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Charge carrier lifetime degradation in Cz silicon through the formation of a boron-rich layer

during BBr3 diffusion processes

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2010 Semicond. Sci. Technol. 25 055001

(http://iopscience.iop.org/0268-1242/25/5/055001)

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IOP PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY

Semicond. Sci. Technol. 25 (2010) 055001 (9pp) doi:10.1088/0268-1242/25/5/055001

Charge carrier lifetime degradation in Czsilicon through the formation of aboron-rich layer during BBr3 diffusionprocessesMichael Andreas Kessler1, Tobias Ohrdes1, Bettina Wolpensinger1

and Nils-Peter Harder1,2

1 Institute for Solar Energy Research, Hameln/Emmerthal (ISFH), Germany2 Institute of Electronic Materials and Devices, University of Hanover, Germany

E-mail: kessler@isfh.de

Received 16 December 2009, in final form 18 December 2009Published 6 April 2010Online at stacks.iop.org/SST/25/055001

AbstractBoron diffusion is commonly associated with the formation of an undesirable boron-rich layer(BRL), which is often made responsible for degradation of the carrier lifetime in the bulk. Weinvestigate the phenomenology of the BRL formation, which results from BBr3 borondiffusion processes, and its impact on sheet resistance and bulk lifetime. Our measurementsshow that boron silicate glass (BSG) and BRL thicknesses vary between 50 and 600 nm and 0and 80 nm respectively within the two-dimensional wafer surface of one sample for onediffusion process. Both thicknesses strongly depend on the gas composition duringcomposition and deposition time. Further results show that BRL formation is favored by highconcentrations of BBr3 vapor and of oxygen during B2O3 deposition. Also, high drive-intemperatures promote the growth of the BRL. We find that a BRL of more than 10 nmthickness causes a degradation of the carrier lifetime in the bulk of the silicon wafer. Inparticular, we show that this bulk lifetime degradation occurs during the cool-down ramp afterthe diffusion process. We show that carrier lifetime degradation can be avoided either bylimiting the process temperature to 850 ◦C and thus preventing BRL formation or throughreconverting the BRL by a drive-in step in oxidizing atmosphere at 920 ◦C.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Open-tube furnace boron diffusion is conceivable forproducing p+-type regions in large-scale industrial productionof high-efficiency silicon solar cells. However, this processis often considered to be technologically challenging. Inparticular, the open-tube boron diffusion process is commonlyassociated with the formation of an undesirable boron-richlayer (BRL), which is often made responsible for degradationof the carrier lifetime in the bulk. These perceptions contributeto the fact that BBr3 boron diffusion processes play a negligiblerole in today’s industrial solar cell production.

1.1. Lifetime degradation at boron diffusion processes

The formation of a BRL generally results in uniform sheetresistance, as a virtually unlimited amount of elemental boronis present at the Si surface [1]. Under certain diffusionconditions e.g. for ‘heavy boron diffusions’, the diffusionprocess can generate defects within the bulk region asinvestigated by Cousins and Cotter [2]. They found boronmisfit dislocations on edges of monitor wafers diffused under2.5% oxygen at 1060 ◦C and on the entire surface of samplesdiffused without oxygen flux during drive-in. They concludedthat the dislocation network arises due to heavy doping(high surface concentration and junctions depth). Lifetime

0268-1242/10/055001+09$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA

Semicond. Sci. Technol. 25 (2010) 055001 M A Kessler et al

measurements have been performed, but not locally dissolved,and show a correlation between the arising dislocation networkand an asymmetric SRH recombination center resulting inbulk lifetime degradation. Prussin [3] reported about thegeneration of dislocations due to boron doping within theemitter region. Ning [4] demonstrated that dislocations formon the diffusion surface and can glide past the diffusionjunction into the bulk region [2]. All authors concluded thatthe tension within the diffused layer increases with increasingamount of dopants and therefore is a function of junction depthand surface concentration. However, an in-depth descriptionof a correlation between BRL formation and bulk lifetimedegradation has not been described. Moreover, we find thatthere is a lack of the literature that discusses in detail thetechnological challenges associated with the preservation ofthe charge carrier lifetime during boron diffusion.

In this paper we therefore investigate

• the BSG and BRL thicknesses;• lateral distributions of sheet resistance and their

correlation with the BSG and BRL thicknesses;• lateral distributions of the bulk carrier lifetime in boron-

diffused industrial-sized wafers and• a correlation of the bulk carrier lifetime in boron-diffused

wafers with BRL thicknesses.

