# Backside metallization affects residual stress and bending strength of the recast layer in laser-diced Si

T. Ziegelwanger, M. Reisinger, K. Matoy, A.A. Medjahed, J. Zalesak, M.

Gruber, M. Meindlhumer, J. Keckes

► To cite this version:

T. Ziegelwanger, M. Reisinger, K. Matoy, A.A. Medjahed, J. Zalesak, et al. Backside metallization affects residual stress and bending strength of the recast layer in laser-diced Si. Materials Science in Semiconductor Processing, 2024, 181, pp.108579. 10.1016/j.mssp.2024.108579. hal-05128608

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# Backside metallization affects residual stress and bending strength of the recast layer in laser-diced Si

<figure><img src="image_3.png" /></figure>

T. Ziegelwanger<sup>a, *</sup>, M. Reisinger<sup>b</sup>, K. Matoy<sup>c</sup>, A.A. Medjahed<sup>d</sup>, J. Zalesak<sup>e</sup>, M. Gruber<sup>f</sup>, M. Meindlhumer<sup>a</sup>, J. Keckes<sup>a</sup>

## ARTICLE INFO

### Keywords:

Ultra-thin Si dies

Nanosecond laser dicing

X-ray nanodiffraction

Local residual stresses

Die bending strength

## ABSTRACT

Thin Silicon dies separated by laser dicing form a thin layer via redeposition of ablated silicon known as recast layer. This work analyzed the influence of the recast layer microstructure and nanoscale residual stress gradients on the bending strength of bare and metalized silicon dies <100 µm. Scanning and transmission electron microscopy revealed an intricate microstructure of ablated silicon and elements of the wafer backside metallization within the recast layer. Refined silicon grains decorated by nanoscopic metallic precipitates at their grain boundaries were observed. Cross-sectional synchrotron X-ray nanodiffraction revealed that the altered microstructure increased the tensile residual stress from 200 to 295 MPa for bare and metalized dies, respectively. Additionally, the metalized die exhibited gradients in residual stress and grain size between the die front- and backside. Despite their similar frontside bending strengths of ~340 MPa, observed in 3-point bending experiments, a considerable strengthening of the backside from 425 up to 957 MPa was measured for bare and metalized die, respectively. The origins of the tensile residual stress and the influence of the backside metallization on the die bending strength are discussed.

## 1. Introduction

Constantly increasing demands on efficiency and performance have triggered the semiconductor industry to develop Si wafers of larger diameters up to 300 mm, while thicknesses decreased to several tens of µm. Consequently, this advancement has reduced resistance-capacitance losses and improved thermal management when stacked into a 3D integrated circuit [1–3]. One crucial milestone for achieving ultra-thin Si wafers of thicknesses below 100 µm was the adoption of laser dicing as a tool for wafer singulation instead of blade dicing. This innovation addressed the issue of chippage caused by blade dicing and increased the cost efficiency for small wafer thicknesses [4].

Laser dicing results in the formation of a heat-affected zone along the Si die sidewall and in the redesolidification of the ablated material as a thin layer, defined as the recast layer. The latter differs significantly from the pristine Si wafer in morphology, microstructure, and composition

and is the object of recent research. Previous investigations [5–7] by transmission electron microscopy (TEM) have revealed inhomogeneous microstructures of the recast layers consisting of (i) amorphous Si, (ii) polycrystalline Si and (iii) SiO₂, whereby their fractions depend on the laser parameters and the die structure. For example, femtosecond laser dicing has been shown to redeposit more amorphous Si [5], whereas picosecond and nanosecond laser dicing has produced more polycrystalline Si for wafers with a thickness of ~20 µm [6,7]. Generally, the recast layer on the chip sidewalls exhibits a high density of structural defects such as pores, grain boundaries, and dislocations.

