ELSEVIER Journal of Crystal Growth 171 (1997) 50-55

, . . . . . . . . C R Y S T A L Q I R O W T H

The mechanism of secondary grain growth in polysilicon films

N.G. Nakhodkin *, T.V. Rodionova Department of Radiophysics, Kieu Taras Shet'chenko University, Vladimirskaya 64, KieL, 252017, Ukraine

Received 16 June 1996

A b s t r a c t

Secondary grain growth in 0.5 ~m thick undoped and phosphorus-doped polysilicon films, produced by low-pressure chemical vapour deposition, is investigated by transmission electron microscopy. Analysis shows that the driving force for secondary grain growth in phosphorus-doped polysilicon films is the difference in dislocation density in the normal and secondary grains for the early stages of secondary grain growth; the driving force is due to surface energy anisotropy for the subsequent growth stages. The most probably secondary grain formation mechanism is grain coalescence assisted by dislocation climb.

1. I n t r o d u c t i o n

Polycrystalline silicon films remain one of the most commonly used materials in integrated-circuit technology. It is well known [1-4] that the grain structure of thin polysilicon films determines signifi- cantly their mechanical, optical and electrical proper- ties. The grain structure can be characterized by grain size, crystallographic orientation and grain size distribution. These are known to change as grain growth occurs during annealing. Therefore, a com- prehensive understanding of grain growth is impor- tant in order to control the grain structure of films subjected to post-deposition annealing. The ultimate goal in both theoretical and experimental investiga- tions [5-13] is to develop means of controlling grain growth in order to produce semiconducting films with desired grain morphology.

* Corresponding author.

Grain growth in polycrystalline silicon films has been the objective of much research [5-13]. There are two variations of grain growth: normal (primary) grain growth and abnormal (secondary) grain growth. In normal grain growth, the grain size distribution remains monomodal. In abnormal grain growth, a bimodal size distribution develops at some time dur- ing the process. That is, at some point there is a population of large grains and a population of smaller grains. In both variations of grain growth, some grains grow larger while other grains shrink and disappear. In abnormal grain growth, the small, nor- mal grains may, on average, continue to grow but the large secondary grains grow at greater rates by con- sumption of surrounding normal grains. This growth continues until the secondary grains consume all normal grains.

Several theoretical models for secondary grain growth in thin semiconductor films are available. The most widespread model [5] treats secondary grain growth as the growth of cylindrical secondary

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grains into a homogeneous matrix of columnar nor- mal grains (two-dimensional model) due to surface anisotropy. This model has been extended to the interpretation of experimental results for secondary grain growth in ultra-thin ( < 0.1 /zm) semiconductor films [9,11 - 13]. Analysis of secondary grain growth in thicker films (namely, 0.5 /xm thick films) is of prime interest for practical applications: these are precisely the films which are required for microelec- tronic technologies. Application of a two-dimen- sional model for secondary grain growth in thick polysilicon films is not straightforward because transmission electron microscopy investigations [14,15] show: (1) normal grains do not all have a size equal to the film thickness up to 1200°C (i.e. the actual film structure is not columnar); (2) grains differ in defect structure and hence the driving force for secondary grain growth that is caused by the difference in dislocation density in the normal and secondary grains, must be taken into account. Ac- cordingly, there is a need to further examine the model [5] as applied to thick ( ~ 0.5 /~m) polysilicon films.

The aim of this paper is to analyse secondary grain formation, growth and defect structure in un- doped and phosphorus-doped 0.5 /xm thick polysili- con films by transmission electron microscopy (TEM).

2. Experimental procedure

Polysilicon films were prepared by low-pressure chemical vapour deposition from a silane/argon mixture. Films were deposited on thermally oxidized (0.1 /zm oxide thickness)(100) single-crystal silicon wafers. The deposition temperature was equal 630°C. The film thickness was 0.5/xm. Samples were doped with phosphorus by thermodiffusion and also under deposition (in situ). Doping concentration was 102t cm ~. Polysilicon films were annealed in a nitrogen atmosphere at a temperature ranging from 900 to 1200°C from 30 rain to 3 h.

The microstructure of polysilicon films was char- acterized by TEM in both cross-sectional and planar directions. Specimens were prepared by chemical etching and ion milling. Any grain larger than twice the film thickness was considered to be a secondary grain.

