEALING TO SOLAR CELL PROCESSING.

Journal de Physique Colloques, 1983, 44 (C5), pp.C5-353-C5-361. �10.1051/jphyscol:1983552�. �jpa-

00223138�

A P P L I C A T I O N S OF T R A N S I E N T ANNEALING TO SOLAR C E L L PROCESSING

G.G. Bentini

C . N . R . I s t i t u t o LAMEL Via Castagnoli 1 , 40126 Bozognu, I t a l y

Resum6 - Ces razsons economiques justifiant l'introduction de les requits a transient dans la fabrication de les cellules solaires ont et6 brievement discut-5. Ces techniques peuvent jouer un r61e importank dan la production a grande echelle, pour leur compatibilite avec la demande de procddes automa- tiques, associe a une bonne qualit6 de la junction p-n.

Les applications des diff6rent techniques de recuite a transient appliqudes a la production des cellules solaires ont et6 discutg en cornparand les r6sul- tats obtenues, soit en phase liquide soit en phase solide, apres Implantation Ionique ou deposition du dopant sur la surface.

La possibilite de realisation de junction p-n la suite de une impulsion laser donne? en une atmosphere contenant un gaz dopant, sera brievement present6 .

Abstract - The economical reasons supporting the introduction of transient annealing in solar cell manufacturing are briefly discussed.

Such techniques may play an important role, as they are compatible with the request of high throughput, automated processing together with the high quality of the p-n junction which are necessary for large scale economical production of photovoltaic energy.

A survey of the applications of the different transient annealing techniques to solar cell processing has been developed by comparing in detail the results obtained up to now the case of solid and liquid phase transient annealing, associated with dry techniques such as Ion Implantation or dopant deposition on the wafer surface.

The possibility of using laser pulses for the formation of the p-n junction by incorporation of dopant atoms from a suitable gaseous environment, has also been examined.

INTRODUCTION

Everybody knows the trend of price reduction and market growth vs. time foreseen by the D.O.E. program and indicating the goal of 0.5 $/Wp for the energy produced by photovoltaic conversion in the year 1985.

In this paper instead of looking at the extrapolation of data in the future, I shall look at the past and I shall consider the comparison between actual market growth and the prevision of the D.O.E. program.

In Fig. 1 the comparison between the actual market growth observed (1) from 1976 to 1982 and the prevision of D.O.E. program is displayed.

It must be pointed out that in spite of the fact that today's price per Watt is higher than in the prevision of the D.O.E. program, the total market has in- creased more than expected, in fact, considering for instance the data of 1982, a business of 105 M$ has been developed, at a price of the order of 6-8 $/W; that is equivalent to a total production of the order of 17 MW/year.

It is interesting to observe that a price of 7 $/W, a production of 2 MWJyear was foreseen by the D.O.E. program; confirming that there is a business increase, vs. cost reduction, with a much greater growth than in the prevision.

To face this impressive market growth it seems reasonable to consider the problems concerning the installation of production lines capable of processing several MW/year, (which will give rise to further price reductions).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983552

JOURNAL DE PHYSIQUE

I I I I I I I I

- PHOTOVOLTAICS

- ^{80 - //-Fig. 1 -} Photovoltaic business growth in c-0 z ^- the period 1974 -

^{1983: actual growth}

(solid line) ; DOE prevision (dashed line).

The numbers in parentheses represent the

/ - cost in US $ per peak watt.

YEAR

To reach an energy production rate of this magnitude, it will be necessary in a short time, to set up production lines capable of processing a number of cells per hour much higher than that currently processed today. An evaluation of the processing time allotted per cell as a function of the yearly energy production in the case of 13% and 14% conversion efficiency is reported in Tab.1.

TAB. I

As the furnace thermal treatments and the subsequent wet chemical stages, are the longest steps in cell manifacture, the interest on any transient annealing technique that may be used alternatively to standard furnace processing is evident.

Different kinds of transient annealing are under study (2,3,4) considering both the p-n junction formation and the contact sintering ( 5 ) . As for dopant introduction, several techniques have been experimented in connection with transient annealing, involving both liquid and solid phase process. In Tab.11 a survey of the different processes usually studied, is reported.

Let us briefly examine the different processes in some detail:

E L E C T R I C A L POWER

1 M h a t t l y e a r

5 MWattlyear

1 0 MWattIyear

1. ION IMPLANTATION

The application of transient annealing to photovoltaics which are by far the most developed, is in connection with Ion Implantation (6,7,8).

