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PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2012; 20:670–680 Published online 5 December 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.1203

PAPER PRESENTED AT 26TH EU PVSEC, HAMBURG, GERMANY 2011

Advanced alignment technique for precise printing over selective emitter in c-Si solar cells Alessandro Voltan*, Marco Galiazzo, Diego Tonini, Alessandro Casarin, Giorgio Cellere and Andrea Baccini Applied Materials Italia S.r.l., via Postumia Ovest 244, 31048 San Biagio di Callalta (TV), Italy

ABSTRACT Significant improvements in c-Si solar cells performance can be achieved acting on the emitter or on the contact formation side. At the moment, an efficiency gain around 0.5–0.6%abs [1] has been demonstrated for selective emitter (SE) cells, mainly caused by the improvement of the blue response of the shallow emitter. Nevertheless, in case of sub-optimal alignment of the printed metal with the deep emitter, the shallow junction depth leads to a significantly narrower firing process window, consequently causing high contact resistance and poor shunting behavior. Moreover, after Anti Reflective Coating (ARC) the contrast between differently doped areas is poor and often prevents the screen printer vision system from correctly recognizing the pattern for metal printing alignment. Thus, we developed a new equipment, high precision selective emitter, for precise alignment over SE able to guarantee recognition of every SE pattern and consequent printing repeatability within +/-30 mm without rejections or misalignments. In this work, results from experimental investigations and mass production are reported and discussed, including also comparison with standard alignment methods and further feasible applications. New equipment for precise alignment over SE able to guarantee recognition of every SE pattern and consequent printing repeatability within +/-30mm without rejections or misalignments has been developed (examples of SE patterns are reported in the following pictures). Test performed at production sites confirm that an efficiency gain of 0.1% and a significant reduction of rejection can result from the increased robustness of the presented method. Finally, also a correlation between sheet resistance of heavily doped area and image contrast has been demonstrated. Copyright © 2011 John Wiley & Sons, Ltd. KEYWORDS selective emitter; screen printing alignment; free carrier absorption; sheet resistance *Correspondence Alessandro Voltan, Applied Materials Italia S.r.l., via Postumia Ovest 244, 31048 San Biagio di Callalta (TV), Italy. E-mail. [email protected] Received 15 May 2011; Revised 13 July 2011; Accepted 26 August 2011

1. INTRODUCTION Nowadays, several different technological strategies for the development of selective emitter (SE) are being established from preliminary study to industrial mass scale production [1]. Despite the existence of different formation technologies, the result of all selective emitters is a differentiation of emitter doping level between lowly doped illuminated areas (LDOP) and the highly doped areas under contact fingers (HDOP). The key advantages achieved with SE are: (1) lower contact resistance in the HDOP regions, resulting in a high fill factor; (2) better blue response in lightly doped illuminated regions with higher Isc and Voc, by means of a better surface passivation and thinner (when not absent) dead layer; (3) a wider firing process window because of the deep emitter in the metallized areas, which results in lower shunting issues. 670

In order to minimize the cost/W, most of the industrial research in SE creation is focused on processes, which require only one additional step compared with standard cell production process flow. A non-exhaustive list of such processes is summarized in Table I. Regardless of how the HDOP area is realized, front contact screen printing over SE requires accurate alignment in order to avoid bad contact behavior. In general, to overcome such problem, HDOP areas up to 400 mm wide, which can accommodate mismatches of the narrower metal fingers (≤100 mm) screen printed using edge alignment are realized. Therefore, the cell area active in light absorption is partially composed of HDOP areas of poor Internal Quantum efficiency (IQE), reducing the benefit achieved using the SE approach. The results of 2D numerical simulations clearly show the strong influence of the HDOP lateral width on the Isc and consequently on the efficiency of the SE cell [9]. A decreased Copyright © 2011 John Wiley & Sons, Ltd.