To evaluate the strength and fracture behavior of brittle components containing different material classes, biaxial [8], and uniaxial [9] testing methods are employed. In the case of the multilayered Si dies, the assessment by the 3-point bending test and expression of bending strength is common [10]. The latter is usually determined for a set of samples to obtain a Weibull distribution [11] to address the failure

<footnote><sup>a</sup> Chair of Materials Physics, Department Materials Science, Montanuniversität Leoben, Franz Josef Straße 18, 8700, Leoben, Austria</footnote>

<footnote><sup>b</sup> KAI – Kompetenzzentrum Automobil- und Industrieelektronik GmbH, Europastraße 8, 95245, Villach, Austria</footnote>

<footnote><sup>c</sup> Infineon Technologies Austria AG, Siemensstraße 2, 9500, Villach, Austria</footnote>

<footnote><sup>d</sup> European Synchrotron Radiation Facility, 71 Avenue de Martyrs, 38043, Grenoble, France</footnote>

<footnote><sup>e</sup> Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringer-Strasse 2A, 5020, Salzburg, Austria</footnote>

<footnote><sup>f</sup> Chair of Structural and Functional Ceramics, Department Materials Science, Montanuniversität Leoben, Franz Josef Straße 18, 8700, Leoben, Austria</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: tobias.ziegelwanger@unileoben.ac.at (T. Ziegelwanger).</footnote>
probability for a given (tensile) stress and effective volume. However, the actual stress present at the critical defect may not equal to the stress applied during the testing. It may be necessary to consider also a local residual stress, which is superimposed on the applied stress during the chip bending. Therefore, the residual stress $\sigma_{res}$ within the recast layer of the sidewall is a factor that could influence the die's bending strength. However, it is rarely evaluated. Experimental data in Ref. [12] represents one of the few instances where the residual stress within laser-diced Si(111) wafers was assessed by micro-Raman spectroscopy, and an average tensile residual stress of 800 MPa with peaks up to 1.4 GPa was reported [12]. Building upon this result, a subsequent study by the same authors [13] used the same method to compare residual stresses after machining with a 30 ns and 150 fs laser and measured average stresses of 2.0 and 1.5 GPa, respectively. However, in both studies [12,13], no mechanical characterization of the Si wafer fracture properties was performed and correlated to the stress values. A link between residual stress and mechanical properties has only been drawn once as micro-Raman spectroscopy was performed on thin Si dies before performing 3-point bending. It was shown that the increasing power of the laser raised the tensile residual stresses within the die sidewall from 200 to 790 MPa, while simultaneously reducing the bending strength from 420 to 240 MPa [14]. However, the empirical nature of this investigation [14] does not address the origin of the residual stress or the influences of different process parameters. Although Raman spectroscopy has proven well suited to characterize the hydrostatic stress components at the surface of single crystals, individual components of the stress tensor are practically impossible to measure using this approach [12]. Furthermore, if the grain size is smaller than the Raman spot size, the grain boundaries are prone to interact with the Raman shift and distort the stress measurement. Comparative measurements on diamond thin films with Raman spectroscopy and X-ray diffraction (XRD) have shown that for small grain sizes, residual stress characterization by XRD is more viable, with a higher reliability of the quantified stress [15,16]. XRD is often selected to assess the magnitude, anisotropy, spatial distribution, and the role of residual stresses within the die recast layer. The recently established method of cross-sectional X-ray nanodiffraction (CSnanoXRD) has proven to be versatile in providing local residual stresses with a spatial resolution of up to 30 nm [17]. It has further given insight into local strain fields, and local phase compositions, even during *in situ* experiments [17–21]. Consequently, conclusions can be drawn on both the *microstructural gradients*, by investigating *intensity* and *full width at half maximum* of XRD peaks, as well as the *residual stress gradients*, by evaluating the ellipticity of Debye-Scherer (DS) rings.

Within this work, the spatial distribution of in-plane residual stresses

and microstructure within the recast layer is evaluated using CSnanoXRD and further correlated with the applied laser parameters and the observed fracture behavior of the chips analyzed by 3-point bending tests. A special focus is drawn to the characterization of local stress gradients along and across the chip sidewalls (between the die front- and backside) and the influence of metallic elements from the backside metallization.