1.2. The BBr3 liquid source boron diffusion process

The BBr3-based open-tube furnace boron diffusion processcan be divided into two sequences: deposition and drive-in. During deposition a gas mixture that is initially at roomtemperature enters the heated furnace, where a batch of siliconwafers is located. The gas mixture is composed of oxygen,nitrogen and boron tribromide (BBr3) vapors. Cousins [5]gives a precise survey of all relevant reactions during borondiffusion.

Upon entering the furnace, the gas mixture heats up andBBr3 and oxygen react according to the following chemicalprocess [6]:

4 BBr3(g) + 3 O2(g) → 2 B2O3(l) + 6 Br2(g). (1)

The liquid B2O3 condenses on the silicon wafer surface (andthe furnace walls). The chemical reaction with the siliconsurface produces SiO2 and elemental boron according toreaction (2):

2 B2O3(l) + 3 Si(s) → 4 B(s) + 3 SiO2(s). (2)

The SiO2 is partly dissolved in the liquid B2O3, resulting ina mixed-phase B2O3–SiO2 system (BSG). Mainly during thesecond step of the diffusion process sequence, during the so-called drive-in, the elemental boron diffuses according to theboron concentration gradient into the silicon wafer as wellas into the BSG system. During this process step a veryhigh concentration of boron (‘boron pile up’) can develop atthe surface of the wafer. The high concentration of boroneffectively transforms a surface layer of the silicon wafer intoa Si–B compound (BRL), according to the reaction [7–9]

Si + 6 B → SiB6. (3)

The composition of this Si–B compound was investigated byvarious authors. Some suggest different compositions likeSiB4 [1] or SiB5 [7], but most authors [1, 6] suggest thiscompound being of SiB6 composition, which also agrees witha rather recent investigation by Aselage [10]. We investigatesystematically how the process conditions affect the formationof the BRL.

BRL formation could be avoided by directly convertingthe excess elemental boron atoms. For example, the drive-insequence can be done under an oxygen-containing atmosphere,making certain that the excess boron atoms react with theoxygen molecules according to the following reaction [1]:

3 O2 + 4 B → 2 B2O3. (4)

The O2 concentration at the Si–BSG interface then dependson the diffusion of oxygen to the Si interface and is a crucialparameter that influences the formation of the undesirableBRL. Deal and Sklar [11] described the temperaturedependence of O2 diffusion through SiO2 as a function ofthe boron concentration within the SiO2. They showedthat O2 diffusion through SiO2 is enhanced by increasedboron concentration within the SiO2 as well as by increasedtemperatures.

Alternatively, the BRL may be oxidized after its formationby a so-called low-temperature oxidation (LTO) step atrelatively low temperatures below 750 ◦C. This LTO istypically performed after the diffusion process and the wet-chemical removal of the BSG. Another possibility is toreconvert the BRL during the tube furnace process by an‘in situ’ oxidation [12]. Both processes, the LTO and the‘in situ’ oxidation, oxidize the SiB6 layer by molecular oxygen:

2 SiB6 + 11 O2 → 6 B2O3 + 2 SiO2 (5)

2. Experimental setup

For our experiments, we perform different sets of diffusionprocesses in an industrial-type diffusion furnace made by LPTEurope R©. The wafers in the silica boats are in an uprightposition perpendicular to the gas flow through the furnacetube. The geometry as well as the exact sample alignmentand position are displayed in figure 1. Note that the wafersare positioned slightly off-centrically from the center line ofthe furnace tube. The process gases such as O2, N2 and BBr3

vapor enter the diffusion furnace at room temperature.

2.1. Silicon samples and sample configuration within thediffusion furnace

In all diffusion processes, we placed 13 silicon wafers inthe silica boat. The wafers were evenly distributed acrossthe boat with a wafer spacing of 1 cm. We used n-type Cz〈1 0 0〉-oriented 125 × 125 mm full square silicon wafers of195–200 μm thickness. Bulk doping was between 6 and 9 �

cm in the case of our lifetime samples, and 3 and 4 � cm in thecase of the samples that we used for measuring sheet resistanceas well as BSG and BRL thicknesses. For characterizing theresults of our diffusion we use the wafers on positions 6 and 7(see figure 1).

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Semicond. Sci. Technol. 25 (2010) 055001 M A Kessler et al

Figure 1. Industrial diffusion furnace, including the sample configuration and position.