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Table I. Most commonly used SE processes. SE process

Additional process step

Position in the process flow

Dopant paste [2]

Screen printing of dopant paste, cleaning

Silicon inkjet [3] Laser doping [4]

Inkjet print of doped Si nanoparticles Laser induced diffusion

Etch back [5]

Screen printing/inkjetting of a protection paste where the highly doped region is to be

Etch paste [6]

Screen printing/inkjetting of an etching paste to create the lowly doped region Deposition of a partially diffusion masking layer and subsequent opening by laser ablation Ion implant, thermal anneal to activate dopants

Oxide masking layer and laser ablation [7] Ion implantation [8]

HDOP width can be highly beneficial for the SE solar cell, as the exposed low IQE area is reduced. Moreover, better matching of HDOP area and metal finger width reduces the dependence of the SE performance on the HDOP profile and hence makes less critical the SE creation process, allowing a larger diffusion process window. Figure 1 shows the simulated efficiency gain obtained by decreasing finger width, plotted versus the surface peak doping concentration (Peak Dose). Generally, two approaches can be used for precise printing alignment over SE: (1) edge alignment and (2) pattern alignment. The first method starts from the recognition of the wafer edges and can be based on different mathematical models. Good SE overlapping is limited by the precise position of the SE pattern related to wafer edges, although by using edge alignment, a printing repeatability of +/-25 mm has been

After texturization. The following POCL3 diffusion serve as both paste drive in and shallow emitter creation After texturization After POCL3 diffusion and before PSG removal. The PSG layer is used as an additional P source After diffusion. Diffusion creates a uniform highly doped emitter; subsequent etching creates the lightly doped emitter by selective etching of unprotected regions After diffusion Before POCL3 diffusion Use an ion implanter to create shallow and deep doped areas in substitution of POCL3 diffusion

demonstrated. On the other hand, pattern alignment is based on the recognition of SE itself and the precise printing over it. Generally speaking, four problems can be faced performing pattern alignment over SE: (1) the low contrast of SE pattern after ARC; (2) the deformation of the SE pattern itself; (3) the unstable reflective behavior; and (4) the image distortions caused by grain boundaries in the case of multi c-Si wafers. Even if grain boundaries can be masked by means of image analysis tools [10], contrast cannot be easily improved by simply using standard cameras and illuminators in the visible range. In order to overcome the previous limitations, we developed a new vision system based on free carrier absorption (FCA). The working principle and test results are reported in the following section.

Figure 1. Simulation results showing the efficiency gain obtained by reducing the HDOP area; data taken from [9]. Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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2. FREE CARRIER ABSORPTION: THEORY Photon absorption mechanisms in silicon devices can be divided into three groups: (1) band-gap or intrinsic absorption; (2) impurity level-to band absorption; (3) free carrier absorption. In FCA, photon energy is absorbed by free carrier in both conduction and valence band and, as FCA effect is proportional to the carrier concentration, it has been used in the past mainly to measure the free carrier concentration in semiconductor devices. According to Schroder et al. [11], the FCA coefficient for bulk silicon can be expressed as Equation (1): a¼

q3 l2 p 4p2 e0 c3 nm2 m

(1)

where l is the wavelength of incident light, p is the density of free carrier (electrons or holes), n is the refractive index, m* is the effective mass and m is the mobility. Furthermore, results of experimental investigations [12] and empirical models [13] show that FCA for HDOP area in n-Si becomes significant at wavelength higher than 1.1 mm whereas the intrinsic absorption can be neglected. Figure 2 shows the dependence of a on the wavelength of incident photons for intrinsic and free carrier absorption coefficients for silicon as reported in [11]. In SE structures, the thickness of the HDOP layer is generally small compared with the entire wafer thickness. Here the transmission ratio (T) of the incident photons through the doped layer can be expressed as Equation (2):

T¼

ð1  RÞ2 ead 1  R2 e2ad

On the basis of these considerations, models to describe correlation between near-IR (Infra Red) absorption and electrical properties of silicon wafers have already been developed and validated by experimental investigations. In [14] a method named infrared lifetime mapping based on an infrared camera is presented for spatially resolved measurement of minority carrier lifetime, whereas lock-in thermography is used in carrier density imaging for minority carrier lifetime [15] and process control emitter structures [16]. Finally in [17,18], transmission of photons with wavelengths between 3 and 5 mm is measured with a 384 x 288-pixel CCD camera for contactless measurement of sheet resistance (sheet resistance imaging) and determination of doping (in)homogeneity. Furthermore, Isenberg already demonstrated in [17] the potentiality of FCA-based IR imaging for SE detection. Here FCA is used to recognize SE pattern on silicon wafers and performing precise printing over it by means of a completely automated alignment and inspection tool for mass production.

3. WORKING PRINCIPLE Based on the aforementioned FCA theory, a preliminary study has been performed using spectral analysis to measure absorption and reflection ratio at different wavelength of incident photons on SE wafers.

(2)

where d is the thickness of the HDOP layer, and R is the reflectivity at the incidence surface.

Figure 2. Intrinsic and free carrier absorption coefficients for silicon as a function of wavelength [11], (Reproduced by permission of IEEE Journal of Solid State Circuits © 1978 IEEE).

