Subsurface Integrity Research in Ultraprecision Manufacturing
Jiwang Yan
Department of Mechanical Systems and Design, Tohoku University
Aoba 6-6-01, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
yanjw@pm.mech.tohoku.ac.jp

Abstract: Although “surface damage” in ultraprecision machining has been investigated extensively in the past decades, “subsurfacedamage”, i.e., internal defects of machined materials, is a new research focus. For an externally “perfect” surface, the depth and nature of thesubsurface damage influences the mechanical, optical and electronic performance of the surface. The production of subsurface damage-freecomponents is an essential task for both manufacturing industry and multidisciplinary researchers. In this talk, fundamental research results on
subsurface integrity from the author’s group will be introduced. Firstly, the formation mechanisms of subsurface damage in ultraprecision
machining will be discussed for various kinds of hard brittle materials, and then the characterization and evaluation techniques of subsurface
damage will be reviewed. Finally, a newly developed laser damage recovery process, which is effective for improving subsurface integrity, will
be introduced, and some future directions in this research area will be briefly mentioned.
Keywords: Ultraprecision machining, subsurface damage, surface integrity

1.

Introduction

Ultraprecision machining of hard brittle materials in the micro-nanoscale is an area of focused research interests. The manufacturing of
defect-free components on these materials is indispensable for
production of precision optics, optoelectronic elements, and microelectromechanical systems. It has been well accepted that a nominallyhard brittle material can be deformed and removed in a ductile mannerin ultraprecision machining processes, such as cutting, grinding andlapping, yielding continuous chips and extremely smooth surfaces [1].
However, these mechanical machining processes will inevitably cause
subsurface damage to the crystalline structures due to the contact stressbetween the tool and the workpiece. It is the depth and nature of thesubsurface damage that influences the mechanical, optical and electronic
performances of the final products. Therefore, “subsurface integrity” isan essential aspect of ultraprecision machining technology. In this talk,experimental and simulation results on microstructures and mechanicalproperties of machining-induced subsurface damage will be presented,
and nondestructive characterization of the subsurface damaged layers
using micro laser Raman spectroscopy will be introduced. Moreover, anano-pulsed laser irradiation technology for reconstructing themachining-damaged lattice structure will be demonstrated. In thisabstract, the results for single crystalline silicon, which is a dominantsemiconductor material, will be shown as an example.
2.

Subsurface damaging mechanism

A number of previous authors used cross-sectional transmission
electron microscopes (TEM) to observe the machined subsurface
structures. TEM studies of diamond-turned silicon surfaces revealed thata thin (a few tens to one hundred and a few tens nm-thick) amorphouslayer was formed by machining, below which is a region of crystal
about a few Pm deep. The depth and the microstructure of thesubsurface damage layer were found to be strongly dependent on themachining conditions, such as tool take angle and depth of cut, andcrystalline orientation of the workpiece [2]. It is interesting that for ahighly negative rake angle, the thickness of the amorphous layer isapproximately equal to the depth of cut.

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An abundance of literature has demonstrated that silicon undergoes
phase transformation under hardness indenters and in other situations
where high hydrostatic pressure exists. It is generally accepted that astructural transformation from diamond cubic (Si-I) into a metallic state

E-Sn (Si-II) that could occur under the indenter during loading due tohigh hydrostatic pressure (10~13 GPa). Material around the indenter
would then become sufficiently ductile to sustain plastic flow. After theindenter is unloaded, the pressure-induced metallic phase does nottransform back to the diamond cubic structure, but instead, changes toan amorphous phase or other metastable phases [3].
Diamond tool

Amorphous chip

Micro-crystalline grains
Threshold pressure forphase transformation(~ 10 GPa)

Edge radius

Metallicphase

Threshold pressure fordislocation initiation
Loading

Amorphous layer
Dislocations
UnloadingFig.1 Schematic model of subsurface damage formation

