Study of SiO2Si interfaces by photocurrent measurements

]OURNAI, OF

ELSEVIER

Journal of Non-CrystallineSolids 216 (1997) 88-94

Study of SiO2-Si interfaces by photocurrent measurements M.L. Polignano *, G. Ferroni, A. Sabbadini, G. Valentini, G. Queirolo SGS-Thomson Microelectronics, Via Olivetti 2, 20041 Agrate Brianza, Milan. Italy

Abstract An experimental study is presented about surface recombination velocity obtained from photocurrent measurements by the Elymat technique. Surface recombination velocity has been studied as a function of process conditions (oxidation cycle, hydrogen annealing, cleaning procedures) with the aim to investigate how this parameter can be used for determining some properties of oxide-silicon interfaces. The data of surface recombination obtained from photocurrent measurements have the expected dependence on the oxidation cycle and on a H2/N z post-oxidation annealing, so we concluded that these data actually reflect the status of the oxide-silicon interface. In addition, these data are shown to be affected by the cleanliness of the oxidation equipment, and they are unaffected by bulk properties of the oxide. By using these measurements, we show that N20 treated interfaces have a better stability under baking treatments than N 2 annealed interfaces. The dependence of surface recombination velocity on injection level can be modeled by analogy with bulk recombination by Shockley-ReadHall recombination statistics. © 1997 Elsevier Science B.V.

1. Introduction Measurements of minority carrier recombination lifetime are sensitive to surface conditions, and often require an effective passivation of surface states to achieve a good sensitivity (i.e., to measure long bulk lifetimes) [1,2]. However, if a method can be found for separating bulkand surface recombination, this feature can be exploited for a description of both bulk and surface properties. Indeed, wafer-level techniques for measuring bulk recombination lifetime are available and well-established [2-5]. These techniques are fast, require little sample preparation, and can easily provide an image of the lateral distribution of bulk contaminants. For these reasons, there is a

* Corresponding author. Fax: + 39-39 603 5233; e-mail: [email protected].

considerable interest in the extension of these techniques to the study of surface properties [6-9]. For instance, photocurrent measurements have been commonly used for a few years for the study of bulk recombination in the Elymat technique (electrolytic metal tracer [5]). This technique can be modified by measuring photocurrent under different conditions of surface passivation, thus allowing surface recombination to be evaluated along with bulk lifetime [10]. It will be shown that this method has a good sensitivity (surface recombination velocities of the order of a few 10 c m / s can be measured), and can provide detailed maps of surface recombination as well as of cartier lifetime. However, at present no experimental results are available linking these data of surface recombination to commonly used parameters for the description of oxide-silicon interfaces (e.g., surface state density). In this paper we present an experimental study of surface recombination vs. process conditions (oxida-

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M.L. Polignano et aL / Journal of Non-C~staUine Solids 216 (1997) 88-94

tion cycle, cleaning procedures, hydrogen annealing) with the aim to investigate how this technique can be used for the description of oxide-silicon interfaces.

2. Experimental details 2.1. Measurement and evaluation procedure

89

where S is the reduced surface recombination velocity, S = sLeaff/D, a is the absorption coefficient of the incident light, d is the wafer thickness and I o is the injected current when the wafer frontside is not immersed in the HF solution. (2) The oxide is etched off and the measurement is repeated with HF passivation, which is assumed to yield a surface with 0 surface recombination. Under this hypothesis, Eq. (1) reduces to