Table 1. Overview of diffusion parameters for experimental series 1with deposition and drive-in parameter variation.

Depositiona Drive-inb

Time O2 flux BBr3 carrier TemperatureDiffusion (min) (L min−1) flux (L min−1) (◦C)

Reference 30 0.4 0.2 9201a 10 0.4 0.2 9201b 60 0.4 0.2 9202a 30 0.04 0.2 9202b 30 1.0 0.2 9203a 30 0.4 0.1 9203b 30 0.4 0.4 9204a 30 0.4 0.2 8504b 30 0.4 0.2 8804c 30 0.4 0.2 9004d 30 0.4 0.2 970

a 850 ◦C, N2 flux: 19.4 L min−1.b 60 min, N2 flux: 10 L min−1.

2.2. Diffusion parameters

Based on a reference diffusion process with a deposition timeof 30 min at 850 ◦C and a drive-in sequence of 60 min at920 ◦C (corresponding gas fluxes are displayed in table 1), westudy two series of diffusions. In the first series (see table 1),we investigate the implications of the drive-in temperature,BBr3 and O2 concentration during deposition and the effect ofdeposition duration.

In the second series (see table 2), we investigatethe implications of an oxidizing atmosphere during thehigh-temperature drive-in step by testing different O2

concentrations and ‘in situ oxidation’ durations. That is, intwo of the diffusion processes with O2 flux during the drive-instep (processes 5b and 6b in table 2), we restrict the durationof the O2 flux to a shorter duration than the duration of thecomplete drive-in step.

All diffusions of series 1 are variations that are basedon the reference diffusion listed in the first column oftable 1. Each process variation differs from this referenceprocess by changing only one process parameter such as gasflow composition during deposition and deposition duration.Diffusions from experimental series 2 vary from the referencediffusion process in terms of an additional oxygen flux duringdrive-in and in terms of an enhanced BBr3 carrier flux duringdeposition (see table 2). The parameters differing from thereference diffusion process are marked with bold letters. Weuse 19.4 L min−1 N2 carrier gas stream during deposition at850 ◦C and 10 L min−1 overall gas flux during drive-in. Thedrive-in of all investigated diffusions was performed for 60 minat 920 ◦C (except diffusion 6b with a prolonged drive-in time

of 90 min, see table 2). The parameters of our referencediffusion yield a boron-doped layer with a sheet resistance ofapproximately 65 � sq.−1. The actual sheet resistances of themodified diffusions are subject to the details of the diffusionprocesses and thus varies upon varying the process parameters.We therefore include measurements of the sheet resistance forcharacterizing our diffusion variations.

2.3. Experimental procedure

Prior to the furnace diffusion process, the silicon wafer sampleshave undergone a saw damage etch in KOH and a standardRCA clean, including the subsequent dip in hydrofluoricacid. The samples were then rinsed in deionized water anddried thoroughly. From each boron diffusion process, twosamples (see figure 1) were taken for analysis. One sampleis first examined in terms of B2O3/SiO2 and BRL thicknessesdirectly after diffusion. Sheet resistance measurements areperformed after HF treatment (10 min in 1% HF), LTO andsubsequent HF treatment (10 min in 1% HF). The othersample got identical HF–LTO–HF treatment. The emitterregion of this sample was then completely (>8 μm) etchedoff by KOH and the surface was passivated with SiNx [1](deposited by PECVD, refractive index: 2.4, thickness: 60–70 nm) after RCA cleaning. The LTO oxidizes the rather thickSiB6. However, the low temperature and short duration of theLTO can oxidize only a very thin layer of less than 2 nm ofcrystalline silicon. Consequently, the LTO does not affect thesheet resistance of the boron-doped surface layer within thecrystalline silicon, which has been penetrated by boron to adepth of approximately 0.2–0.5 μm. Hence, etching off theoxidized BRL in HF hardly removes any crystalline siliconmaterial. Furthermore, the boron diffusion coefficient at thetemperature of 600 ◦C of the LTO is still relatively small, andthus, the additional temperature budget of the LTO process canneither be expected to produce any appreciable re-distributionof boron atoms in the silicon crystal nor any crystal damageswithin the bulk.

