M.S. Amer et al- Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers

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Femtosecond versus nanosecond laser machining: comparisonof induced stresses and structural changes in silicon wafers

M.S. Amera,*, M.A. El-Ashrya, L.R. Dosserb, K.E. Hixb, J.F. Maguirec, Bryan Irwind

a Department of Mechanical and Materials Engineering, Wright State University, 3640 Colonel Glenn HWY, Dayton, OH 45435, USAb Mound Laser and Photonics Center Inc., Miamisburg, OH 45342, USA

c

AFRL, Materials Directorate, WPAFB, OH 45433, USAdSciperio Inc., Stillwater, OK 74075, USA

Received in revised form 12 July 2004; accepted 7 August 2004

Available online 12 October 2004

Abstract

Laser micromachining has proven to be a very successful tool for precision machining and microfabrication with applications in

microelectronics, MEMS, medical device, aerospace, biomedical, and defense applications. Femtosecond (FS) laser micromachining

is usually thought to be of minimal heat-affected zone (HAZ) local to the micromachined feature. The assumption of reduced HAZ is

attributed to the absence of direct coupling of the laser energy into the thermal modes of the material during irradiation. However, a

substantial HAZ is thought to exist when machining with lasers having pulse durations in the nanosecond (NS) regime. In this paper,

we compare the results of micromachining a single crystal silicon wafer using a 150-femtosecond and a 30-nanosecond lasers.

Induced stress and amorphization of the silicon single crystal were monitored using micro-Raman spectroscopy as a function

of the fluence and pulse duration of the incident laser. The onset of average induced stress occurs at lower fluence when

machining with the femtosecond pulse laser. Induced stresses were found to maximize at fluence of 44 J cmÀ2 and 8 J cmÀ2 for

nanosecond and femtosecond pulsed lasers, respectively. In both laser pulse regimes, a maximum induced stress is observed at

which point the induced stress begins to decrease as the fluence is increased. The maximum induced stress was comparable at

2.0 GPa and 1.5 GPa for the two lasers. For the nanosecond pulse laser, the induced amorphization reached a plateau of

approximately 20% for fluence exceeding 22 J cmÀ2. For the femtosecond pulse laser, however, induced amorphization was

approximately 17% independent of the laser fluence within the experimental range. These two values can be considered

nominally the same within experimental error. For femtosecond laser machining, some effect of the laser polarization on the

amount of induced stress and amorphization was also observed.

# 2004 Elsevier B.V. All rights reserved.

PACS: 61.18j Other methods of structural determination; 61.82 fk Radiation effect of semiconductors; 81.65 b Surface treatment

Keywords: Raman spectroscopy; Laser machining; Micromachining; Silicon amorphization; Induced stress; Femtosecond laser machining

www.elsevier.com/locate/apsusc

Applied Surface Science 242 (2005) 162–167

* Corresponding author. Tel.: +1 937 775 5095; fax: +1 937 775 5009.

E-mail address: [email protected] (M.S. Amer).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2004.08.029



1. Introduction

Lasers have been widely utilized in metallic

materials machining since the early 1970s [1]. Morerecently, lasers have been utilized in machining non-

metallic materials such as ceramics, plastics, various

composites, and semiconductors (e.g., silicon, silicon

carbide, etc.) for a number of industrial applications

[2,3]. The ability of lasers, especially pulsed lasers, to

precisely machine micron and sub-micron features in

otherwise hard to machine materials such as ceramics

and semiconductors has created a rapidly growing

interest in understanding the parameters controlling

the limits and the capabilities of this process [4]. A

large number of studies have been devoted to

investigate laser-based micromachining that covered

the different aspects of the machining process [1,4–8]

and the physics of laser/material interaction [9,10].

The development of femtosecond (FS) lasers and their

initial application to the machining of a variety of

materials has created huge interest in their micro-

machining potential.

Current reasoning suggests that the pulse duration

of a femtosecond laser is so short such that there is

not suf ficient time for any of the pulse energy to be

distributed to the substrate in the form of heat. Thus,

particularly for low pulse energies, there should be noheat-affected zone (HAZ) resulting from the proces-

sing. This is in direct contrast to nanosecond (NS)

machining, which has an associated HAZ. The

magnitude of the HAZ is a direct result of the

machining parameters and can be minimized for

nanosecond micromachining. However, the HAZ is

only one aspect of how a material is altered during the

machining process. It is important to also look at the

stresses induced as a function of laser parameters.

Previously, the authors utilized micro-Raman spec-

troscopy to investigate induced stresses and struc-tural changes in single crystal silicon as a result of

nanosecond pulsed laser machining [11]. This paper

reports the results of an initial study to compare the

magnitude of the stresses induced by femtosecond

and nanosecond micromachining of single crystal

silicon.