## 2. Experimental

### 2.1. Sample processing

Structural and mechanical characterization was conducted on two varieties of dies, both separated, in a single pass, from a 12-inch Si(100) wafer using a DISCO DFL 7160 ns laser with a 29 ± 10 ns pulse duration and a wavelength of 355 nm. The laser power was at 6 W resulting in an effective fluence of 465 J/cm². These two types of dies will be subsequently referred to as a bare die and a metalized die. The sole distinction between them lies in the fact that the backside of the latter was coated with a ~500 nm thick contact metallization layer comprising Ti, NiV, and Ag. The final die dimensions were 4.8 × 3.15 mm² at a thickness of <100 µm. For mechanical testing, an additional subset of metalized dies was prepared by etching in a 40 wt% aqueous solution of ferritic chloride ($FeCl_3$) at 60 °C for 60 min, as practiced prior by Marks et al. [22]. The etching process removed the contact metallization on the die backside and according to Ref. [22] leaves the recast layer unaffected. Additional top-view SEM images of the recast layer before and after etching are shown in Suppl. Fi. 1 for comparison, indicating no obvious changes of the microstructure.

### 2.2. Electron microscopy

The morphology of the dies was investigated utilizing a Zeiss LEO 1525 scanning electron microscope (SEM) operated at an acceleration voltage of 3 kV and using an aperture of 30 µm. An electron transparent TEM lamella was prepared using an FEI Helios NanoLab 660 focused ion beam microscope (FIB) equipped with a $Ga^+$ ion source. The FIB operated with acceleration voltages ranging from 30 to 2 kV and ion currents spanning 20 nA to 40 pA. TEM investigations were conducted on an FEI Titan Thermis operating at an acceleration voltage of 300 kV in a scanning S(TEM) mode, primarily focusing on energy dispersive X-ray spectroscopy (EDS). This utilized a four-quadrant detector and a probe current of 1 nA. The results were treated using the Velox software and quantified through integrated standards.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** A schematic of the CSnanoXRD experiments performed at the ID13 beamline. The dies were probed using an X-ray beam with a cross-section of 70 × 70 nm². The samples were characterized in front (a) and edge (b) geometries in order to evaluate lateral and depth gradients of microstructure and stresses across the recast layers (cf. Fig. 2).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. SEM micrographs the recast layer surfaces for bare (a) and metalized (c) dies show similarities in morphology due to similar processing conditions. However the fracture cross-sections reveal vast differences in the cross-sectional microstructure for the bare (b) and metalized (d) dies. While in (b) a rather columnar microstructure is identified, the grains appear globular in the metalized die presented in (d).</figcaption></figure>

## 2.3. Synchrotron X-ray nanodiffraction

Microstructure and residual stress of the polycrystalline fraction of the recast layer were investigated using CSnanoXRD at the ID13 beamline of the European Synchrotron Radiation Facility in Grenoble, France [23,24]. Fig. 1 shows the two complementary sample mounting and CSnanoXRD setups used to investigate the recast layer. In the first setup, the front plane of the die (y-z plane) was investigated (Fig. 1a), and in the second setup, the sample was rotated by 90 deg, and the edge of the die (x-z plane) was investigated. These setups are defined as front and edge geometry going forward, respectively. For the scans in the edge plane, the die was thinned along the lateral direction prior to the synchrotron experiment, using a Hitachi E-3500 ion milling system with an acceleration voltage of 6 kV and a discharge voltage of 4 kV applied for 4 h. Therefore, the sample was thinned to a thickness of a few hundred µm in the beam (y) directions. Suppl. Fig. 2 provides a micrograph of the die sidewall before and after milling for comparison. These two setups were chosen in order to investigate gradients caused by the lateral movement of the laser as well as gradients caused between the die front- and backside.

The probing monochromatic synchrotron X-ray beam had an energy of 13 keV and was focused by a pair of multilayer Laue lenses (MLLs) to a cross-section of 70 × 70 nm². The Si dies were aligned through a sequence of knife-edge absorption scans, which involved performing line scans along the z-axis at various sample orientations φ. This process continued until the absorption contrast between the sample and the surrounding air reached its maximum. Mesh scans with dimensions of y × z = 40 × 10 µm² and x × z = 75 × 15 µm² at a step size of 100 nm were performed for the front and edge geometries, respectively. This resulted in a total of 40.000 detector images per image in front geometry. The diffraction signal was recorded using a Dectris Eiger X 4 M detector with an exposure time of 50 ms. The sample to detector distance and detector geometry were calibrated using a NIST Al₂O₃ powder. The data were evaluated using the software package PyFAI [25,26]. Two full circle Debye-Scherrer (DS) rings were recorded, caused by the diffraction of the Si(220) and Ag(111) crystallographic planes in the recast layer. The latter was only present in the metalized die. These DS rings were divided along the azimuthal angle into Δδ segments of 30 deg and afterward integrated individually. Then, the diffraction peaks were fitted using a Pseudo-Voigt function in each cake g in order to attain the maximum intensity $I^{hkl}(\delta)$, full width at half maxima $\beta^{hkl}(\delta)$, and peak positions $\theta^{hkl}(\delta)$ for every cake g and hkl reflection. Afterward, the 12 maximum

