Document Sample
                                                           Paper M1105

                                                         Mohammed Naeem

                                              GSI Group, Laser Division
                                              Cosford Lane, Swift Valley
                                           Rugby, Warwickshire, CV21 1QN, UK

                                                                                  energy delivered with high directional
                         Abstract                                                  precision
                                                                                   high processing speed
                                                                                  high resolution ,accuracy and high flexibility
Silicon is widely used in a number of industries but
primarily in solar cells and in semiconductor
manufacture, with growing applications in jewelry
and entertainment goods. In the majority of
applications the source material is in the form of                       Silicon wafers are conventionally diced off by a thin
wafers which are typically 0.2-1.5mm (thick and 100-                     diamond blade into individual IC chip, before they
300mm diameter. From the flat faces of the octagons                      are packaged. The problems encountered in blade
they laser cut a flat rectangular wafer that is then                     dicing include chipping, kerf-loss and low
further processed to become a solar cell. Cutting                        productivity [1-2]. Currently green wavelength
speed and yield are important to reducing costs. The                     (frequency doubled) and microjet are been used but
main requirements for cutting of silicon wafers are                      both of these processes are slow and expensive to
dross free and crack free cut edges in a range of                        operate [3-8]. Milliseconds low power pulsed Nd:
thicknesses.                                                             YAG lasers and high beam quality continuous wave
                                                                         fiber lasers are being used to cut these materials [9]
                                                                         but the cut quality is poor i.e. microcracking due to
                                                                         excessive heat input, which can lead to failure of
The cutting trials have been performed in a range of                     some components during process steps and
thicknesses with two different laser sources i.e. with                   associated reduction in yields.. The length of
a high beam quality CW single mode fiber laser and                       microcracks can range from 15µm to 100µm
also with a high beam quality pulsed Nd: YAG laser.                      depending on the laser source being used.
Considering the differences in beam quality and
pulsed performance between the two types of laser,
there are different operating regimes for the two
types of laser. The lamp-pumped YAG laser is                             So what determines a good wafer? A wafer must not
characterised by long high-energy pulses but poorer                      have corner cracks that are larger than a certain
beam quality, and the fibre laser with high repetition                   specification and must meet a 125MPa strength test
rate on-off type modulation, single-mode beam                            [10-15]. Corner cracks are related to the wafer
quality but low pulse energy. From an applications                       cracking away instead of being cut away. The
perspective, both these regimes have their advantages                    strength test is based on a bending force test before
and these are discussed in the following sections.                       breakage and is done on an automated tester. A set
                                                                         of 25 parts is used to find and average. Generally the
                                                                         wafers have strength of about 50MPa after being cut
                                                                         and then the wafers are etched to remove the micro-
                      Introduction                                       cracking at the edge of the cut. Nominally wafers
                                                                         that have been cut with lasers meet the 25MPa spec.
As the cost of the conventional energy continue to                       after about 100 seconds of etching.
increase, the use of solar technology for the
production of alternative source of energy is
becoming more and more important. This has led to
the increase in the production of solar cells and the                    The main requirements for cutting of silicon wafers
demand for the right laser source to cut the                             are dross free and crack free cut edges in a range of
rectangular wafers from the flat faces of the octagons                   thicknesses. The cutting trials have been performed
than further processed to become a solar cell. The                       in a range of thicknesses with two different laser
laser offers a number of advantages when cutting                         sources i.e. with a high beam quality CW single
these very brittle and fragile wafers i.e.                               mode fiber laser and also with a high beam quality
                                                                         pulsed Nd: YAG laser. For a number of years pulsed