Similarly, in ultraprecision machining, the hydrostatic pressure canbe sufficiently high to make silicon undergo phase transformation. Fromthe viewpoint of both phase transformation and dislocation, a model ofsubsurface damage mechanism in ductile machining of silicon is
proposed as schematized in Fig. 1. As soon as the tool advances into thematerial (loading), a phase transformation from diamond cubic structure(Si-I) to metallic phase (Si-II) occurs in the material surrounding thetool tip. This part of the material becomes sufficiently ductile to sustain
plastic flow which facilitates ductile-mode material removal. After thetool has passed (unloading), the metallic phase does not transform backinto the diamond cubic structure, but rather, changes into an amorphous
phase. As a result, the final subsurface damage layer contains anamorphous phase rather than the metallic phase. The metallic phase does
not transform to other metastable phases presumably because the1unloading speed in cutting is by far higher than that used in theindentation tests. In Fig. 1, as the distance from the tool tip increases,
the pressure decreases and finally drops below the threshold pressure
required for phase transformation (~10 GPa). However, the pressure
may still be high enough to facilitate dislocation initiation in silicon if a
sufficiently high shear stress exists. As a result, a dislocation layer isgenerated beneath the amorphous layer as the tool passes [2]. The phasetransformation of silicon during machining has double-faced effects. On
the one hand, due to the transformation from diamond cubic to metallicstructure, the material around the tool becomes sufficiently ductile tosustain plastic flow. This facilitates the ductile regime machining. On
the other hand, the phase transformation from metallic to amorphousgives rise to the residual amorphous layer on the produced surface,which affects the subsurface integrity.
3.

Characterization of subsurface damage

Subsurface damages, especially amorphous layer, will significantly
influence the mechanical, optical and electrical functions of silicon parts.
For example, when considering micromechanical applications wheresurface contacts and/or frictions exist, the mechanical properties of theamorphous layer, such as hardness, elasticity and plasticity, become very
important. The subsurface damages also influence the subsequent wafermanufacturing processes. That is, a machining operation always
involves multiple tool passes due to the cross feed; thus, with the
exception of the first cut, all subsequent cuts are made upon an
amorphous material and not the starting crystalline material. From thispoint of view, it is essential to clarify the mechanical properties of thisamorphous layer and to quantitatively measure the thickness of thislayer.
In a previous work [4, 5], we performed nanoindentation tests on
ultraprecision ductile-machined silicon wafers. As the indentation
behavior is dominated by the machining-induced amorphous layer for
low loads and shallow depths, this situation can be simply considered asa thin film of amorphous silicon formed on a pristine crystalline
substrate. Therefore, if a suitable indentation load is used, it may bepossible to detect the presence and the mechanical property of thisamorphous layer by nanoindentation. We found that when the maximum
load is smaller than a critical value, the hardness of the diamond-turned
silicon wafer becomes distinctly lower than that of the pristine silicon.
In this case, the indentation response will be mainly due to theamorphous layer; thus, it can be concluded from these results that themachining-induced amorphous silicon is softer than diamond-cubic
silicon. This surface-softening phenomenon is very different fromconventional metal machining, where the near-surface layer always
becomes harder due to the work-hardening effects. The work-hardening
effect of metal is due to dislocation activities; while thesurface-softening effect during silicon machining is caused by phasetransformation. It was also found that the machining-induced amorphoussilicon has significantly higher micro plasticity than diamond cubic
silicon. Therefore, it is the property of the amorphous phase that ineffect dominates the machinability of silicon. From this aspect, we cansay that when dealing with ductile machining technology the mechanicalproperties of the amorphous layer should be considered.
To measure the depth of the phase transformation layer resulting
from machining, we performed laser micro Raman spectroscopy
analysis (Fig.2). Laser Raman can detect the presence of amorphoussilicon as well as residual stresses in silicon wafers [6]. For bulk
crystalline silicon (c-Si), the triple degenerate optical phonons display inthe first-order Raman spectrum a sharp peak at the Raman shift of 521
cm-1, and for amorphous silicon (a-Si), the first-order Raman spectrum
reflects the phonon density of states and presents an optical band peak atISUPEN 2012