A variation in both the temporal and spatial

distributions of the laser energy exists in the

experiments performed using the FS and NS laser

systems. The temporal distribution is attributed to the

difference in the laser pulse duration while the spatial

distribution is attributed to different laser focal spot

sizes. Studying the effects of the temporal distribution

is the primary goal of this paper. However, thevariation in the spatial distribution of the laser energy

can be accounted for by normalizing the pulse energy

with respect to the area over which it is delivered.

Thus, the data should be interpreted in terms of energy

density or fluence (J cmÀ2) to understand the

machining effects of variations in the pulse duration.

2. Experimental procedure

2.1. Micromachining

Laser micromachining experiments were per-

formed using solid-state laser systems having pulse

durations in the nanosecond and femtosecond regimes.

The nanosecond machining was performed using the

Spectra Physics frequency tripled YHP40-355

Nd:YVO4 laser. The wavelength was 355 nm and

the pulse duration was nominally 30–50 ns depending

on the pulse repetition rate. The linearly polarized

laser beam was scanned across the stationary<1 1 1>

single crystal Si substrate using the ScanLab

HurrySCAN10-355 galvanometer laser beam scanner.The theoretical focal spot size was 13 mm. A single

pass of the laser beam was used to machine one groove

for each of the experimental conditions. The incident

laser pulse energy was varied from 3.0 mJ to 193.3 mJ.

The pulse repetition rate and scanning speed were

varied to maintain a constant pulse overlap.

The femtosecond laser micromachining was

performed using the Clark MXR laser system. The

wavelength was 775 nm and the pulse duration was

nominally 150 fs. The laser beam was focused to a 50-

mm spot. The machining was performed by translatingthe Si wafer at 1 mm/s through the focal region for all

trials. The pulse repetition rate was held constant at

1 kHz, and the pulse energy was varied from 17.5 mJ

to 950 mJ. Experiments were also performed using a 1/

4-wave plate to circularly polarize the beam. Under

these conditions, the pulse energy was reduced

slightly and experiments were performed from

16.5 mJ to 930 mJ. All grooves were machined using

a single pass of the laser beam. Beam cross sections

were measured using beam profilers. No attempt was

M.S. Amer et al. / Applied Surface Science 242 (2005) 162–167 163



made to change the beam cross sections during

machining.

2.2. Micro-Raman spectroscopy

Micro-Raman measurements were conducted using

a Renishaw1 2000 model. The excitation laser used

was Ar+ ion laser with a wavelength of 514.5 nm.

Laser power at the sample was kept at 2.5 mW to avoid

local heating of the wafer. An area of 100 mm Â

20 mm around each laser-machined groove was

mapped using a step of 1 mm to produce the reported

stress and amorphization maps. The Raman machine

was calibrated using an untreated area of the same

(1 1 1) silicon wafer to determine the stress free peak

position at 520 cmÀ1 [12]. Hence, the stresses

measured and reported represent the stresses directly

resulting from laser machining. Induced stress was

calculated from the shift in the silicon peak position

using the experimentally measured value for the

Raman shift/stress relationship of 1.55 cmÀ1 GPaÀ1

and not the value of 2 cmÀ1 GPaÀ1 obtained from

theoretical calculations [13]. The zero stress value for

silicon peak position was set to the value obtained

from the same silicon wafer away from the laser

machined region (about 10 mms). The local induced

stress was calculated as follows:Local stress (GPa) = (local peak positionÀ520)/

1.55. Details of such calculations are explained

elsewhere [11]. Also, the amorphization induced

was calculated from the relative integrated intensity of

the amorphous silicon (a-Si) peak that occurs at

490 cmÀ1 [14].

3. Results and discussions

Raman spectra obtained from the laser-machined

silicon wafer are best fitted by three peaks. The very

well characterized peak around 520 cmÀ1 resulting

from the triply degenerate optical mode in single

crystal cubic diamond phase of silicon (Si-I) [15], a

lower intensity peak around 510 cmÀ1, and a broad

peak around 490 cmÀ1 that is related to amorphous

silicon [16]. The origin of the peak at 510 cmÀ1 is not

clear. It was previously assigned to either hexagonal

structure of bulk silicon (Si-IV) or nano-crystals of

silicon [17]. A typical Raman spectrum along with the

lorentzian fitting used to determine the characteristics

of each peak is shown in Fig. 1. Fig. 2 shows typical

color maps of the stress distribution measured within

the laser-machined region superimposed on optical

micrographs of the area mapped. It is important to note

that due to the complexity of the relationship between

Raman frequency and individual components of the

stress tensor, a direct estimation of individual

components of the stress tensor (even under theassumption of plane stress situation) is practically

impossible [18]. However, due to the fact that no

splitting in the triply degenerate peak around

M.S. Amer et al. / Applied Surface Science 242 (2005) 162 – 167 164

Fig. 1. Typical Raman spectrum obtained from the machined area. The Raman spectrum was best fitted to three lorentzian peaks.