In the Elymat technique [5] carriers are injected by a laser beam at wafer frontside and are collected in the space charge region of a Schottky contact located at wafer backside or at wafer frontside. Carrier diffusion length, Ldi ff (Ldiff = DC-D-'~,where D is the carrier diffusion coefficient and ~- the is carder lifetime), is extracted from backside photocurrent (IBpc) measurements. Frontside photocurrent (IFpc) measurements are also possible, but they will not be used in this work for measuring carrier diffusion length. The frontside photocurrent is used here as a direct measurement of the total injected current (this identification is correct if diffusion length is large enough). In this work a laser with a 820 nm wavelength was used. The Schottky contact for charge collection is provided by a suitable solution, usually diluted HF, which also has the purpose of suppressing surface recombination [ 1]. In the limit of a negligible surface recombination velocity, the collected photocurrent depends on bulk recombination only. If surface recombination is not suppressed by the HF solution, but determined by an oxide-silicon interface, the collected photocurrent, IBPO depends both on bulk lifetime and on surface recombination velocity, s. Therefore, a modification of the standard Elymat technique allows surface recombination at an oxide-silicon interface to be evaluated from subsequent photocurrent measurements. The evaluation procedure is as follows. (1) Photocurrent is measured with the frontside surface of the sample covered by an oxide. Obviously, in this measurement wafer frontside is not immersed in the HF solution. Under this condition, from the solution of the diffusion equation the backside photocurrent is

where I 0 is the injected current when wafer frontside is immersed in HF, and is obtained by a measurement of lwc. This is the standard Elymat measurement which is generally used to evaluate carrier diffusion length. From Eqs. (1) and (2) surface recombination velocity at the original oxide-silicon interface can also be calculated. For a correct evaluation of surface recombination, it must be taken into account that I o can be different from I o because of the different reflection properties of the wafer surface/solution and wafer surface/air interfaces. Since only I 0 can be directly obtained by a measurement of IFpc, a specific experiment was set up in order to accurately measure the ratio I o / I o. Relatively thick oxides (20 nm) were grown, the oxide at wafer backside was stripped and the backside photocurrent was measured with the frontside surface exposed to air (measurement of IBpc)- Then, samples were immersed in the HF solution and IBPC was measured immediately, before the oxide could be etched by the HF solution. In so doing the injected current was changed from I0" to I 0, but s remained unchanged, since it was still determined by the original silicon-oxide interface. So, according to Eq. (1) the ratio of these measurements yields the ratio ~ = I o / I o. By this method we obtained 7/= 0.90 ___0.03. This datum is used in elaborating data of the following experiments.

l -1- S / ( O~Ldiff) IB*pc = IO cosh(d/Ldiff ) + S s i n h ( d / L d i f f )

If a good sensitivity is required for surface recombination, bulk recombination must obviously be kept as small as possible. Therefore, wafers with a large

,

(1)

1 IBPC = I0 cosh(d/tdiff) '

(2)

2.2. Sample preparation

90

M.L. Polignano et al. / Journal of Non-Crystalline Solids 216 (1997) 88-94

lifetime (small contamination level and no or very little oxygen precipitation) must be used. For this reason, magnetic Czochralski wafers were chosen for these tests. P-type, 10 ~ cm resistivity wafers were used. Prior to oxidation samples were cleaned by a commonly used cleaning sequence (H2SOn/H202 + H 2 0 / H z O 2 / N H 4 O H + H z O / H 2 0 2 / H C 1 ) . Various oxidation cycles have been compared: A dry, high temperature annealed oxide: 950°C, dry 02, with a 1000°C annealing in N 2 or N20. A wet, low temperature annealed oxide: 770°C, wet 02, with a 900°C annealing in N 2 o r N 2 0 . A deposited oxide followed by a thermal oxidation: a high temperature ( = 800°C) deposited oxide (HTO), further oxidized at 770°C in wet O 2, and annealed at 900°C in N 2 o r treated at 900°C in N20. The HTO layer is deposited from SiH2C12 and N20. In all samples the final thickness was 12 nm. The impact of a forming gas annealing (430°C, N 2 + 5 % H 2) w a s also investigated.