470 cm-1. In most of previous works, qualitative detection of structuralchanges of materials has been carried out using Raman shift information,
whereas the intensity information of the Raman scattering has not beeneffectively used. In our work [7], we use the Raman intensity tocharacterize the thin amorphous layers on the crystalline bulk material.
In order to compare the intensities of Raman scattering from theamorphous region and that from the crystalline region, we propose anew parameter, namely Raman intensity ratio, to represent the relative
significance of the two phases. We used the Gaussian distribution for
fitting amorphous peaks and the Lorentzian distribution for fitting
crystalline peaks, respectively. A strong correlation was identified
between the Raman intensity ratio and the depth of amorphous layer,
and the correlation was approximately linear within the experimentalrange. Therefore, by utilizing the linear relationship, it is possible for us
to quantitatively predict the depth of amorphous layers using the Raman
intensity ratio. This method can be used for quantitative measurement ofsubsurface damage depth of silicon. This measurement method is very
quick, inexpensive and completely nondestructive. It can be done atroom temperature, in air and without contacts, and can be used foron-machine measurement and mapping of specific surface areas.

Anti-Stokes
scatteringQQRIncident rayQ

RayleighscatteringQ

Stokesscattering
QQR

Atoms ofsubstanceFig.2 Principle of laser Raman analysis for subsurface damage

4.

Subsurface damage recovery by laser irradiation

Removing subsurface damage from silicon substrates is essential for
producing high-reliability silicon parts. This issue has become a subjectof concentrated research interests from multidisciplinary researchcommunities and industries. Currently, after the machining processes,
etching and chemo-mechanical polishing (CMP) processes are carriedout to remove the subsurface damaged layers. However, due to the poor
controllability of processing depth, the deterioration of substrate form
accuracy becomes a big problem. Other issues, such as increase inproduction cost, and environmental pollution due to the chemical waste
fluids, are also serious problems.
In our group, we proposed reconstructing the lattice structure of thedamaged layer by laser irradiation [8, 9]. The processing mechanism isschematized in Fig. 3. As amorphous silicon has a remarkably higher
absorption coefficient of laser light than crystalline silicon, there will besufficient absorption of laser in the near-surface layer to form a thinliquid silicon film. The liquid layer is metallic and has a much higher
absorption rate, and thus becomes thicker and thicker. The top-down
melted liquid phase finally extends below the deeper dislocated region.
During the period of melting, an initially rough surface becomes asmooth one due to the surface tension effect of the liquid layer. This issimilar to the manner in which a free droplet of liquid naturally assumes
a spherical shape to achieve a minimum surface area to volume ratio.
For a plane wafer, the surface area reaches a minimum when the liquid2thin film becomes completely flat at the surface. After the laser pulse
then, environmental cooling will result in a bottom-up epitaxial
regrowth from the defect-free crystalline region which serves as a seed
for crystal growth. The epitaxial regrowth mechanism in this case isdifferent from that in laser annealing technology, where amorphoussilicon is usually sputtered on substrates of different materials, such as
glass or sapphire. In such cases, no lattice-matched crystal seed exists,
thus poly-crystal structures will be formed. In this work, it is expectedthat by using the material self-organization phenomena induced by thenanosecond laser pulses, we might be able to achieve both a perfect
single-crystal subsurface structure and an extremely smooth surface atthe same time.
Nanosecond pulsed laser

Amorphous layer

Dislocation layer
Top-down melting

Surface flattening

(b)

(c)

processes: (i) It involves no material removal thus preserves thedimensions of the workpiece; (ii) It generates no pollutants; (iii) It
enables selective processing and processing of complex shapes likeaspherical and diffractive optical elements.
5.