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Figure 3. Spectral analysis of Etch back SE wafer. Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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In Figure 3, results of spectral analysis over an Etch back SE pattern on multi c-Si wafers are reported. The graphs, obtained from certified laboratory, show that HDOP areas are more visible at long wavelength, whereas the reflectance is decreased because of increased free carrier absorption. On the other side, reflectance differences at short wavelength are probably caused by differences in texture. These results confirm what was expected by the theory on FCA and the correlation with the FCA coefficient presented in Figure 2. Inputs from the spectral analysis have been used for the development of a dedicated vision system for alignment over SE using both cameras and illuminators working in the near IR. The presented technique is based on measuring the IR signal coming from the wafer, where the differences in “escape reflectance” (light that has entered the wafer but is lost again through the front) is generated by FCA. Working principle and sketch of mechanical layout are reported in Figure 4 [19]. The SE wafer is illuminated by an IR source whereas the SE pattern is detected in four points by means of four low-resolution IR cameras working in the same reference system. Even if the IR source is represented above the SE wafer, the alignment system is able to work in both reflection (illuminators over the wafer) and transmission mode (illuminators under the wafer). As the newly developed near IR vision system can also be used for recognition of standard printing pattern, it can be easily integrated in automatic printing lines as upgrade of existing pattern alignment technologies [20], such as those used for double printing of metal fingers [21]. Even if IR cameras are used for the alignment, identification of SE pattern is extremely fast, and the standard print cycle time (in our case less than 2.5 s) is not affected by the implementation of the system. Actually, because of the absence of manual adjustment, set-up procedures are even faster than in edge alignment (for example during the screen replacement), resulting in an overall increased throughput and uptime of the printing line.

Furthermore, considering that wafer front side is printed before firing and BSF formation, image quality is not influenced by the back side of the cell. For this reason, the High Precision Selective Emitter (HPSE) system can be used for both Back-Back-Front and Front-Back-Back metallization processes without any modifications.

4. ALIGNMENT CAPABILITY The first prototype of the HPSE has been integrated and tested into an automatic screen printing line in order to evaluate alignment repeatability and performance. Two different tests have been performed: (1) evaluation of intrinsic repeatability of the system without taking into account the effect of SE; (2) evaluation of performance degradation caused by SE instability. In test (1), two subsequent prints with a defined off-set have been carried out, using the first as reference pattern for the second one (Figure 5a). Then, the repeatability of the alignment has been measured by optical microscope as X and Y components of the distance between the centers of the two printed patterns. For the test, 40 wafers have been printed using the four loading devices (nests) commonly used in Applied Materials Baccini Softline (Figure 4). As reported in Figure 6, the alignment repeatability has been measured to be within +/-15 mm for each nest. In test (2), two subsequent prints with a defined off-set have been performed over SE structure using the SE pattern as reference (Figure 5b). Also, in this case and the distance between the two centers of the printed pattern has been measured in X and Y direction for the evaluation of alignment repeatability. The test has been performed using Etch back SE pattern on multicrystaline wafers, which was not detectable using standard camera and visible light (Figure 7). Finally, as well as in test (1) also in this case, 40 wafers have been printed using the four nest obtaining repeatability within +/-25 mm for each nest and an overall

Figure 4. Schematic of advanced vision system used for alignment over SE [adapted from 19]. Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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Figure 5. Repeatability test.

Figure 7. Etch back SE pattern on multi-wafers and related gray scale analysis (profile measured along the line). a) Standard vision system, b) IR vision system and pattern recognition.

Figure 6. Results of test i), deviation from average off-set.

accuracy lower than 7 mm. Even if the results are reliable, it has to be considered that, if compared with test (1), the degradation of performance are caused by the instability of SE pattern itself, and to the double alignment over SE with off-set in opposite directions (Figure 8). Considering the different behavior of the four nests in terms of accuracy, on overall printing repeatability of +/-30 mm has been stated for the entire system. 674

Figure 8. Results of test ii), deviation from average off-set. Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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Table II. Results from mass production. Average Alignment method HPSE Edge

Voc (V)

Isc (A)

Rshunt (Ω)

FF (%)

Efficiency (%)

Irev1 (A)

Irev2 (A)

N. of samples

0.619 0.617

8.24 8.23

195 203

78.50 78.21

16.49 16.38

0.11 0.13

0.20 0.22

5027 5002

Efficiency (%) 0.43 0.51

Irev1 (A) 0.096 0.211

Irev2 (A) 0.169 0.380

N. of samples 5027 5002

Standard deviation Alignment method HPSE Edge

Voc (V) 0.006 0.006

Isc (A) 0.122 0.124

Rshunt (Ω) 56 73

FF (%) 0.64 1.33

Figure 9. Efficiency distribution. Results from mass production.