(a): Bulk diffusion length 500pm [] [] [] [] [] [] []

[] [] []

950#m (b): Surface recombination velocity 7" 10 4 c m / s [] [] [] [] [] [] []

3. Experimental results In order to determine if the measurement procedure in the previous section is actually able to discriminate between bulk and surface recombination, it is useful to compare wafer maps of bulk diffusion length and surface recombination velocity (Fig. 1). It is immediately recognized that these maps have completely different geometries, confirming that the quantities obtained by this method are really independent of each other. Since noise-free maps of surface recombination are obtained down to s = 20 c m / s , the sensitivity of this technique is estimated to be of the order of 10 cm/s. So, from the point of view of sensitivity, the present method, though destructive, is better than the non-destructive approach proposed in Ref. [6]. 3.1. Surface recombination vs. oxidation cycle Figs. 2 - 4 show distributions of surface recombination velocity for different oxidation cycles. Oxidized wafer samples and wafer samples receiving a post-oxidation annealing in an H 2 / N 2 environment are compared. As expected, in all cases there is an

[] [] []

6- 103 cm/s Fig. 1. Maps of diffusion length (a) and of surface recombination velocity (b) in a wafer oxidized in wet 02 at 770°C and annealed in N 2 at 900°C.

effect of the annealing in forming gas, which reduces surface recombination velocity. Besides, these data are also sensitive to the oxidation cycle. The dry 02 grown oxide annealed at the high temperature in N 2 has an interface with a moderate surface recombination even in the absence of a hydrogen annealing, and this fact is somewhat expected according to well-known data [11]. These results confirm that these surface recombination data are actually related to surface states at the oxide-silicon interface. A less expected result concerns dry 02 grown oxides with N20 treated interfaces. In the absence of the hydrogen annealing, these samples have higher surface recombination velocities than samples grown under the same conditions and N 2 annealed (com-

M.L. Polignano et al. / Journal of Non-Crystalline Solids 216 (1997) 88-94

detection 0.7 ~ l i i oxidation:950 C,dry02

detection

1.0 o x i d a t i o n : '950 C, d r y 02 + 1000 C, Nz 0.8 430C Ha annealing

__

0

~O~0) 0.4~0"6 __]~Iii __ no Ha annealing ~-~ 0.2

91

~D

r--Ib I

0.6

+ 1000 C, NsO

0.5

_ _ 4 3 0 C He annealing _ _no H~ annealing

0.4

0.2

r--

I 0.1

_l

L_l----q_

I

I _

o.o 10 °

L

I 101

.

l 10 a

.

.

.

II 10 a

I 10 4

o.o

j I

i0°

10 5

101

I

i 0a

I

i

I

i0a

104

s (em/s) Fig. 2. Distributions of surface recombination velocity for dry 0 2 grown, high temperature annealed oxides.

pare Figs. 2 and 3). However, the surface recombination of N20 treated interfaces is completely suppressed by a hydrogen annealing, irrespective of the oxidation cycle. The stability of the hydrogen passivation is discussed in the following section. Table 1 collects the results of surface recombination velocity for various oxidation cycles. It is worth noting that these samples had approximately the same bulk diffusion length, of the order of 1 mm. On the contrary, surface recombination velocity varies from a few tens of c m / s to about 10 4 c m / s , depending on the oxidation cycle and on the forming gas annealing.

Fig. 3. Distributions of surface recombination velocity for dry O 2 grown, high temperature N20 treated oxides.

Data in Table 1 also show that the oxidation of HTO layers approximately yields the same value of surface recombination velocity as a fully thermal oxide sample provided the temperature and the environment of the oxidation and of the annealing are same. Since the bulk properties of the HTO layer are different from those of a thermal oxide sample [12], this fact confirms that (as expected) data of surface recombination velocity are affected by interface properties only, while they are roughly independent of bulk properties of the oxide.