We examine the influence of varying diffusion parametersduring the boron diffusion process by measuring the B2O3–SiO2 and BRL thicknesses as well as the sheet resistance asa function of the wafer position. The local existence of aBRL is determined by the hydrophilic behavior of the siliconsurface after HF treatment. Additionally, we perform carrierlifetime mappings on corresponding wafers. We measure theBRL/BSG thickness and sheet resistance along the verticalaxis of symmetry across our samples, as shown in figure 2. Wealso detect the BRL coverage and measure the thickness of theBRL and BSG along this axis of symmetry of the experimental

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Table 2. Overview of diffusion parameters for experimental series 2 where an additional ‘in situ oxidation’ step was performed during thedrive-in sequence.

Depositiona Drive-inb

BBr3 carrier Drive-in Duration of O2 flux N2 fluxDiffusion flux (L min−1) time (min) O2 flux (min) (L min−1) (L min−1)

5a 0.2 60 60 5 55b 0.2 60 15 5 55c 0.2 60 60 1 96a 0.4 60 60 5 56b 0.4 90 30 5 5

a 850 ◦C, 30 min, O2 flux: 0.4 L min−1, N2 flux: 19.4 L min−1.b 920 ◦C, O2 flux always at the end of the drive-in step.

Figure 2. Location of measurements taken on our full-squaredboron-diffused samples. We scan the wafer along this verticalsymmetry line for the BRL and BSG layer thickness, boron sheetresistance and bulk lifetime.

setup. The reduction of the lateral properties to this one-dimensional line scan still represents the lateral distributionsquite well, yet at the same time facilitates the simultaneousgraphical display and comparison of several samples.

2.4. B2O3–SiO2 and BRL thicknesses measurements

We employ two methods for determining the overall thicknessof the B2O3–SiO2/BRL system. In the first method, wecompare the color appearance of the BSG–BRL with a colorchart for determining the thickness of SiO2 layers. Byassuming that the refractive index of the B2O3–SiO2 layeris similar to that of a pure SiO2 layer, we obtain a value for theB2O3–SiO2 layer thickness from the comparison with the SiO2

color chart. The second method is determining the B2O3–SiO2/BRL thickness from secondary electron microscopy(SEM) images of the cleaved edge of the silicon wafers.Figure 3 shows a SEM micrograph of a wafer from thereference diffusion at the position indicated in figure 5.

The two-phase BRL/B2O3–SiO2 layer system can beclearly seen in figure 3. The overall thickness of the BRL/BSGstack is 300 nm and the BRL 90 nm. The visual inspectionand comparison with a SiO2 thickness chart yields a thicknessvalue of 330–350 nm and thus only slightly overestimates thethickness value of the SEM investigation by 10–15%.

Figure 4 shows the results of both BSG thicknessmeasurement methods near the wafer top, plotted as a functionof the position along the diagonal line as shown in figure 2.

The overall BSG–BRL thickness from optical assessmentonly slightly overestimates the layer thickness measured by

Figure 3. Micrograph of the two-phase layer system 20 μm fromthe wafer top of the sample from reference diffusion.

Figure 4. BRL and BSG thicknesses of a sample from diffusion 1b.The BSG thickness values are determined by both SEM and visualinspection and compared to a SiO2 color chart. The BRL thicknessis determined by SEM only.

means of SEM. In all cases the error is less than 20%,and we discover a consistent overestimation through opticalassessment. Therefore, we use the fast and simple opticalassessment for characterizing the overall layer thickness on allother sample wafers. In all cases where we make explicitreference to the BRL thicknesses only (as opposed to thethickness of the BRL/BSG layer system), we report BRLthicknesses as determined by SEM on cleaved sample edges.

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Figure 5. Overall BSG/BRL thickness, on the silicon wafer surfaceplotted as a function of the position.

3. Results and discussion

3.1. Layer thickness measurements

Figure 5 shows the results of the layer thickness measurementsfor the following diffusions:

• diffusion 1a;• diffusion 1b;• reference diffusion.

The details of these diffusions are given in table 1. Thisset of diffusions represents an investigation of the effect ofthe deposition duration. Regardless of the diffusion process,the BSG/BRL thickness shows a pronounced peak at the topcorner of the wafer. This location on the sample is representedin figure 5 by the x-axis value ‘0 cm’, the layer thicknessthen rapidly decreases toward the middle of the wafer, whereit forms a nearly constant plateau for major parts of thetotal line scan. We compared front and rear sides of thewafer and found that both sides show almost identical layerthicknesses. After thickness measurements, we etch thesesamples in hydrofluoric acid and determine the BRL coveragefrom the lateral distribution of hydrophilic properties. Thecorresponding BRL coverage is indicated by a continuous solidline that interconnects the thickness measurement points in thehydrophilic regions. We find that the total layer thickness forextended deposition time increases and furthermore is stronglydependent on the position on the wafer.