Nd: YAG lasers have been the laser of choice for                              reliable and compact solution for microcutting and
cutting silicon wafers. At wavelengths of around                              micro joining. The diode- pumped technology offers
1µm, focusing optics is smaller and simpler to enable                         low maintenance cycles and high conversion
smaller spot sizes than equivalent CO2 lasers. The                            efficiency. Theoretical pump- light conversions of
need for more efficient, compact and high beam                                more than 80% are possible [16] but typical optical
quality lasers for very fine micromachining has                               conversion efficiencies for Ytterbium double- clad
fuelled the rapid growth for developing fiber lasers.                         fiber lasers are 60-70% [17]. Average power levels
                                                                              up to 100W are possible with air- cooling. Since the
                                                                              overall efficiency is high, most fiber lasers are
Single mode (SM) fiber lasers                                                 powered by standard 110V/230Vsupplies. The
                                                                              benefits of the all-fibre design have been widely
Unlike a conventional laser which is constructed                              discussed elsewhere [18], with two of the principal
from components such as mirrors, rods and lenses a                            advantages being the absence of optical alignment
fiber laser is an all bulk material device. The laser rod                     and exposed optical surfaces. Chief amongst these
is effectively substituted by a length of doped active                        enabling components are the pump combiner and
fiber several meters long. The rest of the components                         Bragg grating reflector.
are also all fiber devices each of which are spliced
together to form a laser resonator. An immediate
advantage of this is that there is no need for
alignment, adjustment or subsequent cleaning of                               Nd: YAG laser              vs.    fiber    laser       overall
optical surfaces once the laser has been built, which                         differences
means little or no maintenance requirement. Figure 1
below shows a schematic of such a fibre laser cavity
of GSI design. The output fibre has a single mode                             An area where there is a significant difference
core with a diameter of less than 10 microns which                            between lamp-pumped YAG and fiber laser
ensures a high beam quality output. This architecture                         performance is pulsed operation. Lamp-pumped
is designed to be capable of producing output powers                          lasers are capable of producing long, multi-ms, pulses
of up to 500W with a 100,000 hour diode ensemble                              with peak powers many times the rated average
lifetime in a water-cooled heatsink configuration and                         power of the laser, provided that the duty cycle is
up to 120W using forced air-cooling at ambient air                            sufficiently low (Figure 2). This ability stems from
temperatures of up to 35 deg C with the same level of                         the flash-lamp itself which is often more constrained
diode reliability.                                                            by the maximum average thermal load than the peak
                                                                              power output.

                                                                              By contrast, while the semiconductor laser diodes
                                Yb                                            used to pump a fiber laser can be on-off modulated
   Fused                       Doped                                          over a wide frequency range as shown in Figure 3
   Pump                         DC                                            (from DC to tens of kHz in most industrial
  Combine                                                                     applications), they cannot typically be over-driven for
                                                             Beam             long periods (multi-ms), in the same way as a flash-
     r                                                      Delivery          lamp, without reducing the lifetime of the device to
                                                             Optic            an un-acceptable level.

                                                                              Considering the differences in beam quality and
                                                                              pulsed performance between the two types of laser,
                                                                              there are different operating regimes for the two
                                                                              types of laser. The lamp-pumped YAG laser is
                                                                              characterised by long high-energy pulses but poorer
                                                                              beam quality, and the fibre laser with high repetition
                                                                              rate on-off type modulation, single-mode beam
                                                                              quality but low pulse energy. From an applications
  Figure 1 Schematic diagram of single mode fiber                             perspective, both these regimes have their advantages
                      laser                                                   and these are discussed in the following sections.

These low power fiber lasers are very compact and
robust and has an edge over lamp pumped Nd: YAG
lasers in terms of beam quality and wall plug
efficiency (approx 20%). Current investigations show
that the single –mode fiber laser is an efficient,

         5            Standard                            Shaped Pulse
                                                                                                           Table 1: JK100P performance data
                                                        18 Joules                                        Laser parameters                                                       JK100P
                      12 Joules                         4msec@ 2kW
 Peak                 6msec,                            10msec @ 1kW
Power                 2kW peak                                                                      Maximum average power                                                         100W
                                                                                                     Maximum pulse energy                                                             0.25J

                                                                                                        Pulse width range                                                     15µs-200µs
             0                 10                20                  30         40                       Frequency range                                                      350-2000Hz

                                    Time (msec)                                                             Peak power                                                            10kW

        Figure 2: Basic laser pulse of LPSS laser with                                                    Beam quality1                                                       5mm.mrad
              an initial spike and shaped pulse
                                                                                                          Fiber diameter                                                         100µm
                                                                                                  Half angle radius


        Po w er (W)





                  0      100        200   300                 400   500   600    700

                                                Tim e (m s)

                                                                                                           Figure 4: JK100P (lamp pumped pulsed
            Figure 3: Fiber laser- modulation characteristics                                                          Nd: YAG laser)

                                                                                                                                   Typical long pulse 450Hz, 520V, 6.5J/P


                                    Experimental work                                                              0.01

                                                                                                              Relative Intensity
        The cutting tests were carried out with two different                                                     0.006

        laser sources i.e. lamp pumped pulsed Nd: YAG laser                                                       0.004

        and SM fiber laser respectively. The wafer                                                                0.002

        thicknesses ranged from 0.25mm up to 2mm thick.
                                                                                                                          -50      0           50           100         150     200      250

        Pulsed Nd: YAG laser
                                                                                                                                                        Time usec

        The silicon wafer cutting with a pulsed Nd: YAG
        laser was carried out with GSI latest low power
        pulsed laser (JK100P, Figure 4) and Figure 5 shows
        typical output pulse form and the beam profile.
        Combination of highest beam quality, small spot
        sizes, high peak power (10kW), short pulses (15-
        200µs) and high repetition rate (2000Hz) makes this
        laser ideal for cutting thick sections of silicon wafers.
        The full laser specification is highlighted in Table 1.