520 cmÀ1 was observed, the shift in the Raman peak

position can be assumed to be due to uniaxial stress

normal to the groove direction [19].The focus spot size of the two lasers was

considerably different (13 mm for nanosecond and

50 mm for femtosecond), and therefore the data were

plotted as a function of fluence (J cmÀ2) to normalize

the results to the same focal area. The resulting data

are shown in Figs. 3a–c and 4a–c. Fig. 3a–c shows that

the stress induced by the FS laser reaches a maximum

before that of the NS laser. This is reasonable since the

laser energy delivered by the femtosecond laser occurs

over a time period nearly five orders of magnitude

shorter than the nanosecond laser, which results in avery high peak power at the substrate surface

(TWcmÀ2).

At fluence levels that are only slightly above the

damage threshold for the substrate, the femtosecond

laser–material interaction is purely ablative. It is

interesting that the induced stress in this region is

greater than for the nanosecond laser and increases

more rapidly to a maximum value of $1.1–1.4 GPa

before diminishing to a value of $1.0 GPa. It is

believed that the initial increase in stress for the FS


Fig. 2. Colored map of the stress distribution in and around the

machined groove and an optical micrograph of the mapped area.

Fig. 3. Average induced stress as a function of laser fluence for (a)

femtosecond linearly polarized, (b) femtosecond circularly polar-

ized, and (c) nanosecond laser. Lines are added to guide the eye.



laser machining may be due to laser induced shock and

that the decrease in stress at higher fluences may be the

result of plasma heating providing a stress-relief or

annealing mechanism. It is interesting to note thatalthough the femtosecond interaction induces stress

more rapidly, the nanosecond interaction ultimately

induces more stress at the higher fluences. It is

believed that this is due to the thermal component

(excessive thermal loading of the substrate) of the

nanosecond laser/matter interaction.

The experimental results showed also that lower

stresses were induced when the FS laser was circularly

polarized. This is very interesting result, however, the

exact relationship between laser polarization and

induced stresses in the machined substrate is not clear

at this stage and will be addressed in future

investigations.

From the experimental data it is clear that it is

more dif ficult to avoid induced stresses when

machining with the femtosecond laser. Thus, it is

very important to machine the substrate near the

threshold for ablation. The data also suggest that more

experiments should be performed at lower fluences.

The optimal processing window for low induced-

stress machining requires relatively low fluence.

Although nanosecond lasers can induce more stress,

they can be readily controlled to maintain the inducedstress level at or below that of the femtosecond laser.

Ultimately, the selection of the appropriate laser for a

particular machining application will be made based

upon the significance of induced stress in the final

product and time required to complete the machining

operations.

Fig. 4a–c shows the dependence of the induced

amorphization as a function of laser fluence.

Amorphization is constant with respect to fluence

for the femtosecond laser over the experimental range

studied in this paper. Likewise, the induced amorphi-zation is also constant for the nanosecond interaction

except for fluence less than 20 J cmÀ2, in which case it

decreases sharply. Knowing that the 514.5 nm laser

optical penetration depth in silicon is around 770 nm

and assuming the Raman cross section for both

crystalline and amorphous silicon are equal, the 20 Æ

5% amorphization observed can be translated into

amorphous layer thickness of 150 Æ 40 nm. The

thickness of such amorphous layer will definitely

affect the induced stress in the silicon substrate.


Fig. 4. Averageinduced amorphization as a function of laser fluence

for (a) femtosecond linearly polarized, (b) femtosecond circularly

polarized, and (c) nanosecond laser. Lines are added to guide the

eye.



4. Conclusions

Micro-Raman spectroscopy was utilized to investi-

gate induced stresses and amorphization in lasermachined silicon wafers and to compare such induced

changes for femtosecond and nanosecond laser machin-

ing. Contrary to expectations, femtosecond laser was

found to induce significant stress and amorphization in

single crystal silicon. Results showed that induced

stress depends on the laser fluence and reaches a

maximum around 50 J cmÀ2 and 25 J cmÀ2 for nano-

second and femtosecond lasers, respectively. The

maximum stress observed for nanosecond laser was

higher than that observed for femtosecond lasers. It was

also observed that circularly polarized femtosecond

laser induced less stresses that the linearly polarized

laser. Amorphization induced ranged around 20 Æ 5%.

There could be a polarization effect for femtosecond

laser machining on induced changes in the substrate.

Such effects require further investigation.

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M.S. Amer et al- Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers