The author would like to thank Shannon Fields and Kevin Hart for assistance with depositions and Vicky DiNetta for AFM measurements, all of whom are from ...
Mater. Res. Soc. Symp. Proc. Vol. 989 © 2007 Materials Research Society
0989-A18-06
Hydrofluoric Acid Treatment of Amorphous Silicon Films for Photovoltaic Processing M. Burrows1,2, U. Das1, M. Lu1, S. Bowden1, R. Opila2, and R. Birkmire1 1 Institute of Energy Conversion, University of Delaware, 451 Wyoming Rd., Newark, DE, 19716 2 Materials Science and Engineering, University of Delaware, 201 Dupont Hall, Newark, DE, 19716
ABSTRACT Hydrofluoric acid (HF) is commonly used in Si wafer processing as a surface treatment to remove surface oxide and provide a H-terminated surface passivation that resists contamination within short time scales. During silicon heterojunction (SHJ) device fabrication a similar oxide removal and surface passivation is desired for doped and intrinsic hydrogenated amorphous silicon (aSi) films and therefore studied as follows. X-ray photoelectron spectroscopy is employed to evaluate surface chemical composition, especially with regard to oxygen removal, resistance to hydrocarbon adsorption, and fluorine incorporation post HF treatment. Variable angle spectroscopic ellipsometry is used to determine the growth rate of native oxide and terminal oxide thickness. The electrical effects of aSi native oxide at contact interfaces of SHJ cells are evaluated with current-voltage measurements. HF treatment is effective for oxide removal and provides surface passivation of aSi similar to the crystalline counter-part. Further, fluorine bonding is enhanced for p-type aSi films and control of native oxide thickness below 20Å is not essential for typical electrical contacts of the SHJ photovoltaic cell. INTRODUCTION Silicon heterojunction devices which involve aSi films deposited on crystalline Si wafer (cSi) are being produced industrially claiming 6% of the total photovoltaic market and studied by a number of groups internationally [1]. Excellent cSi wafer surface passivation can be achieved by thin intrinsic aSi layers and doped aSi films provide the emitter and back contact. Device contacts typically consist of a transparent conducting oxide for the illuminated junction and metallization for the rear. Due to the numerous deposition steps required it is difficult to realize this completed device structure without exposure to atmosphere. The work herein is designed to answer the question of what is on the a-Si surface after a simple wet chemical H-termination procedure and thus compliments the body of literature on cSi surface treatment and furthers that of SHJ processing. EXPERIMENT Two types of double-side polished float zone (111) silicon wafer have been used in this work: phosphorous doped silicon (n-cSi) of 2.5-3Ωcm resistivity and 300µm thickness and boron doped silicon (p-cSi) of identical resistivity and thickness. Four different aSi film conditions deposited by plasma enhance chemical vapor deposition are analyzed. These include 2% gas phase concentration PH3/SiH4 (n-aSi), 2% gas phase concentration B2H6/SiH4 (p-aSi), undoped or intrinsic films (i-aSi) deposited by DC plasma and a i-aSi film deposited by 13.56MHz RF
plasma. Other deposition conditions include substrate temperature of 200°C, pressure of 1.25Torr, .078mA/cm2 discharge current for DC plasma and 19mW/cm2 power for RF plasma. The silane gas flow rate of 20sccm is constant throughout and H2 to SiH4 gas flow rate ratio of ~6 is also maintained. Atomic force microscopy (AFM) measurements were performed on a Digital Instruments Dimension 3100 Scanning Probe Microscope. X-ray Photoelectron Spectroscopy (XPS) was performed using a Physical Electronics PHI 5600 using Al Kα radiation with a base pressure in the low 10-10 range. The electron take-off angle was approximately 30° from the surface normal and the high resolution quantitative scan pass energy was set to 23.5eV. A variable angle spectroscopic ellipsometer (VASE) from J. A. Woollam Co. was used and corresponding optical models were built and analyzed in the WVASE32 software. The ellipsometer was fit with a N2 purge source directed at the sample stage and cover to protect from room turbulence during the 12min scan time. Two cleaning processes were used immediately prior to deposition or analysis. A three step approach including a pre-etch ultrasonic clean to remove debris and dissolve organic contamination; a wet-chemical oxidation that both dissolve impurities including ionic, metallic, and surface hydrocarbon as well as causing further oxidization into the Si bulk; finally this surface oxide is etched and a new H-terminated Si surface is created using an HF solution. Table 1 contains the cleaning procedures’ details. Table 1. Cleaning procedure prior to deposition or analysis. Deposition Step Pre-etch Ultrasonic Clean
Wet -Chemical Oxidation Oxide etch and H-termination
† ‡ *
Time
Temp
Description
Analysis Step
Time
Temp
(min)
(°C)
Description
(min)
(°C)
1
2
80
ultrasonic soap solution clean
1
4
RT
ultrasonic acetone clean
2
3
70
ultrasonic DIH2O‡ rinse
2
4
RT
ultrasonic methanol clean
3
10
85
hot flowing air dry
4
60-70
piranha† etch
4
RT
ultrasonic 18MΩ DIH2O clean
3
RT
flowing 18MΩ DIH2O rinse
5
5
73
piranha etch
5
RT
flowing 18MΩ DIH2O rinse
6
5
RT
flowing 18MΩ DIH2O rinse
6
1
RT
10% HF etch
7
1
RT
10% HF etch
7
0.33
RT
N2 jet dry
8
0.05
RT
18MΩ DIH2O rinse
9
0.33
RT
N2 jet dry
5
5
3 4
*
Piranha etch is 4:1 H2SO4 to H2O2 allowed to self heat to 60-70°C or held at 73°C in a hot water bath DIH2O is de-ionized water RT denotes room temperature
RESULTS AND DISCUSSION Sample Roughness The root mean square surface roughness (Rq) of a sample set of 200nm DC i-aSi / n-cSi and bare n-cSi in four stages of extended clean processing were evaluated: after pre-etch ultrasonic clean and after 1, 2, and 3 series of oxidation in piranha solution followed by HF dip. For both the deposited film (Rq = 4.4-6.0Å) and wafer (Rq = 1.3-2.0Å) there is no detectable roughening upon single or repeated etching. Also it was observed that at the above deposition conditions surface roughness is independent of film thickness up to a few hundred nanometers.