The comparison of the diffusions 1a and 1b with ourreference diffusion suggests that the formation of a BRL ispromoted by rather thick BSG deposition and also by thespecific conditions that are found near the edges of a wafer.However, there are also areas on the wafer without BRLformation in spite of rather thick BSG layer coverage ofaround 200 nm (see figure 5). We conclude that the BRLformation process is rather complex and that BSG depositionthickness is only one of the parameters that influence theBRL formation. Busnaina et al [14] performed simulations

Table 3. Averaged measurement data of the total BSG/BRLthickness, BRL coverage, sheet resistance and carrier lifetime for allof the diffusion processes investigated in this paper. (BSG/BRLthickness and Rsheet taken in the center of the wafer, lifetimemeasurements are average values from the entire wafer.)

BSG/BRL BRL Rsheet LifetimeDiffusion thickness (nm)a coverage (%)b (� sq.−1) (μs)

Reference 100 38 66 33451a 60 6 64 30611b 180 76 62 9792a – – – 23262b 120 100 75 10803a 70 12 65 33013b 180 100 64 6484a 90 0 171 25894b 110 6 96 27824c 105 6 74 33944d 100 71 38 18895a 130 0 280 28215b 125 6 99 28375c 130 6 151 28996a 245 0 337 33996b 250 1 95 3114

a Median value representing the plateau thickness.b Estimation with respect to the line scan. Small values of BRLcoverage are completely linked to BRL formation at the wafer top.

of the gas flow patterns and of the temperature distribution fora comparable furnace and wafer configuration. The resultsof these simulations showed a non-axis-symmetric gas flowwithin the furnace and non-uniform wafer heating during theprocess. As this situation should also be expected for ourfurnace, we associate the occurrence of instable gas flowpatterns and temperature variations with the BRL formationnear the wafer edges.

The BSG/BRL thickness profiles of diffusions 2–6 showqualitatively the same characteristics as the curvesdisplayed in figure 5. They show a pronounced peak of theBSG thickness near the uppermost corner of the wafer, a BSGthickness plateau over most part of the sample and a decreasingthickness at the lower corner of the wafer. Table 3 provides alist of the thicknesses of the plateau regions for all diffusions.

BSG thickness profiles from diffusions 4a–d (variation ofdrive-in temperature) do not differ significantly from thicknessprofile obtained from the reference diffusion. However,enhanced O2 flux (diffusion 2b) as well as increased BBR3

vapor flux (diffusion 3b) during the deposition step result inincreased BRL/BSG layer thickness as well as increased BRLformation. The oxygen supply during drive-in results in anincreased BSG layer thickness, as was expectable from thereaction equation (4) and (5). However, we furthermore cansee from table 3 that the oxygen supply during drive-in can beused for suppressing the BRL formation (compare referencediffusion with diffusions 5a–c and diffusion 3b with diffusions6a and b). On the other hand, while the oxygen supply maybe beneficial for avoiding the BRL formation we can see thatthe oxidation consumes such a large fraction of the total boronconcentration that the sheet resistance of the diffusions 5 and6 is markedly higher than that of any of the other diffusions

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Semicond. Sci. Technol. 25 (2010) 055001 M A Kessler et al

Figure 6. Sheet resistance measurements for diffusions 5a–c andthe reference diffusion process.

with a drive-in temperature of 920 ◦C. Interestingly, this effectcannot be reduced by a deposition process with increased BBr3

vapor flux and thus higher BSG layer thickness (comparisonof diffusions 5a and 6a in table 2)

3.2. Sheet resistance measurements

The sheet resistance was measured by a Four Point Probe(Napson Corporation, Model RT-70/RG-7) along the verticalsymmetry line across the wafer (see figure 2). The sheetresistance distribution on wafers of all diffusions are quitehomogeneous (< ±15%), and the corresponding averagevalues are shown in table 3. In particular, we do notobserve any correlation between the sheet resistance andthe BSG/BRL thickness within the range of our processparameters. Diffusion 2b shows an increased sheet resistanceof approximately 15% yet does not feature either a particularlythin or thick BSG/BRL thickness. We associate this decreaseof boron concentration within the silicon with a change ofthe B2O3–SiO2 phase system. The enhanced O2 flux duringthe deposition of diffusion 2 oxidizes part of the elementalboron and reduces the supply of boron to the Si surface.Due to the low oxygen flux during deposition, there is nodeposition of B2O3 on the sample during diffusion 2a andtherefore no emitter is formed during the diffusion process.Sheet resistance values for diffusions 4a–d show the expectedincrease in emitter doping for increasing drive-in temperature.