3. Results and discussion

Fig. 1 shows the normalized grain size distribu- tion in phosphorus-doped polysilicon films both as deposited and annealed at various temperatures. Grain size distribution for each sample has been normal- ized by dividing the amount of grains of a particular size Np by the total amount of grains N t. The grain size distribution and TEM studies show that sec- ondary grain growth is absent in undoped polysilicon films with a fibrous structure up to annealing tem- peratures of 1200°C for 3 h (the fibrous structure of polysilicon films is considered in Ref. [14] in detail). As deposition phosphorus-doped polysilicon films, however, exhibit a homogeneous fine-grained struc- ture with an average grain size of about 0.05 /zm (Fig. l a and Fig. 2a), that is much less than the film thickness (0,5 /zm). Annealing at 900°C for 30 rain causes an increase in the average grain size to 0.17 ~m but secondary grains are absent (see Fig. Ib). At

0.6

0.4

0.2

0.6

0.~

0.2

0.6

0.%

~ 0.2

O.G

0.4

0.2

0.6

0.~

0.2

b

L

d

a . . l " 7 . . , . . r d b - ~ . L . . . . . . . . . .

0.5 I.O 1.5 2.0 2.5

Fig. 1. Normalized grain size distribution in phosphorus-doped, 0.5 p~m polysilicon films as deposited (a), and annealed at: (b) 900°C; (c) 1040°C; (d) 1120°C; (e) 1200°C for 30 rain.

52 N. G. Nakhodkin, T. V. Rodionot,a / Journal of Crystal Growth 171 (1997) 50-55

Fig. 2. Plan-view TEM micrographs of phosphorus-doped polysilicon films: (a) as deposited; annealed at 1150°C for (b) 15 min, (c) 30 rain and (d) 3 h. Marker represents 0.2 /xm.

an annealing temperature of 1040°C, for 30 min in parallel with the normal grain growth (average size of normal grains increases to 0.52 /zm) secondary grain growth is observed in phosphorus-doped polysilicon films. Average size of secondary grains is equal to 1.1 /~m.

As may be seen from Figs. l c - le , as annealing temperature increases, grain size distribution be- comes bimodal. At a temperature of 1200°C for 30 min secondary grain size reaches 3.0/zm, which is 6 times larger than the film thickness. Examination by cross-sectional TEM clearly shows (Fig. 3) that sec- ondary grain growth in phosphorus-doped polysili- con films has begun before a columnar grain struc- ture is achieved, which does not agree with the theoretical model [5].

TEM studies show that in parallel with the in- crease in secondary grain size with increasing an- nealing temperature there also has been a rise in their number. The annealing temperature dependence of

the number of secondary grains per unit area N, obtained by TEM, is

N=Noexp(-QN),--~ (1)

were QN is an activation energy for secondary grain formation; k is the Boltzmann constant; T is anneal-

- $ t

oo)

Fig. 3. Cross-sectional TEM micrograph of a phosphorus-doped polysilicon film annealed at > IO00°C. Marker represents 0.2 /zm.

N.G. Nakhadkin T.V. Rodionol,a / Journal of Crystal Growth 171 (1997) 50-55 53

I

o

IO 9 8 7 6

5

4

I ' I ' I

k t \ @

\

6.5

\ \

, I J I i I ,

7.o 7.5 ~.o

z/• (zo -4 ~-z)

Fig. 4. Annealing temperature dependence of the number of secondary grains per unit area N for phosphorus-doped polysili- con films.

ing temperature; N O is a temperature-independent constant. By plotting lnN versus I/T (Fig. 4) Qy is equal ~ 1.0 eV. This value is near the migration energy of vacancies in grain boundaries in phospho- rus-doped Si (0.6 eV [6]), suggesting that formation of secondary grains is a diffusive process. This conclusion is supported by the absence of secondary grain growth in undoped polysilicon films, resulting from poor concentration of vacancies (including charged vacancies). It is well known that phosphorus doping enhances the diffusive process by increasing the number of charged vacancies and therefore the total number of vacancies [4,6].

In order to determine the activation energy for secondary grain growth Qr, secondary grain sizes were measured after annealing at temperatures for which secondary grains do not impinge (1040- 1200°C). By approximating the temperature depen- dence of the average grain size r by the expression

(r 0 is a temperature-independent constant) and using the data of Fig. 5, we obtain Qr= 0.6 eV. The obtained value of Qr is close to the value 1 eV [16], derived in a similar way. The value of Qr is some- what lower than previously reported (3.3 eV [9], 3.4 eV [12]), obtained by using the two-dimensional Johnson-Mehl-Avrami analysis. It has been sug- gested that this distinction is determined by the fact that values of activation energy obtained from a two-dimensional analysis represent the sum of the activation energies for both formation and growth of secondary grains (i.e. Qy + Qr ). Furthermore, in re- ports [9,12] we are dealing with thinner polysilicon films, lesser doping concentration (102o cm -3) and other means of doping (ion implantation). Thus, comparison and interpretation of different values for the activation energy are complex.