intensities $I^{hkl}(\delta)$ were averaged in order to obtain the local peak intensity for the Si 220 and Ag 111 diffraction peaks.

$$I^{hkl} = \frac{\sum_{g=1}^{12} r^{hkl}(\delta)}{12} \qquad (\text{Eq. 1})$$

The $\beta^{hkl}(\delta)$, were averaged similarly to attain $\beta^{hkl}$. The $\beta^{hkl}$ of the diffraction peaks correlates to the size of the coherently diffracting domains and density of structural defects such as dislocations and lattice distortions. By using the Scherrer Equation [27] a lower limit of the size coherently diffracting domains can be estimated:

$$D_{hkl} = \frac{K \times \lambda}{\beta^{hkl} \times \cos(\theta^{hkl})} \qquad (\text{Eq. 2})$$

K is a shape factor for the crystallite size and method of determination for the peak width, and is recommended to be K = 0.9 for this evaluation procedure [28]. λ is the wavelength of the probing X-ray.

For the evaluation of residual stresses within the recast layers, the assumption of an equibiaxial stress state was made, with approximately identical in-plane stress components, namely $\sigma_{xx} = \sigma_{yy}$. This simplification allowed to disregard the out-of-plane stress component $\sigma_{zz}$ as well as shear stresses $\sigma_{ij}$, in the interest of simplicity. First the fitted peak positions are transformed into lattice spacings using Bragg's law:

$$d^{hkl}(\delta) = \frac{\lambda}{2 \times \sin[\theta^{hkl}(\delta)]} \qquad (\text{Eq. 3})$$

Second the in-plane isotropic residual stress $\sigma_r = \sigma_{xx} = \sigma_{yy}$ is calculated from the ellipticity of the DS rings according to Ref. [29]:

$$\frac{d^{hkl}(\delta)}{\partial \sin^2 \delta} = \frac{1}{2} S_2^{hkl} \times d_0^{hkl} \times \cos(\theta)^2 \times \sigma_r \qquad (\text{Eq. 4})$$

In the case of Si 220 reflection, the X-ray elastic constant $\frac{1}{2}S_2^{220} = 0.725 \times 10^{-5}$ MPa⁻¹ was calculated using the Hill grain interaction model as reported in Ref. [30]. The unstressed lattice parameter of $d_0^{220} = 1.92$ Å was used. δ represents the azimuthal orientation along the DS ring (Fig. 1).

For comparison of different maps or different regions within a single map, histograms of the distribution of $\beta^{Si 220}$ and $\sigma_{yy}^{Si 220}$ were calculated. Therefore, the values of individual voxels of a specified area were grouped into segments of 0.01 deg and 50 MPa for $\beta^{Si 220}$ and $\sigma_{yy}^{Si 220}$, respectively. The number of voxels in each group was divided by the
total number of voxels counted, leading to a “normalized count”. These normalized counts are displayed as a function of the mean value of individual segments.

## 2.4. Mechanical testing

The die bending strength was characterized using uniaxial 3-point bending tests (3 PB). The tested dies were evenly picked from different wafer regions to avoid influences of the wafer processing. A universal testing machine Messphysik MIDI 10-5/6x11 equipped with a 20 N load cell of an accuracy of 1 % at 0.02 N was used to perform the tests. The samples were loaded at a rate of 0.5 mm/min, and the load span was chosen to be 4.2 mm. Both types of dies were tested at the front- and backside, indicating the side of maximum tensile strength. The applied stress was calculated as follows [10]:

$$ \sigma_{\text{app}} = \frac{3Pl}{2bh^2} \qquad (\text{Eq. 5}) $$

where *l* is defined as the loading span of the setup, and *b*, *h* are the width and thickness of the die, respectively, and *P* is the applied maximum load. All load-displacement curves showed a linear-elastic response. The maximum likelihood method was employed to derive the characteristic bending strength *σ*₀ and Weibull modulus *m*, using a minimum of 20 specimens per sample. These parameters represent the stress at which the probability of failure reaches approximately 63 %, and the slope of the Weibull fit, which signifies the scatter in bending strength.