This technique, already well established in device technology, is now consi- dered by many people as being the most promising non traditional tool for p-n

T I M E 1

IVAFER) (sec) 1) = 13%

1 0 . 2

2 . 0

1

1 ) = 14%

9.5

1.9

0.95

TAB. I1

Today high current Ion Implantation machines are available, even if the system is still considered too expensive for cell manufacture.

Alternative low cost machnies without mass analysis, have been studied and developed

ANNEALING D E V I C E

c w laser, e-beam i n c o h e r e n t l i g h t

Q-switched laser, e l e c t r o n p u l s e

i c x c i m e r AI-F (193nm))

Fig. 2 - Low cost prototype accelerator without mass selection, suitable for solar cell processing.

ANNEALING PROCESS

s o l i d phase

l i q u i d phase

I

DOPANT I N T R O D U C T I U N

I o n I m p l a n t a t i o n w i t h o r w i t h o u t mass selection

Dopant deposition o n t h e surface

R e c r y s t a l l i z a t i o n of d o p e d amorphous f i l m

D i r e c t r e a c t i o n

These machines produce ion beams containing the dopant ion together with molecular species of the dopant itself. The presence of contaminant species such as F, C1, H, introduced with the dopant beam could affect the device performance negatively, nevertheless by choosing suitable ion beam composition and annealing conditions, good quality cells can be produced ( 8 ) .

In any case, whichever implantation system has been chosen, the annealing of the damage induced by the ions and the dopant activation can successfully be achie- ved by transient annealing techniques both in liquid and solid phase regime.

Z ( s e c ) 10-'-10

1 0 - ~ - 1 0 - ~

1.1 Liquid phase regime

Liquid phase annealing is generally performed by laser or electron pulses

which have a time duration ranging from 10-100 nsec; this process gives rise to a

very good crystalline quality of the melted region and to a complete dopant acti-

JOURNAL DE PHYSIQUE

vation; the disadvantages are mainly represented by an uncontrolled diffusion of the dopant and by some deep residual damage, frozen below the melted region, probably originated by the channelled tail of the implanted ions. Generally it is believed that a supplementary annealing stage at 450'-500°C must be performed to increase the cell efficiency (12,131.

Prototype production equipment, based on electron pulses, is now available capable of processing up to 1200 wafers per hour (i.e. 3 sec per 4 in wafer ( 4 ) ) . 1.2 Solid phase regime

The solid phase transient annealing, having a time duration which ranges roughly from 1 msec to 10 sec, is generally achieved by electron beams or incohe- rent light sources.

Fig. 3 - Time necessary: a) to Fig.4 - Stress profiles induced regrow an amorphous layer 1000 W in a silicon wafer during short thick (solid line) ; b) to anneal transient annealing in solid dislocation loops having 1000 W phase regime: T = transient du- diameter (dashed line) ration, P = power density.

This technique gives rise to a very good dopant activation almost without changes in doping profile, the main disadvantage is represented by dislocation loops that are usually left in the implanted regions. In fact to anneal out ex- teoded defects such as dislocation loops, much harder time-temperature conditions are necessary (131, if compared to a simple SPEG process as shown in Fig. 3 dis- playing that in transient conditions (i.e. t<10 sec) temperature higher than 1400 K are necessary to anneal 1000 W dislocation loops; m reover t can be seen that the useful annealing time must range roughly between and 10 sec. jr

Whichever the energy source chosen is, a big problem concerning solid phase transient annealing arises in connection with therma ly induced stresses. In fact -3

when the annealing pulse is shorter than about 10 sec, a thermal gradient is

the stress induced on the sample, changes from tensile to compressive as a function of depth. If longer times are used (1-10 sec), by heating the wafer surface unifor- mly, a radial thermal gradient is generated giving rise to thermal stresses near the edge of the sample as shown in Fig. 5a and 5b (G.G. Bentini, L. Correra, C.

Donolato, unpublished).

These stresses are responsible for the slips which are often observed in Rapid Isothermal Annealed samples. In spite of these limitations it must be said that efficient solar cells can be produced by this method (6,7,8) and that recent works (15) based on very intense ( > 100 KW) incoherent light sources, seem to confirm the possibility of finding a "window" in the annealing conditions, allowing for the formation of recrystallized layers which are almost free from residual extended defects, without introducing thermoelastic stresses exceeding the elastic limit.

A comparison of typical doping profiles obtained after transient annealing of implanted layers, performed by using different techniques involving solid and liquid phase regimes is reported in Fig. 6.