TEM studies show (Fig. 2b) that for a short annealing time in contrast to the normal grains, the secondary grains are characterized by irregular grain boundaries, complex grain configuration and a high dislocation density. This structure of secondary grains may be explained in terms of a growth mechanism involving coarsening of neighbouring normal grains, i.e. coalescence [17]. In this case, grain boundary migration is controlled by grain boundary diffusion. The most probable atomic mechanism of coalescence

2.0 1.9

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1.6

1.5

1.4

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1.2

I.I

' I ~ I I

~A \ A \

A

A \

I , I i I ,

6.5 7.o 7.5 8.o

I/~ (zo -~ K -I)

Fig. 5. Annealing temperature dependence of the average size r for secondary grain growth in phosphorus-doped polysilicon films.

54 N.G. Nakhodkin, T. V. RodionoL,a / Journal qf Cr~'stal Growth 171 (1997) 50-55

(grain coarsening) is the climb of grain boundary dislocations and the ensuing material transfer is car- ried out by diffusion of atoms or vacancies to the dislocation lines. A great number of dislocations remain at the site of a consumed grain [17]. In addition, some secondary grains contain twins and staking faults on the (l 1 l) planes (Fig. 2c). As can be seen from Fig. 2d, dislocations practically disap- pear with an increasing annealing time up to 3 h. Thus, secondary grain structure drastically changes as both annealing temperature and time increase and it may be suggested that this process is accompanied by a change of driving force for secondary grain growth.

The total driving force can be expressed by sum of components [ 14,15],

F= FT + Fg, (3)

where F T is due to surface energy anisotropy and grain boundary energy; /78 is caused by different dislocation density in adjacent grains. The first com- ponent was considered in detail [5] and the expres- sion for the F T was obtained as

- 2 A T - Y~b FT h ' (4)

where Ay is the difference between the average surface energy of normal grains and the average surface energy of secondary grains, assumed to be the minimum surface energy; ~gb is the grain bound- ary energy per unit area; h is the film thickness. Using the values Ay = 0.5 J / m 2, Y~b = 0.3 J / m 2 [5] and h = 0.5 /~m, we estimate F v = 2.6 × 106 J / m 3.

As discussed above, different dislocation densities in neighbouring grains cause the various interior stresses in grains and leads to the rise of the driving force component Fg. The value of Fg can be approx- imately estimated through the relation [14,15]

Fg = Gb2Ap, (5)

where G is the shear modulus; b is the Burgers dislocation vector (for silicon G = 75.5 GPa, b = 3.84 × l0 -~° m [18]); Ap is the difference in dislo- cation densities in different grains. Estimations show that Ap = 10 j5 m 2 for an annealing time of 15 rain and Ap ~ 1014 m -2 for 30 min (Figs. 2b and 2c) and values of Fg respectively equal to ~ 107 and 106j/m 3. Hence, for a short annealing time the

value of F~ exceeds the value of Fv. In this situation film thickness plays no part in the secondary grain growth because Fg is independent of film thickness.

Since dislocation density drastically decreases with increasing annealing time it may be suggested that the driving force component F~ plays a domi- nant role in the initial formation of secondary grains and F v is dominant in the subsequent stages of secondary grain growth. Thus a theoretical model [5] is appropriate for describing the secondary grain growth in more recent stages for an appreciable length of time.

4. C o n c l u s i o n

The following features of secondary grain growth in phosphorus-doped 0.5 /~m thick polysilicon films were established:

(1) The number of secondary grains increases with increasing annealing temperature; the activation energy for secondary grain formation is equal ~ 1.0 eV; the activation energy for secondary grain growth is equal ~ 0.6 eV;

(2) For short annealing times the driving force for secondary grain growth is caused by the difference in dislocation density in the normal and secondary grains;

(3) For the subsequent growth stages, the driving force is due to surface energy anisotropy;

(4) Secondary grain growth is observed in the absence of columnar grain morphology.

In undoped 0.5 /zm thick polysilicon films no secondary grains are observed up to 1200°C because diffusive processes are retarded due to deficiency of vacancy concentration.

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