# 3. Results

## 3.1. SEM analysis

SEM micrographs of both the bare and metalized dies displaying the surface morphology of the recast layer are presented in Fig. 2. Both show similar periodic grooves every ~25 µm along the surface, which are expected to be predetermined by laser machining speed and laser pulse duration. For closer investigation, fracture cross sections of the recast layers are presented in Fig. 2b and d for the bare and metalized dies, respectively. In both cases, the recast layers exhibited a distinct interface with the Si wafer and displayed varying thicknesses ranging from 500 nm to 2 µm. The fracture cross-section of the bare die (Fig. 2b) showed facets representing cleaved Si grains or remnants of former grain boundaries. Furthermore, smaller facets near the Si wafer interface gradually merge into larger facets towards the surface of the recast layer. This gradual microstructure indicates a competitive grain growth of the recast layer during the cooling down after dicing. Contrasting this, the recast layer of the metalized die (Fig. 2d), while of comparable size, exhibited a notably distinct microstructure at the die's backside. The observed grain size is considerably smaller than those in the bare die (Fig. 2b) and is decorated by a seam of bright features along the grain boundaries of the polycrystalline Si. This gave a reason for further TEM investigations.

## 3.2. TEM analysis

Further insights into the microstructure and composition of the recast layer of the metalized die were gained by evaluating a high angle annular dark-filed imaging (HAADF) micrograph in Fig. 3a and the complementary local elemental EDS maps in Fig. 3b-f. The micrograph shows a recast layer of ~1 µm in thickness, where the competitive grain growth results in the gradual increase of the grain size towards the recast layer surface (in agreement with Fig. 2d). Bright features of higher atomic mass (Z contrast) are visible in Fig. 3a, which are distributed preferably at the interface to the Si wafer, within the first 200 nm of the recast layer and along the grain boundaries of the relatively large columnar Si grains.

<figure><img src="image_6.png" /><figcaption>Fig. 3. The recast layer of the metalized die was investigated using HAADF-TEM (a) with a subsequent EDS characterization (b-f). The bright features along the grain boundaries are identified to be precipitates stemming from the backside metallization, either continuous (c-e) or globular (f).</figcaption></figure>

The EDS maps of the recast layer in Fig. 3b-f reveal several phenomena. Initially, the anticipated dominant concentration of Si is evident within both the wafer and the recast layer (Fig. 3b). However, there is a discernible depletion of Si along the interface between these two regions, as well as at the grain boundaries (Fig. 3b). EDS spectra analysis further revealed the presence of Ni, Ag, Ti, and V, alongside Si, although in diminishing quantities (Fig. 3c-f). These elements are anticipated to originate from the backside metallization process. The localized EDS maps for these four elements provide complementary information to the zones of Si depletion identified in the Si map of Fig. 3b. Notably, Ni, Ti, and V exhibit elevated concentrations at the interface between the wafer and the recast layer before forming a continuous network through segregation at the grain boundaries of the polycrystalline Si (Fig. 3b-f). This arrangement is likely to disrupt columnar grain growth and prompt renucleation of the ablated Si during the recast layer growth. Furthermore, the EDS map for Ag unveils distinct globular Ag precipitates embedded in areas of the Ni, Ti, and V phases, presenting an additional layer of complexity.

## 3.3. CSnanoXRD - front geometry

Results from the CSnanoXRD characterization in front geometry (Fig. 1a), containing 2D distributions of diffraction intensities, full width
at half maximum, and residual stress, are displayed in Fig. 4 for the bare and metalized dies, respectively.