R= 3.81 cm (wafer radius)

~ ~ 0 . 5 0 (emissivity) 1510 K= 0.25W/cm'K (thermal

conductivity at 1573 'K)

tang.=%.. b)]

stress component

E 8 radial = a,, o ,

I

0 ,

-'.

I4'O - COMPUTED TEMPERATURE PROFILE -

I I I I I I I l l ,

I I I I I I I I I I

-8 1 COMPUTED STRESS PROFILE 1

- >

0 0 -4

lo2'

R, actwe dose

R/o

low

13

l0I5

~.FURNAEEIPO 088

r -> c-LASER

64 2 0

E,

^{43 19}

U

z Q lo'9

6

n z

W U

Z

00 ^10"

10"

TENSILE

- ^: &.:_i-+ ; - ' ' A d -

- COMPRESSIVE -

Ne

10'~ I I I

0 0.1 0.2 0.3 0.4 0.5 0.6

DEPTH [pm]

Fig. 5 - a) radial temperature Fig. 6 - dependence of the doping pro- profile across a wafer during files in implanted silicon after dif- Rapid Isothermal Annealing, b) ferent types of transient annealing:

stress distribution across the a) multiscanning e-beam (MEBA) ; b) fur- wafer when the temperature pro- nace 750°C, 30 min; c) Q-switched ruby file of Fig. 5a) is present. laser; d) pulsed e-beam (PEBA).

The typical cell efficiencies reached by joining Ion Implantation and tran-

sient annealing techniques, are reported in Table I11 showing that efficiency of

the order of 14% - 15% can be reached. This demonstrates the feasibility of

processes based on transient annealing which have a good conversion efficiency.

C5-358 JOURNAL DE PHYSIQUE

I t must be p o i n t e d o u t t h a t t h e c e l l s c l a s s i f i e d a s "MEBA" i n Tab I11 have been manufactured by vacuum d e p o s i t i n g t h e m e t a l c o n t a c t a f t e r t h e i m p l a n t a t i o n s t a g e ; t h e s u c c e s s i v e t r a n s i e n t a n n e a l i n g performed t h e dopant a c t i v a t i o n and t h e c o n t a c t s i n t e r i n g a t t h e same time ( 6 ) .

TAB I11

2. DIFFUSION OF DEPOSITED LAYERS

IMPLANTED SPECIES

P 1 0 k e V

P 10 k e V

P 10 k e V

P 10+40 k e V

-

PF5 10 k f V

PF5 10 k e V

An a l t e r n a t i v e method e x p l o r e d t o o b t a i n p-n j u n c t i o n by t r a n s i e n t a n n e a l i n g ( s e e Tab.111, i s based on t h e i r r a d i a t i o n of a t h i n f i l m of dopant m a t e r i a l deposi- t e d on t h e wafer s u r f a c e (16,17,18) by u s i n g l a s e r o r e l e c t r o n p u l s e s . I n t h i s c a s e t h e a n n e a l i n g must t a k e p l a c e i n l i q u i d phase and t h e dopant i s d r i v e n i n s i d e t h e sample by f a s t d i f f u s i o n i n t h e melted l a y e r .

I n p r i n c i p l e t h e system i s v e r y a t t r a c t i v e because it a v o i d s a r a t h e r expen- s i v e p r o c e s s l i k e Ion I m p l a n t a t i o n . The dopant m e t e r i a l can be d e p o s i t e d on t h e s u r f a c e by d i f f e r e n t t e c h n i q u e s such a s vacuum e v a p o r a t i o n , spin-on, spray-on, s p u t t e r i n g and d e p o s i t i o n of a doped amorphous s i l i c o n f i l m , a s shown i n Tab. I V . TAB. I V

ANNEALING CONDITIONS

P u l s e d e-beam (PEBA)

Q-switched laser

M u l t i s c a n n i n g e-beam ( M E B A )

I c o h e r e n t l l y h t (3 s e c l e-beam (10 sec) f u r n a c e (G50°C. 30'1

Laser N d : Y A G 3.0 ~ i c m ~

F u r n a c e (GOO°C. 30'1

Vacuum Evsp.

7] 8 ( A M I )

15.3

12.5

14.0

12.5 13.5 13.6

13.4

13.6

over wide areas 14-6 in. wafers) can also be a problem. In any case a wet stage for etching the undiffused layer is necessary after the annealing process.

The best results have been obtained by evaporation technique or by depositing a thin doped layer of amorphous silicon on the surface (as the introduction of unwanted material is avoided); a comparison among the conversion efficiencies of solar cells manufactured with the different deposition techniques is shown in Tab.

