DISCO Technical Review Feb. 2016

Thermal effect of stealth laser dicing
Engineering R&D Division, Laser Engineering Department/ Sales Engineering Department

Abstract
Stealth dicing (SD) equipment has features such as dry processing, a narrow kerf width, and no chipping.
Thus, SD processing is employed for cutting LED chips and MEMS. The SD processing method, which
irradiates a high transparence laser beam on the workpiece, is not able to fully absorb the laser using a
focus point. As a result, the thermal effect on the dice has been attracting interest. This review presents
the absorption features near the focus point when performing SD processing on silicon and simulates thethermal effect on the device surface.

1. Introduction
Stealth dicing (SD) is an innovative dicing
technology to cut a workpiece in two stages. In
the first stage, a pulsed laser beam transmissive
to wafers is focused at the inside of a workpiece,
thereby causing noticeable nonlinear absorption
only in the vicinity of the focal point. A localized
high-temperature region is generated as a resultand forms a high-transfer-density layer called amodified layer. At the same time, a vertical crackdevelops[1]. In the second stage, the crack
extends upward and downward to the front andrear surfaces when an external force is applied.

the focus point, and part of the beam penetrates
into the bottom surface because a highly
permeable laser is employed. Fig. 1 is aschematic view showing the method ofirradiating a laser beam from the backside, whichis required for the process of stealth dicing
before grinding (SDBG). Since the laser beam isirradiated from the opposite side of the surface
on which devices are integrated, it is necessary toconsider the thermal effect of a through beam on
the device surface, in addition to the effectcaused by the heat generation near the focalpoint.

SD has features such as no chipping, a narrow
kerf width, and completely dry processing,compared with conventional blade dicing. Thus,
SD has been attracting recent attention as aunique way of cutting various types ofsemiconductor device wafers.

Beam absorption and thermal diffusion in the SD
process occur in a tiny area of several tens of
micrometers in a short time of several
microseconds. Thus, it is extremely difficult tomeasure transitional temperature distribution
through an experiment. In recent years, however,numerous research studies were conducted on thedependency of the absorption coefficient of thelaser wavelength on temperature based on
nonlinear absorption theory. The mechanism of
the SD process has been further clarified throughsimulations using thermal conductivity analysis
models that take the absorption, heat generation,and the thermal diffusion of the laser beam intoaccount[1][2].

The SD process, however, has a problem: theirradiated laser beam is not fully absorbed at、

1lncidentbeam
Modifiedlayer

入HIIHIIIIItlll
HIHI

At DISCO, with reference to the methods
mentioned above, we examine physical modelsof the SD process for various materials such as
silicon (Si), sapphire, and silicon carbide (SiC) toevaluate the thermal effect of a through beam on
the device surface, in addition to the temperature
increase at the focus point. This review presents
simulation results using Si wafers.

Siwafer

Fig.1 Schematic view of the SD laser irradiationfrom backside

1DISCO Technical Review Feb. 2016
where I(t) is the intensity of the laser beam attime t, I0 is the peak intensity, tp is the pulse
width, and θ is the waveform adjustment
coefficient.

2. Overview of a laser absorption andthermal analysis model
In a physical model of the SD process, a laser
beam propagates as a Gaussian beam, and linear
absorption occurs near the focal point. It is
assumed that a through beam is absorbed by amechanical layer (copper (Cu) film) provided on
the bottom surface, and there is no energy loss
other than lens aberration and beam absorption
such as scattering.

Further, the intensity distribution D(r, z) of thelaser beam at position (r, z) in the single-crystal
Si is calculated as shown below, in accordancewith the Gaussian beam-propagation equation:
D (r , z ) =dりーー

Imagine a column-type coordinate system withthe z-axis as an optical axis and the r-axis in theradial direction in the Si wafer. As shown in Fig.
2, the single-crystal Si wafer is divided into
many microvolume elements. The beamabsorption amount and the thermal diffusion
amount from the peripheral for each volume
element per unit of time are calculated using thefinite difference method. Then, temperature
variations in Si and the bottom Cu layer are
calculated along the elapsed time of the pulsed
beam irradiation.

2 r 2 exp  −2 
πR ( z ) 2
 R ( z )  (Equation 2)
2R( z ) = R0 1 + (λπR02) 2 (z − z 0 )2(Equation 3)

where Z0 is the focal point position, and R(z) andR0 are the laser beam diameters at z and z0,respectively.
The beam absorption amount at position (r, z) attime t is expressed as Equation 4, in accordancewith Equation 1, Equation 2, and theBeer-Lambert law:

Q(r, z, t) = A(r, z)I (t)D(r, z)(1 − exp(− α (r, z, t) × dz))
(Equation 4)

The beam absorption amount and thermal
diffusion amount are mainly calculated based on
the following equations.

where α(r, z, t) is the beam absorption coefficient,and A(r, z) is the coefficient to satisfy a law ofenergy conservation.

The pulse waveform (time distribution of energy)of the laser beam can be described with thefollowing equation:

Temperature T of the volume element at time t iscalculated by solving the following thermal
conductivity equation:1/ 2
 (t − 2t )2 ln 2 pt exp  −I(t) = I 0 
3 ―t 2ln 2  2π J()

θ



ρC(Equation 1)∂T 1 ∂  ∂T  ∂  ∂T = rK  +  K  + Q(r, z, t)∂t r ∂r  ∂r  ∂z  ∂z (Equation 5)

where ρ is the density, C is the specific heat, andK is the coefficient of thermal conductivity.