Table 1 Surface recombination velocity for various oxidation cycles. The impact of a forming gas annealing is also shown Oxidation cycle and annealing 950°C, dry 02, + N 2, 1000°C

Hydrogen

s (cm/s)

no

100-200 0-30 100-1000 0-50 1.2 × 10 -4 =10 5 × 10-3-1.4 × 10 -4 0-50 5 . 3 + 2 . 3 × 103 49 + 34 7.6 + 3.2 × 103 =10

yes 950°C, dry O 2, + N 2 0 , 1000°C

no

yes 770°C, wet 02 , + N 2, 900°C

no

yes 770°C, wet O 2, + N 2 0 , 900°C

no

yes HTO, 7 nm, 770°C, wet 02, + N 2, 900°C

no

yes HTO, 7 nm, 770°C, wet 02, + N 2 0 , 900°C

10 ~

s (cm/s)

no

yes

M.L. Polignano et al. / Journal of Non-Crystalline Solids 216 (1997) 88-94

92

I

3.2. Stability of the hydrogen passivation

I

I

950 ° d r y 0 z o x i d a t i o n

The stability of the passivation of surface states obtained by forming gas annealing is an important issue, because devices can undergo additional thermal treatments after N 2 / H 2 aluminum alloy (for instance during packaging). By this technique it is easy to investigate this subject. Oxides grown at 950°C in dry 02 environment and N 2 annealed or N20 treated at 1000°C were chosen for this study. After oxidation some samples were annealed in forming gas at 430°C. The backside photocurrent was measured without etching the oxide at the frontside (lffpc measurement) of as-prepared samples and after subsequently baking these samples for 2 h at temperatures in the range 100400°C. N 2 annealed and N20 treated samples were baked together, in order to exclude any random variation of baking temperature and environment. Finally, the oxide was stripped and the backside photocurrent was measured with HF passivation, in order to allow the calculation of surface recombination velocity and bulk lifetime. One week was spent between each baking treatment and the subsequent measurement, in order to avoid bulk-related phenomena (splitting of the iron-boron pair) that might confuse our data. In any case, these samples had small bulk contamination (very long bulk lifetime, of the order of 300 p~s).

detection 0.8

I

I

I

oxidation: 770C, wet 0 a + 900C, Nz0 _ _ 4 3 0 C H2 a n n e x i n g 0.6 - _ _no Hz annealing

im

I I I

g 0.4

300 -_._ 1000oc ' N2 ~,_ 1000°C, Nz0

"~" 200

Nz/H z a n n e a l i n g

(,q

100

/

t--~---~.---

o

0

I

I

I

100

200

300

400

Tb.~g (°C) Fig. 5. Surface recombination velocity vs. baking temperature in N 2 annealed and N20 treated interfaces. Lines are drawn as guides for the eye.

It was found (Fig. 5) that the hydrogen passivation is stable to 300°C, while at 400°C some degradation of the oxide-silicon interface is observed. It is interesting to note that N20 treated interfaces have a better stability than N 2 annealed interfaces, since the degradation is less pronounced in N20 treated sampies.

3.3. Oxidation carrier cleaning As an application of this technique, we investigated the impact of a cleaning procedure of oxidation carriers over surface recombination and bulk lifetime. SiC, polysilicon-coated carriers were cleaned by silicon etching and a subsequent silicon deposition. Wafer samples were oxidized in a single oxidation cycle by using both cleaned and uncleaned careers. In this test the oxidation process was 10 min at 1000°C in a dry 02 environment. The results of this test are shown in Table 2 and show that the cleaning procedure had no impact on

~D

0.2

0.0

I 10 °

10 a

10 2

I

I

10 a

104

Table 2 The impact of a cleaning procedure of oxidation carriers on bulk diffusion length and surface recombination velocity 105

s (ore/s) Fig. 4. Distributions of surface recombination velocity for wet O 2 grown, low temperature N20 treated oxides.

Carrier cleaning

Ldiff (p,m)

s (cm/s)

Yes No

778:1:111 785 + 48

0-80 200-2 × 104

M.L Polignano et al. / Journal of Non-Crystalline Solids 216 (1997) 88-94

the bulk lifetime. On the contrary, surface recombination velocity was affected by the cleaning procedure. Wafer samples oxidized in clean carders had much smaller surface recombination velocity than wafer samples in not cleaned carriers. It is not possible at the moment to identify the reason for this difference, which is sensitive and is definitely related to the cleaning procedure, since wafer samples were processed together.