                                                                                                  Figure 5: Laser output pulse and the beam profile at
                                                                                                     the focus position of JK100P Nd: YAG laser

Single mode fiber laser                                          The output from the SM fibers can be either CW or
                                                                 can be modulated at high frequency i.e. up to 50 kHz.
Cutting tests were carried out with GSI’s SM                     GSI low power (up to 200W) SM fiber laser also
Ytterbium fiber laser operating at 1080nm                        have peak power enhancement capabilities which
wavelength emits a gaussian beam with an M2 < 1.10               under certain conditions offers significant benefits in
(Figure 6). Typical beam profiles of the SM fiber                processing speed and quality without affecting diode
laser at 100W, 200W and 400W are highlighted in                  performance. Figure 8 shows modulated and peak
Figure 7. The high brightness of the fiber laser enable          power enhancement output wave forms.
high power densities even at modest power levels,
which is sufficient for cutting a range of thin metals,
capable of welding of various materials including                      160

high reflective material and can also be used for                      140

abating aerospace materials.                                           120


                                                                   Power 80




                                                                              0               50   100           150      200         250          300   350        400   450   500
                                                                                                                                Time use cs

                                                                                          Peak power enhancement waveform, 50W
                                                                                           average power, 120W peak power
      Figure 6: JK SM fiber laser (50W-400W)
                                                                                                              Typical pulse with relaxtion oscillation




                                                                             Power W



               100W SM fiber laser M2<1.05                                               30              35                40                 45               50          55         60
                                                                                                                                         Time use cs

                                                                                                   Modulation output waveform

                                                                              Figure 8: Typical output waveform of SM fiber laser

                                                                 Cutting trials

                200W SM fiber laser M2<1.06                      Both fiber and pulsed Nd: YAG lasers were fitted
                                                                 with GSI micro cutting nozzle the calculated spot
                                                                 size for the fiber laser with this arrangement was
                                                                 approx 10-15µm and approx 60µm for pulsed Nd:
                                                                 YAG laser. The nozzle assembly was fitted with a
                                                                 copper cutting nozzle and adjusted to give ~ 1mm
                                                                 standoff height between the nozzle tip and the
                                                                 workpiece. Straight line cuts were made in a range of
                                                                 thicknesses with nitrogen assist gas. The cut edges
                                                                 were examined under SEM for microcracking and the
                                                                 amount of dross attached to the underside of each cut.

                 400W SM fiber laser M2<1.1

        Figure 7: Beam profiles of SM fiber lasers

               Results and discussion                               Table 2: Typical cutting speeds with SM fiber laser
SM fiber laser cuts
                                                                   Thickness      Laser       Output         Peak       Cut
The SM fiber with its high beam quality is cable of                 (mm)          power      waveform       power      speed
giving very small spot (10-20µm diameter) idea for                                 (W)                       (W)      (m/min)
producing good quality cuts in a range of materials                                100                         -        >3.5
including silicon wafers. The initial cutting trials with            0.25                    CW
                                                                                    80                       130        >4.5
the SM fiber laser up to 200W are very encouraging                   0.25                     PP+
                                                                                   100                         -        >2.5
up to 1mm thick wafers in terms of cutting speed and                 0.30                    CW
                                                                                    80                       150        >4.0
micro cracks <5µm. The edge quality was slightly                     0.30                     PP+
                                                                                   170                         -        >1.5
batter than that achieved with pulsed Nd: YAG lasers                 1.0                    Modulated*
                                                                                   170                         -        >0.5
because striations appeared to be orders of magnitude                1.5                    Modulated*
                                                                                   170                         -        >0.2
less than those with Nd: YAG lasers.                                 1.8                    Modulated*
                                                                                   170                         -       >0.15
                                                                     2.0                    Modulated*
                                                                                   350                         -        >5.5
                                                                     1.5                    Modulated*
                                                                                   350                         -        >3.5
                                                                     1.8                    Modulated*
Tests were also carried out to cut thick section wafers                            350                         -        >2.5
                                                                     2.0                    Modulated*
(up to 2.0mm). The results show that it is only
possible to cut this thickness with modulated output.
The single mode fiber laser used in this work can also
be pulsed by gating at high frequency (50 kHz) and                                 +
                                                                              Peak power enhancement
pulse widths as short as 5µs can be achieved. With                    *Modulated frequency range between 5-15 kHz
CW output the cut quality was poor in terms of
attached dross and microcracking because of
excessive heat input. The modulated output produced
better edge quality but the cut edge quality was still
not good as the thin sections (1mm thick), indicating
that the power density at workpiece is not sufficient
enough to give clean cut. There was a marked
improvement in the edge quality of the thick sections
as the average power was increased to 400W.

The results also show that with peak power
enhancement which is feature of GSI’s 100W SM
fiber, the cutting speed for 0.25mm thick wafer at
100W increased by 29% and edge quality was
characterised by no dross compare to the CW cut.