5. SYSTEM VALIDATION AND RESULTS FROM MASS PRODUCTION Results related to alignment capability and precision have been extended to mass production. One week test has been performed at a production site to validate the system, and the final solution has been released. Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

Because of the limitation of standard pattern alignment (Figure 7), Etch back SE process over multi-wafers has been chosen as test case. Indeed, by using the HPSE system, the pattern contrast is significantly improved, and the effect of grain boundaries is filtered out. To evaluate the benefit of the advanced alignment system in terms of advancement in process stability and improvement of electrical parameters of printed wafers, a comparative test 675

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Whereas benefits of advanced alignment method have been demonstrated in comparison with edge alignment, even better improvements are expected with the reduction of SE finger width, mainly related to efficiency gain. Feasibility of precise alignment over 200 mm finger width SE pattern has been demonstrated successfully both at small and large scale production. In Figure 10a, an example of precise screen printing over SE where part of the silver pattern has been masked to allow evaluation of overlapping is reported. As the HPSE system is able to recognize also standard printed pattern and SE pattern at the same time, it could further be used also for post-print inspection in order to assure precise overlapping and correct drift of the print process. In this way, effects of unavoidable screen deformation and instability (see ref. [22,23] for detailed analysis and modeling of screen printing process) could be compensated or mitigated without increase in the average SE finger width. An example is reported in Figure 10b where the SE finger width is 350 mm.

6. FURTHER DEVELOPMENT AND APPLICATIONS

Figure 10. Results of pattern alignment. a) SE finger width lower than 200_m, b) post print inspection.

between standard edge alignment and advanced vision system has been carried out. Test has been performed using two subsequent sets of 5 K wafers, each with 350 mm HDOP finger width, coming from the same production batch (standard HDOP finger width in production: 400 mm). The improvement of electrical parameters caused by the use of the new advanced alignment is unraveled by the results of the comparison reported in Table II. As found in other similar tests conducted both in small and large scale production, the efficiency gain was about 01% abs caused by the improvement of alignment precision over SE pattern, with an overall narrower distribution of final data (decreased standard deviation, Table II). The efficiency gain is coming mainly by the increase in FF, Voc and Isc related to improved alignment with respect to edge alignment. As reported in Figure 9 for the efficiency analysis, the increased average value is generally caused by the reduction of outliers and to a higher mode value in the statistical distribution. As another major advantage, because of the accurate alignment over SE pattern, the risk of misalignment is significantly reduced using the advanced pattern alignment method. Indeed, during the 1-week test, no misalignment or rejections have been observed, with a reduction of rejection rate caused by leakage current Irev >2 of 0.1% with respect to standard production. 676

To investigate the range of potential applications of the described system, we tested at laboratory level its applicability to different technologies of SE pattern and c-Si substrate. Because the results of the proposed method are not affected by the optical properties of SE pattern, and the effect of grain boundaries are significantly reduced, the same system can be used for pattern alignment over different SE processes with mono, multi, and even mono casting c-Si wafers without any modifications. Some examples are reported in Figure 11 whereas in Table III, the most commonly used alignment methods for precise printing over SE in c-Si solar cells are compared with respect to the different SE processes (Table IV). Please note that in Figure 11, even if all the SE patterns can be easily recognized and used for the alignment, not all the processes give same results in terms of contrast and image quality. Because the reason of such behavior is generally caused by the different concentration of free carriers, the correlation between image quality and Rsheet has been analyzed on different samples. In Figure 12, an example of different results of the same SE process is reported. Images are related to SE pattern obtained using Dopant paste on multi-c-Si wafer. After the images have been acquired with the IR system, the differences of contrast between the HDOP area and the lightly doped area have been quantified by means of Grayscale analysis. Notably, in the same areas also, the difference of Rsheet has been measured with fourpoint probing. For the SE pattern reported in Figures 12a, 12b and 12c, the measured difference of average Rsheet is, respectively, 6.2 Ω/square, 10.3 Ω/square, and 18.5 Ω/square confirming that the increased quality of the image contrast, strictly correlates with the quality of the SE Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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Figure 11. Examples of SE pattern recognition with the advanced vision system.