Figure 6 shows the sheet resistance line scans for three ofthe diffusions with oxygen flux during drive-in (5a,b,c). Forcomparison, the sheet resistance line scan of our referencediffusion is plotted in the same graph. Again, the occurrenceof BRL is indicated by a solid line that interconnects themeasurement points in the regions with BRL coverage.

We can see that oxygen flux during drive-in decreases theboron doping with respect to the reference diffusion. Thesheet resistance distributions of all of these diffusions arequite homogeneous showing a maximal deviation of 16%

from the mean value, except for the uppermost corner ofthe wafers of diffusion 5c. Higher oxygen concentrationas well as a longer duration of the O2 flux resulted in thedesired decrease of BRL formation for all of the three testdiffusions 5a, b and c compared to our reference process(see figure 6). However, only the samples of diffusion 5ahave been completely BRL free, yet it suffers from a ratherstrong loss of boron doping in the diffused surface layer.This loss of boron doping during a drive-in under oxygen-containing atmosphere can be explained by two processes.First of all, the redistribution of boron into the B2O3–SiO2

system at the Si interface. Elemental O2 diffuses throughthe BSG layer and produces an oxygen-rich phase near theSi interface. This gives rise to reactions (4) and (5). Thesolubility of boron is higher within SiO2 than within siliconand consequently causes a redistribution and depletion ofboron in the wafer. Secondly, the boron-doped silicon atthe silicon–BSG interface is oxidized like in any conventionaloxidation process and therefore the boron concentration withinthe silicon decreases. A similar boron depletion effect can beobserved when oxidizing boron-diffused wafers in a separateoxidation process [12, 15].

The drop of sheet resistance at the uppermost corner ofthe wafer of diffusion 5c (see figure 6) is a result of highBSG/BRL thickness near the wafer top (compare figure 5).The high BRL and BSG layer thickness function as a barrierfor oxygen diffusion at this part of the wafer. Consequently, theO2 diffusion to the Si interface is limited and thus the effect ofthe oxidation during drive-in minimized. As a result, the sheetresistance remains at the same low level than without oxygen-containing atmosphere (or with only short oxygen exposure,diffusion 5b).

We find that for very thick BSG/BRL rather short oxygenexposure times during drive-in (diffusion 5b) or only very lowoxygen concentrations during drive-in (diffusion 5c) are notsufficient for avoiding the BRL formation completely. BRLformation was generally avoided or reversed in regions ofoverall BSG/BRL thicknesses less than 200 nm. Shorteningthe deposition time to only 10 min produces BSG/BRLthicknesses less than 150 nm (compare diffusion 1 a, table 3).The goal of BRL avoidance with concurrent stable sheetresistance level was not achieved due to either remainingBRL formation near the wafer top or a drop in boron dopingconcentration indicated by higher sheet resistance values. Acareful control of the O2 concentration and time frame ofoxygen penetration and the avoidance of strongly varying BSGlayer thicknesses in boundary regions on the wafer could leadto proper process parameters.

3.3. Carrier lifetime measurements

Lifetime mappings are performed by means of microwave-detected photoconductance decay mappings (MWPCD,Semilab WT 2000). We verified the material quality prior toprocessing our boron diffusions. We found carrier lifetimes ofmore than 2500 μs for all wafers. Any post-processing lifetimeless than 2500 μs can therefore be regarded as degradation ofthe carrier lifetime during boron diffusion.

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Figure 7. MWPCD lifetime mappings of a wafer diffused in ourreference diffusion (left) and of a wafer of diffusion 1b (right). Thebulk carrier lifetime after the reference diffusion is on average3345 μs, while the bulk carrier lifetime after diffusion 1b is reducedto 979 μs.

We characterize the bulk carrier lifetime of our wafersafter boron diffusion in the following manner. We etch off theboron-diffused surface layer with KOH and passivate thesesurfaces with SiNx [13]. The effective lifetime measured onthese samples is taken as the lower limit of the bulk carrierlifetime after boron diffusion. For simplicity, we will referto these measurements of the effective lifetime of the etchedsamples in the remainder of this paper as ‘bulk carrier lifetime’or simply ‘carrier lifetime’. Figure 7 shows lifetime mappingsof samples of our reference diffusion and of diffusion 1b,which feature a prolonged deposition time. The upper rightcorner in the figure corresponds to the upper corner of thesample within the diffusion furnace (see figures 1 and 2).

Figures 8(a) and (b) display line scans of the carrierlifetime mappings as indicated in figure 7. Additionally,the sheet resistance and the BSG/BRL thickness areshown, with the BRL coverage being indicated by asolid interconnection line between the layer thickness datapoints.

We can see for both diffusions, a homogeneousdistribution of the sheet resistance. The reference diffusionallows maintaining a very high carrier lifetime across almostthe entire sample area. However, we find a degraded lifetimeat the upper corner of the sample, where the BSG/BRL wasthicker than ∼120 nm. As opposed to the reference diffusion,which essentially allowed us to maintain the bulk carrierlifetime, we find for diffusion 1b (prolonged BSG depositiontime) a lifetime degradation down to values of ∼500 μs inlarge parts of the wafer. Nevertheless, the sample of diffusion1b shows a correlation between the absence of BRL formationand high carrier lifetime (∼3000 μs, see figure 8(b)).

We find this correlation between the absence of BRL andpreservation of bulk carrier lifetime for all of the diffusionprocesses. Diffusions 2b and 3b are the diffusions withan increased O2 gas flux and increased BBr3 vapor flux,respectively. The samples of these diffusions were coveredcompletely by a BRL. Nevertheless, qualitatively we find asimilar peak for the lifetime distribution between 2 and 8 cmto that shown for diffusion 1b in figure 8(b) except that themaximum bulk carrier lifetimes of samples of diffusions 2band 3b were lower than that of samples from diffusion 1b.

(a)

(b)

Figure 8. (a) Line scan of the carrier lifetime mapping of figure 7(reference diffusion). (b) Line scan of the carrier lifetime mappingof figure 7 (diffusion 1b).

We find the peak carrier lifetime of the wafer of diffusion 2bto be 2500 μs, and the peak carrier lifetime of the sample ofdiffusion 3b to be 1000 μs.

The wafer of diffusion 1b shows a strong lateral variationof the bulk carrier lifetime, yet the sheet resistance distributionis very homogeneous at 60 � sq.−1. Taking the sheet resistanceas a good qualitative measure for the dopant concentration,we conclude that under the investigated diffusion conditions,the bulk carrier lifetime degradation does not occur due tohigh dopant concentration. Instead, we ascribe the lifetimedegradation to the formation of a BRL.

In order to analyze the degradation process in more detailwe modified diffusion 3b, which showed the lowest averagebulk lifetime, as follows. We perform the drive-in sequenceunder oxidizing atmosphere with an O2 concentration of 50%during the entire drive-in step (diffusion 6a). Secondly, weadd an additional 30 min oxidation step between the drive-in sequence and the cool-down ramp (diffusion 6b), keepingall other diffusion parameters constant in both cases (seetable 1—all three relevant diffusions 3b, 6a and b are shadedfor a better clarity). In both cases, the samples have beenalmost completely BRL free (see BRL coverage in table 3).In the case of diffusion 6a, the BRL either did not form at

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Figure 9. Correlation of MWPCD measured carrier lifetimes versusBRL thickness. SEM images were used for determining the BRLthickness. The lifetime data were taken from wafers which wereplaced on directly adjacent positions during boron diffusion. Thelifetime was measured after etching off the boron-diffused surfacelayer and SiNx passivation of the surfaces. Values next to datapoints indicate the position from wafer top on the sample whereboth measurements are taken.

all during drive-in or is directly reconverted within the drive-in sequence under oxygen. In the case of diffusion 6b, thealready existing BRL after drive-in is reconverted during thesubsequent oxidation step.

By comparing average lifetime values from diffusion3b (648 μs) and diffusion 6a (3399 μs, see table 3), onecan clearly see that by avoiding the formation of the BRL,lifetime remains high. From the high average lifetime valueof diffusion 6b (3114 μs) we conclude that the observedlifetime degradation most probably occurs during the cool-down ramp of the diffusion process if a BRL of a certainthickness is present during the ramp. Diffusions 3b and 6bjust differ by an additional oxidation sequence for 30 mindirectly after the drive-in sequence (‘in situ oxidation’ indiffusion 6b). This oxidation sequence converts most of theBRL of the sample from diffusion 6b, which is assumed tobe already present after the high-temperature drive-in of bothdiffusions 3b and 6b. We propose that the presence of the BRLduring the subsequent cool down cycle (diffusion 3b) resultsin mechanical stress between the SiB6 layer and the siliconsubstrate. This mechanical stress is assumed to generatedislocations that glide into the silicon bulk and devastigatereduce the bulk carrier lifetime.

However, our results show that the mere BRL existencealone is not a sufficient criterion or cause for lifetimedegradation. A general correlation between the existence ofa BRL and lifetime degradation is not possible, as samples ofthe reference diffusion and of diffusion 1b feature regions ofBRL coverage and at the same time high bulk carrier lifetimesin those regions. We therefore investigate the correlationbetween bulk carrier lifetime and BRL thickness. Figure 9displays the correlation of BRL thickness and bulk carrierlifetimes as measured by MWPCD. The BRL thickness wasdetermined from SEM images, and the MWPCD lifetime

mappings were taken from a different wafer, which wasplaced on a directly adjacent position during the same borondiffusion. We verified that the sheet resistance and BSGthickness distributions were identical on such adjacent wafersand we therefore can directly correlate the SEM data to thelifetime data. SiNx-passivated reference samples that have notbeen boron diffused show a laterally constant lifetime signalfor all positions on the sample that are further than 1 cm awayfrom the wafer edge. In order to avoid influences from any ofsuch edge effects (poorer SiNx passivation, tweezer handling,crystal defects/cracks near the wafer edge), we have restrictedour collection of BRL thickness and lifetime data to positionson the wafer that were at least 1 cm away from the edges. AsBRL thicknesses of less than 5 nm are difficult to detect bymeans of SEM, the very low BRL thicknesses below 5 nm aresubject to some uncertainty.

Figure 9 shows a very clear correlation between BRLthickness and the bulk carrier lifetime. We find a decrease ofthe lifetime with increasing BRL thickness. The graph showsthat BRL thicknesses of more than 5–10 nm cause the bulkcarrier lifetime to drop below 1000 μs. Correspondingly,we did not detect any regions with BRL thicker than5–10 nm where no lifetime degradation occurs. We thereforeconclude that for boron diffusions, a BRL thickness of 10 nmor more causes a degradation of the bulk carrier lifetime andthat this degradation process takes place during the cool-downramp of the diffusion process.

Area-averaged bulk carrier lifetime values aresummarized in table 3. Lifetime measurements on wafers ofthe diffusion series 5 and 6 generally do not show reducedcarrier lifetime except in boundary regions. The preservationof the high carrier lifetime can be explained by the factthat there is no BRL present during the cool-down ramp(except in small areas near the wafer top). On the other hand,table 3 shows that samples with high BRL coverage showsignificantly reduced average bulk carrier lifetime values. Itis well understandable from the results shown in figure 9 thatsamples with similar BRL coverage (diffusions 2b and 3b aswell as 1b and 4d) as listed in table 3 still can exhibit differentdegrees of lifetime degradation.

4. Summary

Overall, BSG layer thicknesses of 50–60 nm are sufficientto serve as unlimited doping source for 60 � sq.−1 emitterdiffusion processes, which can be reached by a depositionstep as short as 10 min. BSG layer thicknesses can varysignificantly with position on the wafer. In the case of ourfurnace setup, we find that high BSG and BRL thicknessesoccur in boundary regions, particularly near the wafer top.BRL formation is generally favored by high BSG layerthicknesses as well as high drive-in temperatures. BRLformation and its thickness correlate with carrier lifetimedegradation in the bulk of the wafer. In particular, wefind that BRL thicknesses exceeding 10 nm are markedlydetrimental for the bulk carrier lifetime. The degradationprocess occurs during the cool-down ramp. We associatethis lifetime degradation with the creation of crystal defects

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Semicond. Sci. Technol. 25 (2010) 055001 M A Kessler et al

due to mechanical stress resulting from differences betweenthe thermal expansion coefficients of SiB6 and silicon.BRL formation can be avoided or reversed by a drive-in sequence under oxidizing atmosphere without significantlifetime degradation. However, our results show that thistechnique is prone to compromising the sheet resistance ofthe diffused layer. We shall note that all measured parametersare only evaluated for samples that were processed on oneparticular position within the diffusion furnace. As theprocess appears to be sensitive to seemingly minute changes,we consider it to be possible that quantitative results mightbe influenced when varying the wafer position within thefurnace—even when holding all other parameters constant.

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