4. Discussion

i0 s

93

I

[

,-.., 10 4

n o Hz/N 2 a n n e a l i n g

o m

i0 s

o 9 5 0 C, d r y 02 + 1000 C, N20 I0"

I 10 -2

I 10 -I injection

In the Elymat technique the injection level (defined as ~n/p o, where ~n is the concentration of injected minority carders and P0 is the equilibrium majority carrier density) can be varied by varying the intensity of the laser beam. It was previously shown [13] that the dependence of bulk recombination lifetime upon injection level can be correctly predicted by Shockley-Read-Hall recombination statistics by using literature data of energy levels and carrier cross-sections. Here surface recombination is also studied as a function of the injection level, and an attempt is made to calculate the expected s(~n/p o) curve. Surface recombination is written as a function of the injection level by analogy with bulk recombination, and surface recombination velocity is obtained from an integration over the energy gap. The injection level at the surface is easily calculated from the solution of the diffusion equation as ~n(x = O)/p o. The dependence of the injection level on surface recombination velocity and bulk lifetime has been taken into account. Surface state density, N~, is used as a fitting parameter. For simplicity, a few assumptions have been made. First, a typical distribution of surface states [14] and carrier cross-sections from the literature [15] have been assumed. Since both the distribution of surface states and the carrier cross-sections are known to depend on details of the oxidation cycle, the results of these calculations may only give a qualitative indication of the behavior to be expected. A similar approach to Ref. [16] was followed, i.e., each trap was assumed to act independently of the others and flat-band conditions have been assumed. This latter assumption can only hold if the injection level sup-

I

770C, w e t 03 + 900C, N20

I 100 level

Fig. 6. Surface recombination velocity vs. injection level. Lines are best-fit curves with Nss as the fitting parameter.

presses band bending, so our calculations are expected to fail at small injection levels. Fig. 6 compares calculated curves and experimental data for a sample with a moderate surface recombination velocity ( 1 0 0 < s ( c m / s ) < 1000) and a sample with a high surface recombination velocity (s = 104 cm/s). It is worth noting that the curves in Fig. 4 were obtained in the same range of injected currents, but because of the large difference in surface recombination velocity the range of the injection level is different in these measurements. The trend of s(Sn/p o) data is correctly predicted by calculation, however there is a distinct difference between the examples under consideration. In the sample with a moderate value of s (100 to 1000 cm/s), the injection level is always rather large and calculated curves agree with experimental data. In the sample with a large combination velocity, the injection level is less and the assumption of flat-band conditions may fail, thus producing some discrepancy between calculated curves and experimental data.

5. Conclusions The results of this study show that surface recombination velocity, as obtained by photocurrent measurements, is a useful parameter for the description of the oxide-silicon interface. This parameter has the expected dependence on the oxidation and annealing temperature and environment and on a post-

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M.L. Polignano et al. / Journal of Non-C~stalline Solids 216 (1997) 88-94

oxidation forming gas annealing, so we conclude that surface recombination velocity is affected by the state of the oxide-silicon interface and is unaffected by bulk properties of the oxide. By using these measurements we show that N 2 0 treated interfaces have a better stability under baking treatments than N 2 annealed interfaces. In addition, surface recombination velocity is found to be affected by the cleanliness of the oxidation equipment, so this parameter could be used for monitoring the quality of the oxidation process. The dependence of surface recombination velocity on injection level can be modeled by analogy with bulk recombination by using S h o c k l e y - R e a d - H a l l recombination statistics. Available data at the moment do not allow us to build a microscopic model of the S i - S i O z interface and of its evolutions upon the treatments under consideration. This failure is partially due to the fact that surface recombination velocity comes from an integral over the energy gap, so that specific contributions to this quantity cannot be easily resolved in these experiments.

Acknowledgements This work was partially supported by the JESSI project E66 'Silicon wafers for submicron technology'.

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