Table III. Review of most commonly used alignment methods for precise printing over SE in c-Si solar cells.

SE process Etch back

Dopant paste

Edge alignment Repeatability. +/25 mm Risk of bad contact behavior and misalignment • Repeatability. +/25 mm • Risk of bad contact behavior and misalignment

High precision standard pattern alignment (HP)

•

•

•

•

Silicon inkjet

Repeatability. +/25 mm • Risk of bad contact behavior and misalignment

Etch paste

Repeatability. +/25 mm • Risk of bad contact behavior and misalignment

•

• •

High precision advanced pattern alignment (HPSE)

Intrinsic repeatability. +/15 mm Not feasible with multi-wafer

•

Intrinsic repeatability. +/ 15 mm Dedicated illuminators necessary

•

•

•

•

•

•

•

Possible rejection with multi-wafers Intrinsic repeatability. +/15 mm • Dedicated illuminators necessary

Intrinsic repeatability. +/15 mm For both mono and multi-wafers Intrinsic repeatability. +/15 mm For both mono and multi-wafers

Not affected by PSG removal Intrinsic repeatability. +/15 mm • For both mono and multi-wafers

• •

Possible rejection with multi-wafers Intrinsic repeatability. +/15 mm • Dedicated illuminators necessary •

• •

Intrinsic repeatability. +/15 mm For both mono and multi-wafers

•

Laser doping

Repeatability. +/25 mm • Risk of bad contact behavior and misalignment •

Possible rejection with multi-wafers Intrinsic repeatability. +/15 mm • High contrast fiducials necessary for stable alignment •

process. In order to evaluate the overall performance of the system in terms of Rsheet analysis, the investigation has been also extended to the samples reported in Figures 11a, 11b, and 11c. Results are summarized and analyzed in Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

•

Intrinsic repeatability. +/15 mm For both mono and multi-wafers

•

Feasible with mono-casting wafers

•

Figure 13 where it is clearly recognizable that a linear relationship between the two variables with Rsheet lower than 40 Ω/square exists, whereas the difference in image contrast becomes stable at higher levels of Rsheet. 677

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Table IV. Correlation between ΔRsheet and Image Contrast (ΔGrayscale level).

Dopant paste (Figure 12a) Dopant paste (Figure 12b) Dopant paste (Figure 12c) Dopant paste (Figure 11b) Etch back (Figure 11a) Silicon inkjet (Figure 11c)

ΔRsheet (Ω/square)

ΔGrayscale level

A

6.2

10.2

B

10.3

20.0

C

18.5

33.7

D

31.7

93.7

79.2

93.0

19.9

43.6

vision system has been developed based on Near InfraRed Free Carrier Absorption in heavily doped silicon layers. The intrinsic precision (repeatability) of the system has been proved to be within +/-15 mm. However, because of instability of SE process, a printing precision within +/-30 mm without rejections or misalignments can be guaranteed in mass production. Test performed at production sites confirm that an efficiency gain of 0.1 abs % (with respect to a standard edge alignment technique) and a significant reduction of rejection rate can result from the increased robustness of the presented method. In addition, because of absence of manual adjustment, also the uptime and throughput of the printing line can be significantly improved with the HPSE system. Finally, from a comparative analysis on different SE processes, a correlation between sheet resistance of heavily doped area and near infra-red image contrast has been

Figure 12. Grayscale analysis, profile measured along the line.

Even if the feasibility of a contactless spatially resolved measurement of Rsheet has been already demonstrated in [17,18] using the HPSE system, both alignment over SE and process control of HDOP area can be carried out at the same time without affecting the standard cycle time. In this direction, further investigations are under development to assess and properly quantify the relationship between image contrast and Rsheet for the most commonly used SE processes.

7. CONCLUSIONS Here, an advanced alignment technology for precise printing over SE in c-Si solar cells has been presented and compared with existing alignment technologies. The new 678

Figure 13. Correlation between _Rsheet and Image Contrast (ΔGrayscale level). Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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demonstrated. Further investigations are under development to employ the same system for both qualitative analysis of heavily doped area and accurate alignment over SE pattern.

Precies alignment over Selective Emitter in c-Si solar cells

9.

ACKNOWLEDGEMENTS The authors would like to thank R. De Rose and the ARCES-DEIS, University of Bologna group for performing the numerical simulations and for the fruitful discussions, and the R&D department of XGroup S.p.a. for the technical support and suggestions during the experimental investigation.

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11.

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Prog. Photovolt: Res. Appl. 2012; 20:670–680 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/pip