The diffraction intensities of Si 220 reflections obtained from the recast layer of the *bare die* in Fig. 4a exhibited alternating intensity along the *y*-direction. The amount of polycrystalline Si is higher in the region between 0 < *y* < 10 µm and *y* > 30 µm. The *β*<sup>Si 220</sup> values shown in Fig. 4b are within the range of 0.1–0.2 deg, exhibiting an increased *β*<sup>Si 220</sup> in the area of lower Si 200 intensities (cf. Fig. 4a and b). The local residual stresses *σ*<sub>yy</sub><sup>Si 220</sup> (Fig. 4c) are generally tensile and at around 150 MPa, except for the center, where increased stresses of up to 600 MPa are present and align well with an overall lower intensity of the Si 220 reflection and higher *β*<sup>Si 220</sup> values.

The recast layer of the *metalized die* was analyzed in a similar manner, and the Si 220 peak intensities are depicted in Fig. 4d. Therein, alternating intensities comparable to those seen in the bare die (Fig. 4d) were evident. This is reasonable given the shared laser dicing parameters. The local β<sup>Si 220</sup> values along the recast layer are illustrated in Fig. 4e, spanning from 0.1 to 0.3 deg and increasing in regions of low Si 200 peak intensity. In Fig. 4f, the local σ<sub>yy</sub><sup>Si 220</sup> is overall higher than for the bare die in Fig. 4c, displaying regions of elevated stress which align with regions of increased β<sup>Si 220</sup>.

To facilitate a comparison between the full width at half maxima and residual stress data for both dies, the distributions were charted in two histograms in Fig. 5. The average β<sup>Si 220</sup> values ascend from approximately 0.16 deg in the bare die to around 0.19 deg in the metalized die, with a broader distribution evident in the latter (cf. Fig. 5a). This finding suggests a more refined, but less uniform distribution of Si crystallite sizes within the recast layers of the metalized die. The mean σ<sub>yy</sub><sup>Si 220</sup> increases from 200 to 295 MPa for the bare and metalized dies, respectively (Fig. 5b). The differences in full width at half maximum and residual stress between the two recast layers will be further discussed in Sec. 4.

### 3.4. CSnanoXRD – edge geometry

CSnanoXRD was used in edge geometry to evaluate microstructure and stress changes along the die thickness (x-axis in Fig. 1b). Since the probed area has a thickness of about 300 µm in *y* direction, local variation such as the previously described grooves (Fig. 2) are averaged in this setup.

The micrographs of the thinned *bare die* recast layer are shown in

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** The distributions of Si 220 peak intensities of the bare die (a) and the metalized die (d) provide insight into the morphology of the recast layers. The β<sup>Si 220</sup> in the bare (b) die shows little trends whereas the residual stress of this sample (c) shows an area of enhanced stress at *x* ≈ 20 µm.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** The distribution of β<sup>Si 220</sup> (a) and residual stress (b) for Fig. 4, allow for a comparison of the two samples. A shift of the distribution to higher β<sup>Si 220</sup> from bare to metalized die is accompanied by a similar shift in residual stress.</figcaption></figure>

Fig. 6, starting with the Si 220 peak intensities displayed in Fig. 6a. Therein, a relatively thick recast layer is observed near the frontside of the die (at *x* ≈ 5 µm) which first remains constant to *x* ≈ 30 µm and then decreases gradually in size and intensity to the die backside. The local β<sup>Si 220</sup> of the Si 220 peak is presented in Fig. 6b and ranges between 0.1 and 0.2 deg. The local σ<sub>xx</sub><sup>Si 220</sup> is displayed in Fig. 6c and varies mostly between 0 and 300 MPa except for some enhanced area at *x* ≈ 60 µm. Both properties lack any distinguishable gradient between die front- and backside and correspond well to the results obtained from the 2D maps

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Results from CSnanoXRD performed on the bare die show a thicker recast layer at the die frontside. Analysis of the $\beta^{Si 220}$ and the residual stress show a uniform distribution with no gradients between front- and backsides of the device.</figcaption></figure>

in the front geometry (cf. Fig. 4a-c). Noticeably, the observed Si 220 intensities extend (in x) to a width of ~80 µm, which is larger than the die thickness observed in the SEM image (cf. Fig. 2a). It can be estimated that due to a relatively large sample thickness of ~300 µm in the x-direction, a sample misalignment of ~5 deg is the reason and influenced the evaluated $\beta^{Si 220}$ and $\sigma_{xx}^{Si 220}$.