三3. Analysis example
A thermal analysis is simulated of the inside of
the Si workpiece having the thickness of 750 µm
and the Cu layer on the bottom surface when thelaser is irradiated from the Si surface under theprocessing parameters in Table 1.

Table 1 SD processing parameterson.lS

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EnginePulse energy

Focal point position(depth from thesurface)

SDE06

21.1 µJ

700 µm

Fig.2 SD physical model2

DISCO Technical Review Feb. 2016a)
140

1E3dE
3』＝
3（ジ ）
」~

120

100︵0086A↓

40，0＼二,‑
20 ‑
I5 ‑
IO ‑
505IO
15Distancefrombot
tomcenter(μm)

20Time(relative)

Fig.4 Radial distribution of the maximum
temperature of the bottom Cu layer⑥⑤④③①) t→ 宕 コ OSL,0S95dap百ュg←b②

1
401
10℃0℃

↓↓Eュ ON ←mmμμ0022↑↑Fig. 4 shows the distribution of the maximum
temperature of the bottom Cu layer in the radialdirection. The center of the through beam
(positioned directly underneath the focal point) is
the start point. The maximum temperature at thecenter position is approximately 700ºC, which isrelatively high, but the temperature drastically
drops at the area distant from the center. Thetemperature at a distance from the center of 10µm (assumed to have a street width of 20 µm) is
approximately 60ºC, and is only 40ºC when its
distance from the center is 15 µm. Thus, it can beconsidered that there is almost no thermal effectof the through beam on the device surface unlessthe street is too narrow (20 µm or less).

701)
()℃

1000℃
500℃

Fig.3 a) Waveforms of the through beam andincident beam. b) Changes of temperatures withtime near the focal point and bottom center ofthe workpiece

Fig. 3 a) shows the waveform from the start tothe end of one pulse irradiation. As shown in Fig.
3 b), in the initial stage of irradiation ((1) and (2)),

As shown in Fig. 5, in accordance with the factthat the focal point position (depth from theincident surface, see Fig. 2) becomes shallower
from 700 to 680, and to 570 µm, the temperature
at the center on the bottom drops drastically. For
example, by making the focal point position
shallower by only 20 µm from 700 to 680 µm,
the temperature at the center on the bottom
becomes almost half its previous value. On theother hand, the distribution in the radiusa temperature increase in the vicinity of the focal point
is not high enough, so noticeable nonlinear absorption
does not occur, and most of the incident beam
penetrates into the bottom surface. As a result, the Cu
at the bottom layer absorbs the through beam, and its
temperature rapidly rises. In accordance with theJ.
L︵ ビ て 目 で JdUI

elapsed irradiation time, the through beam issuddenly weakened at the timing of (3). This isbecause the processing transfers to a phase of anabrupt increase in temperature (nonlinearabsorption) where the absorption coefficient
becomes sufficiently high and absorption anddiffusion rapidly increase. Then most of the laser
beam is absorbed. As a result, a modified layer(SD layer) is formed inside the Si. On the otherhand, the temperature of the Cu layer gradually
decreases owing to thermal diffusion because
heat generation by the through beam becomesalmost zero. Immediately after the end ofirradiation (6), heat remains around the focal
point and on the bottom surface. However, since
the temperature returns almost to room
temperature before the arrival of the next pulse,it is anticipated that the effect of heataccumulation is minimal.

‑20‑15‑10‑505101520Distancefrombot
tomcenter(μm)

Fig.5 Radial distribution of the bottom surface
temperature at the different focal point positions

3

DISCO Technical Review Feb. 2016
direction of the energy density becomes broader
as the focal point position becomes shallower. In
the end, the optical energy entering the area apartfrom the center increases, which means thetemperature becomes higher.
In order to suppress the temperature increase at
the center of the bottom surface, the focal point
position can be made shallower. However, thisincurs a risk in which the temperature of an area
apart from the center may slightly increase.

4. Conclusion
In the SD process using a laser beam highly
transmissive to semiconductor wafers, a laser
beam partially penetrates around the position
directly underneath the focal point. Simulation
results reveal that when the focal point is
specified at a place 50 µm above the bottom
surface, the maximum temperature at the point
directly underneath the bottom surface (Cu layer)
exceeds 700ºC. However, it is assumed that thetemperatures can be suppressed at low levels:
approximately 60ºC at a distance from the centerof 10 µm, and 40ºC at a distance of 15 µm.
Generally speaking, it is considered that a higher
temperature does not affect the device surfaces
as long as the street width is 20 µm or more.
The simulation also reveals that the effect of heatgenerated by the through beam is greatly
mitigated when the focal point position is raised
from the bottom surface.
At DISCO, we select processing parameters,such as laser power and focal point, which are
optimal for customer products through test cuts
while aiming at establishing a more reliable SDprocess.

Reference[1] K. Fukumitsu, et al., Online Proc.4th Int.
Cong. Laser Adv. Mat. Proc., 2006.
[2] E. Ohmura . “HeatTransfer - Engineering
Applications”, Vyacheslav Vikhrenko (Ed.),
InTech, 24

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