IV, showing that the technique is promising even if more work is needed to select the optimum deposition conditions and the pulse annealing parameters.

3. DIRECT REACTION INDUCED BY LASER

The most recent technique, reported in Tab. 11, applying transient annealing for p-n junction formation, is a one-step process based on the direct reaction induced by a laser pulse in doping gaseus environment. The reaction is induced when the wafer is hit by a laser pulse that performs, the photolysis of gas molecules and the melting of a surface layer at the same time.

Fig. 7 - Experimental set-up used

to irradiate samples in a dopant HOMOGENIZER atmosphere to produce p-n junc-

tions with a one-step process.

\ / GAS-VACUUM

This has been early achieved by using U.V. (Ar, F ) excimer lasers irradiating the sample in a suitable atmosphere; more recently other types of laser have also been used (18,19,20) and the effect takes place even if the laser wavelength is too

Fig. 8 -

" ^{0 0.2 0.4 0 0.2 0.4 0.6}

DEPTH [pm]

Doping profiles of boron (a) and phosphorus (b) obtained by laser

irradiation in dopant gaseus environment

C5-360 JOURNAL DE PHYSIQUE

long t o produce molecular p h o t o l y s i s . Gaseus dopant compounds l i k e BC13 o r phosphi- ne, a t a p r e s s u r e r a n g i n g from 50 t o 500 tor?i7can2be used. During t h e m e l t i n g time a number o f molecules n = P/ 4- ^{= 3x10 /cm} s t r i k e s t h e l i q u i d s u r f a c e , and it i s s u f f i c i e n t t h a t l e s s t h a n 1% of them a r e d i s s o l v e d i n t h e melted r e g i o n , t o o b t a i n t h e p-n j u n c t i o n .

The experimental s e t up i s sketched i n Fig. 6 and t h e t y p i c a l doping p r o f i l e s showing v e r y h i g h a c t i v a t i o n l e v e l s a r e shown i n F i g . 7a) and 7 b ) , r e p o r t i n g t h e r e s u l t s o b t a i n e d i n t h e c a s e of B and P doping.

Although t h e t e c h n i q u e i s only i n i t s e a r l y s t a g e s it h a s a l r e a d y been found t h a t s o l a r c e l l s produced i n t h i s way had an e f f i c i e n c y ranging between 8% and 10%

without A.R.C., demostrating t h a t t h i s one-step t e c h n i q u e could be v e r y promising i n c e l l manufacturing, because i n p r i n c i p l e , it could be joined t o a p r o c e s s of continuous growth of a s i l i c o n ribbon, g i v i n g r i s e t o t h e p r o d u c t i o n of a b a s e m a t e r i a l i n c l u d i n g t h e p-n j u n c t i o n .

CONCLUSIONS

I n c o n c l u s i o n , it can be s a i d t h a t t r a n s i e n t a n n e a l i n g seems t o be a very promising t o o l t o s a t i s f y t h e r e q u e s t of h i g h throughput, r e l i a b l e , a l l d r y pro- c e s s , a b l e t o o b t a i n a good conversion e f f i c i e n c y (average 14%-15%), even i f it seems much t o e a r l y i n t h e development of t h e s e t e c h n i q u e s t o a s s e r t t h a t any one approach i s c l e a r l y b e t t e r t h a n t h e o t h e r s .

I n any c a s e , b e f o r e t r a n s i e n t a n n e a l i n g can be i n t r o d u c e d t o p h o t o v o l t a i c technology, it i s necessary t o perform f u r t h e r i n v e s t i g a t i o n s t o answer s e v e r a l open q u e s t i o n s concerning a n n e a l i n g i n b o t h s o l i d and l i q u i d phase regimes; i n p a r t i c u l a r , a b e t t e r knowledge i s n e c e s s a r y on t h e f o l l o w i n g p o i n t s :

a ) P o s s i b i l i t y of o b t a i n i n g good e f f i c i e n c y c e l l s by l i q u i d phase a n n e a l i n g w i t h o u t p o s t a n n e a l i n g s t a g e s .

b) P o s s i b i l i t y o f o b t a i n i n g implanted l a y e r s almost d e f e c t - f r e e i n s o l i d phase regime.

C ) C o n t a c t s s i n t e r i n g d u r i n g dopant a c t i v a t i o n .

d) F e a s i b i l i t y of growing b a s e m a t e r i a l having p-n j u n c t i o n b u i l t i n by u s i n g l a s e r induced r e a c t i o n i n dopant environment.

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