Typical cutting speeds achieved with the SM fiber
laser up to 400W are highlighted in Table 2 and
micrographs of the cut edge with different outputs are
shown in Figures 9-12.

                                                                            Figure 9: SM fiber cut; 2.0 mm thick wafer,
                                                                            10 kHz, 180W, >0.15m/min, nitrogen assist gas

                                                              CW output (100W), cutting speed>3.5m/min

 Figure 10: SM fiber cut; 1.8 mm thick wafer,
10 kHz, 350W, >3.5m/min; nitrogen assist gas
                                                       Peak power enhancement; average power 80W; peak power
                                                               130W; 5 kHz; cutting speed>4.5m/min

                                                       Figure 12: Cut quality comparison between CW and peak
                                                        power enhancement, 0.25mm thick, nitrogen assist gas

                                                       Pulsed Nd: YAG laser cuts

                                                       The combination of high peak power (10kW), short
                                                       pulses (15-200µs) and high repetition rate (2000Hz)
                                                       makes this laser ideal for cutting thick sections of
                                                       silicon wafers (up to 2mm thick) compare to SM 100
                                                       and 200W fiber lasers. High power density at
                                                       workpiece allows this laser to cut thick section with
                                                       very good edge quality. There is no evidence of any
                                                       dross or resolidified molten material along the cut
                                                       edge. The microcracking was very similar to the cuts
                                                       made with SM fiber laser i.e.≥10µm in length and
                                                       >1µm wide.

                                                        Table 3 highlights the cutting speeds for different
                                                       thicknesses and Figures 13-14 show typical
                                                       micrographs of the cut edge.

 Figure 11: SM fiber cut; 2.0 mm thick wafer,
10 kHz, 350W, >2.5m/min; nitrogen assist gas

Table 3: Typical cutting speeds with pulsed Nd: YAG
                   laser (JK100P)

Thickness (mm)      Average      power    Speed (m/min)

      0.25                  80                   >2.7

      0.40                  80                   >1.2

      0.50                  80                   >0.8

      0.60                  80                  >0.65

      0.80                 100                   >0.5

       1.0                 100                   >0.4

       1.4                 100                   >0.3

       1.8                 100                   >0.2

       2.0                 100                  >0.15

                                                                    Figure 14: Pulsed laser cut; 2.0 mm thick wafer,
                                                                           .>0.15m/min; nitrogen assist gas


                                                                 Cutting tests carried out with both pulsed Nd: YAG
                                                                 and SM fiber lasers on mono and polycrystalline
                                                                 wafers has shown that:

                                                                 Both SM fiber and pulsed Nd: YAG lasers are
                                                                 capable of cutting these materials successfully

                                                                 100W SM fiber laser with its very small spot size is
                                                                 very good for producing very smooth cut edges in
                                                                 thin sections <1.0mm

                                                                 To cut thick section wafers (i.e. >1mm thick) with
                                                                 SM fiber laser, high power (200-400W) is required to
                                                                 produce good quality cuts i.e. dross free and
                                                                 reasonably high cut speeds.

                                                                 With its high peak power and pulse energy pulsed
                                                                 100W Nd: YAG laser is well suited for cutting a
                                                                 range of thicknesses (up to 2mm).

                                                                 The microcracking of the cut edges was very similar
                                                                 for both LPSS and SM fiber laser.
  Figure 13: Pulsed laser cut; 1.4 mm thick wafer,
          .>0.45m/min; nitrogen assist gas

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[4] Richerzhagen, B, Delacrétaz, G and Salathé, R
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[8] Chang, J, Warner, B, Dragon, E and Martinez, M,                                 Meet the author
“Precision micromachining with pulsed green lasers”;
Journal of Laser Applications; Volume 10, Issue 6,              Dr. Mohammed Naeem is Materials Process
pp. 285-291; December 1998                                      Development Group Leader. He received an MTech
                                                                degree in metallurgical quality control from Brunel
                                                                University (UK) in 1981 and a Ph.D. in glass fibre
[9] Naeem, M, “Precision Material Processing with
                                                                composites from Loughborough University of
Fiber Delivered Nd: YAG Laser, Conference
                                                                Technology (UK) in 1985. He has over 18 years of
Proceeding LAMP, Kyoto Research Park, Kyoto,
                                                                experience in the support of industrial lasers with
Japan; May 16-19, 2006
                                                                GSI Group, Laser Division and has published over
                                                                150 papers on laser material processing. He has
[10] Tool, G and Geerligs, L; “ Quantifying surface             previously served as Materials Processing Manager
damage by measuring mechanical strength of silicon              and held several Important Engineering Development
wafers”; 20th EPVSEC, Barcelona, June 2005                      roles.


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