Results from the recast layer of the metalized die are presented (Figs. 7-9), where the Si 220 peak intensities are complemented by Ag 111 peak intensities. The observed Si 220 intensities (Fig. 7a) are comparable to the bare die as a thicker recast layer (Fig. 6a), and higher intensities are measured at the die frontside before declining to the

backside. Here, the measured width of the Si 220 peak intensities fits quite well to the die thickness observed in the SEM image (cf. Fig. 2c). Contrarily, the intensity of Ag 111 peaks is higher at the die backside where the contact metallization is located (Fig. 6b). However, it extends into the recast layer, meeting 10 % of the maximum peak intensity at $x \approx 35$ µm.

The local $\beta^{Si 220}$ are depicted in Fig. 8a and span in a range between 0.1 and 0.35 deg. For a better comparison, histograms were generated for dedicated regions and are displayed in Fig. 8b-d. The $\beta^{Si 220}$ commences with an average of 0.16° within the 10-15 µm range of the x-axis, then progressively climbs to an average of 0.28° within the 55-60

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Results from the CSnanoXRD on the metalized die show a recast layer of similar smaller elongation compared to the bare die (Fig. 6). The intensity of Ag 111 peak (b) has an inverse gradient to the Si 220 peak (a), extending from the backside metallization into the recast layer meeting 1 % of the maximum peak intensity at $x \approx 35$ µm.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 8. The $\beta^{\text{Si 220}}$ (a) exhibits a gradient from device frontside to the backside, in agreement with the Ag 111 peak intensity in Fig. 7b. $\beta^{\text{Si 220}}$ distributions of selected areas (b–d) show that the average $\beta^{\text{Si 220}}$ increases from an average of 0.16 deg at x = 10–15 µm up to 0.28 deg at x = 55–60 µm.</figcaption></figure>

µm range. This corresponds to a minimum size of coherently diffracting domains of 35 nm for the frontside and 20 nm for the die backside, according to Eq. (4).

Finally, the distribution of $\sigma_{xx}^{\text{Si 220}}$ is presented in Fig. 9a. It maintains an overall tensile state within the range of 0–600 MPa, similar to the stress distribution measured in front geometry (Fig. 4), therefore fitting the assumption of equibiaxial stress. A discernible gradient proportional to the rise of the Ag 111 peak intensities is observed, increasing from the die's frontside to its backside, further detailed through dedicated histograms in Fig. 9b–d. Within the initial 10–15 µm span of the mesh scan, the recast layer exhibits a mean stress of about 150 MPa, and subsequently elevates to approximately 390 MPa within the $x \approx 55–60$ µm range near the die's backside.

### 3.5. Mechanical properties

The bending strength of three types of bare, metalized, and etched metalized dies were characterized using 3-point bending experiments and applying force on either the die front- or backsides. For each category of sample, both the frontside and backside bending strengths were evaluated using a Weibull distribution approach, employing 20 samples for every die variant. The respective $\sigma_{\text{front}}$ and $\sigma_{\text{back}}$ distributions are depicted in Fig. 10, while the characteristic bending strength and Weibull modulus values are provided in Table 1 together with their 90 % confidence intervals.

The characteristic frontside bending strength of the bare and metalized die are 335 ± 10 and 343 ± 10 MPa, respectively. Since the 90 % confidence intervals are overlapping the difference in strength may not be of statistical difference, which is reasonable since both front sides have been processed similarly. As for the characteristic backside

bending strength, the bare die displayed a moderate increase up to 425 MPa, but the metalized die exhibited a characteristic strength of 957 MPa. This surpasses more than double the value of the bare die and nearly triples the frontside bending strength. Additional measurements conducted after eliminating the metallization from the backside (“etched” specimens in Table 1) revealed that both the frontside bending strength and backside bending strength decreased to 267 MPa and 861 MPa, yet still maintaining the strengthening observed.

### 4. Discussion

The aim of this work was to identify possible correlations between lateral and cross-sectional gradients of microstructure and residual stress within the die's recast layers and their bending strength. These correlations as well as the influence of the backside metallization will be discussed in this section.