Summary

Ultraprecision machining of hard brittle materials in the micro-nanoscale is technologically and economically important in modern society.
Subsurface damage due to high-pressure phase transformation is an
essential issue to deal with in ductile machining of semiconductormaterials. Micro laser Raman spectroscopy has been demonstrated to bean effective means of nondestructive evaluation for the damaged
subsurface layers. Laser is also effective to repair the subsurface damage.
Perfect single-crystalline structure can be reconstructed on amachining-damaged silicon substrate through a single nanosecond pulse
laser irradiation. The combination of ductile machining and laser
irradiation may lead to significant improvement of subsurface integrityin manufacturing industry.

Single crystalline bulk(a)Further reading[1]

Bottom-up
epitaxial regrowth
Damage-freesingle crystal
Seed forcrystal growth

Flat surface
Melting into
deep region[2](d)(e)(f)

Fig.3 Mechanism of laser recovery of subsurface damage
[3]

[4]

[5]

Fig.4 Cross-sectional TEM of silicon before and after later recovery[6]
Fig.4 shows a cross-sectional TEM micrograph of silicon sample
before and after laser irradiation. We can see that before laser irradiation,
there is an amorphous layer beneath the ground surface, and below theamorphous layer there is a layer of dislocations. After laser irradiation,
however, the amorphous layer and the dislocations have completelydisappeared, indicating that the recrystallization of silicon wasperformed successfully. A perfect single crystalline structure identical tothat of the bulk material has been achieved. From the results, we canalso see that after laser irradiation, the surface roughness has beenremarkably improved. That is, the grinding-induced tool marks on thewafer surface have been significantly flattened. The surface flattening
phenomenon might be a result of surface tension effect of the melted
silicon during the laser pulse. The proposed laser processing techniqueoffers a number of advantages over the conventional chemo-mechanical

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[7][8][9]J. Yan, M. Yoshino, T. Kuriagawa, T. Shirakashi, K. Syoji and R.
Komanduri: On the Ductile Machining of Silicon for Micro
Electro-mechanical Systems (MEMS), Opto-electronic and Optical
Applications, Materials Science and Engineering A, 297, 1-2(2001) 230-234.
J. Yan, T. Asami, H. Harada, and T. Kuriyagawa: Fundamental
investigation on subsurface damage in single crystalline siliconcaused by diamond machining, Precision Engineering, 33, 4(2009) 378-386.
J. Yan, H. Takahashi, X. Gai, H. Harada, J. Tamaki and T.
Kuriyagawa: Load effects on the phase transformation ofsingle-crystal silicon during nanoindentation tests, Materials
Science and Engineering A, 423, 1-2 (2006) 19-23.
J. Yan, H. Takahashi, J. Tamaki, X. Gai, H. Harada, and J. Patten:
Nanoindentation tests on diamond-machined silicon wafers,
Applied Physics Letters, 86, 5 (2005) 181913, pp. 1-3.
J. Yan, H. Takahashi, J. Tamaki, X. Gai, and T. Kuriyagawa:Transmission
electronmicroscopic
observation
ofnanoindentations made on ductile-machined silicon wafers,
Applied Physics Letters, 87, 11 (2005) 211901, pp.1-3.
J. Yan: Laser micro-Raman spectroscopy of single-point diamondmachined silicon substrates, Journal of Applied Physics, 95, 4(2004) 2094-2101.
J. Yan, T. Asami and T. Kuriyagawa: Nondestructive measurementof the machining-induced amorphous layers in single-crystal
silicon by laser micro-Raman spectroscopy, Precision Engineering,32, 3 (2008) 186-195.
J. Yan, T. Asami and T. Kuriyagawa: Response ofmachining-damaged single-crystalline silicon wafers tonanosecond pulsed laser irradiation, Semiconductor Science andTechnology, 22, 4 (2007) 392-395.
J. Yan, S. Sakai, H. Isogai, and K. Izunome: Characterization of
microparticles and oxide layers generated by laser irradiation of
diamond-machined silicon wafers, Semiconductor Science andTechnology, 26, 2 (2011) 025006, pp.1-7.

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