n-aSi Film
no x po x H F
no x po x H F
no x po x H F
no x po x H F
no x po x H F
Atomic Surface Concentration and Bonding XPS was used to quantify oxidation, hydrocarbon adsorption, and residual fluorine for five identically treated samples at various stages of the cleaning process. Importantly the time between the last N2 jet dry step and evacuation of the XPS load-lock to the 10-5Torr range is 4-5 minutes and matches the typical time window required for substrate loading and evacuation of the PECVD deposition system. Figure 1(a) is a summary of the results; ‘nox’ is short for native oxide, ‘pox’ for piranha etch oxide, and ‘HF’ is HF dipped and loaded as described above. In all cases the piranha etch oxide step is seen to effectively decrease the carbon signal and increase the oxygen. The O fraction increases from 35.8±1.3% to 45.4±1.1% (a 27% increase) on average for the four aSi films and from 35.6% to 40.2% for the n-cSi wafer. Conversely the C fraction decreases from 12.0±2.3% to 4.1±0.6% on average for the four aSi films and 10.7% to 5.4% for the n-cSi wafer. The increase in oxygen signal is believed to be from two sources; the more complete oxidation (greater fraction of SiO2 relative to SiOz, z < 2) and further oxidation into the Si bulk. Considering the aSi films and assuming a fixed sampling depth we can estimate that the average increase of SiO2 to SiO ratio upon piranha etch (about 0.67 to 0.76) to account for a 5% gain in oxygen signal, the remaining 22% must come from further oxidation of Si bulk. A similar argument holds for the n-cSi wafer. Figure 1(b) illustrates the Si2p region of the n-aSi sample and is a good illustration of the variation in intensity of oxide state from native to piranha etched oxide to finally the HF treated where no shifted components are detectable. A single Gaussian is applied to fit the elemental, sub-oxide, and dioxide peaks. It is not reasonable to deconvolute the Si region into +1 to +4 oxidation states in our experimental set-up.
100
) % ( 90 no it ar tn 80 ec no C 70 ic m ot 60 A ec af ru 50 S
Si O C
) . u . a (
HF treated
y t i s n e t n I
Piranha Oxide
F
Native Oxide
40
108 n-aSi
DC i-aSi
RF i-aSi
p-aSi
105
102
99
96
93
n-cSi Bind ing Ene rgy ( eV)
(a) (b) Figure 1. (a) Graph of surface atomic concentration at various conditions. Note that for higher resolution in the region of interest the plot is truncated below 40% with the remainder understood to be Si. ‘nox’ is short for native oxide, ‘pox’ for piranha etch oxide, and HF is post HF dip. (b) Si2p region of 200nm 2% n-aSi film with Shirley background correction then deconvolution into elemental, monoxide, and dioxide regions for native oxide, piranha etch oxide, and HF treated.
Three trends can be distinguished post HF dip. n-aSi, DC i-aSi, and RF i-aSi can be considered to have identical results within our experimental resolution, 2.2% O, 4.0% C, and 0.8% F. The
n-cSi wafer was measured to have extremely low contamination with 1.3% O, 2.3% C and F below detection (less than 0.5%). Comparable XPS wafer clean studies reported in literature often contain greater than 5% O, 10% C and at least 1%F [1-3]. It is proposed that two important controls employed are the sources of this improvement. The final three second DIH2O dip has been shown to reduce F contamination by several percent absolute [3]. Also the final post HF carbon concentration is significantly reduced and O to a lesser extent when the piranha etch temperature is maintained at 73°C versus a self-heating strategy. Lastly it is interesting to note that the p-aSi film with 4.6% O, 4.9% C, and 3.1%F contains twice the O and over three times the F of the n- and i-aSi films. In all cases the F is preferentially bonded to Si (binding energy of 685.9eV) versus adsorbed F (binding energy of 687.4eV) [3]. Resistance to Oxide Growth In VASE analysis by ellipsometric modeling a number of important features are used as fitting parameters including surface roughness, oxide thickness, aSi film thickness and optical constants. However in this study the focus is not the aSi film bulk optical properties that are of interest but rather the changing surface upon HF treatment and subsequent exposure to air.
14Å
dSiOx daSi
Model Fit Parameters
50%Void + 50%[(1-y)*SHyd + y*SiOx]
y = 0.086
SiOx
x = 0.75 (EMA of 75%SiO2 + 25%SiO) dSiOx = 0Å
aSi Film
RF i-aSi Cody-Lorentz Oscillator Parameters: Amp=90.3 En=3.76 Br=2.42 Eg=1.65 Ep=1.33 Et=0 Eu=0.05 daSi = 1996Å
_______________________________________________________
________________________________________________________
cSi Substrate Figure 2. Schematic of model structure used in ellipsometry analysis. Also shown is best fit of RF i-aSi 5min post HF treatment resulting in MSE of 1.386. Note that during oxide fitting only the italicized dSiOx and y, 0< y