#### 4.1. Morphology and microstructure

The dies characterized within this contribution exhibited recast layers of similar surface morphologies and periodic grooves with 25 µm intervals (Fig. 2a and c). These are commonly associated with the laser dicing process, and their period is predetermined by the laser's time profile and the lateral speed of the laser movement during dicing [31]. It results in a projection of the laser intensity profile on the die sidewall and forming the observed periodic grooves (Fig. 2).

The cross-sectional microstructure of the recast layer of the bare die exhibited an increasing grain size with growing layer thickness (Fig. 2b), which is typically also observed in physical vapor deposited thin films and can be attributed to competitive grain growth during deposition
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** The residual stresses in Si (a) are rising from device frontside to the backside, in agreement with the $\beta^{Si 220}$ in Fig. 8a and the Ag 111 peak intensity in Fig. 7b. Local stress distributions (b–d) quantify the average residuals stress at 190 MPa for the range of $x = 10–15$ µm, increasing to 390 MPa in the range of $x = 55–60$ µm.</figcaption></figure>

[32]. Close to the interface of the Si wafer, the cooling rate is the highest, leading to a preference for grain nucleation over grain growth. However, the cooling rate decreases as the recast layer gets thicker, giving enough time for grain growth. The facets with the fastest growth dominate and lead to an elongated grain columnar microstructure, as observed for the bare die (Fig. 2b).

For the *metalized die*, the grain size of Si crystallites within the recast layer close to the die backside was significantly smaller, as documented by the SEM micrographs (Fig. 2d) and the $\beta^{Si 220}$ data (Fig. 8). More precisely, the size of coherently diffracted domains evaluated from XRD data was 35 nm and 20 nm for the die front- and backside respectively, which is considerably lower than the ~100 nm grain size observed in Fig. 2d by SEM. While the statistics derived from the SEM micrograph are limited in scale for making precise quantitative statements, they suggest that additional factors may have contributed to the diffraction peak broadening. Since $\beta^{Si 220}$ is also sensitive to strains of 2nd and 3rd order caused by e.g. dislocations and lattice distortions, the difference between the grain size obtained by SEM (Fig. 2d) and the $\beta^{Si 220}$ data (Fig. 8) indicates a high amount of lattice defects within the Si crystallites.

Both the smaller grain size and higher strains of 2nd and 3rd order can be explained by the additional heat dissipation and consequent higher cooling rates provided by the backside metallization. Also, the formation of nanoscopic metallic precipitates along the grain boundaries of Si grains was observed (Fig. 2d and 3). The phase diagrams of Si and the diverse metals can provide further insight. The Si–Ti and Si–V phase diagrams exhibit eutectic points at high Si contents between Si and TiSi₂ or VSi₂, respectively [33,34], as well as a peritectic point for Ni [35]. This indicates a tendency to first form large amounts of Si grains till the concentration of the remaining metal is high enough to form silicides,

most presumably along the grain boundaries of the recast layer. The difference in the globular morphology of Ag can further be explained as Ag has no solubility with Si and a lower melting temperature than all other constituents [36]. As a result, Ag agglomerates at the grain boundaries and minimizes its interfacial area to the other silicide by forming globular particles (Figs. 2 and 3).

## 4.2. Residual stress gradients

Residual stresses in thin films are typically the result of the combined intrinsic and extrinsic residual stress contributions, denoted as $\sigma_i$ and $\sigma_e$, respectively [37]. The intrinsic stress (sometime denoted as growth stress) results from atomistic processes within the growing film, e.g., by adatom mobility, irradiation effects, microstructural defects, and grain boundary coalescence. Extrinsic stresses arise from changes in the physical environment of the film material following the growth. Typically, they develop after the deposition due to structural misfit between the film and substrate, mismatches in coefficients of thermal expansion (CTE), and physical-chemical loads applied to the thin film.

During the recast layers' growth at the laser dicing's high temperatures, a high adatom mobility and a high strain rate resulted in the formation of columnar grain microstructures (Figs. 2 and 3). It is expected that the grain growth and coalescence resulted in the formation of intrinsic stresses. Additionally, thermal stresses, a part of extrinsic stresses, were formed in the nanocrystalline recast layers during cooling down to room temperature due to the CTE mismatch. Grain boundaries within the nanocrystalline recast layers, which contain primarily weakly bonded atoms of shallow and asymmetric interatomic potentials increase the CTE [38]. In previous investigations, the CTE of single crystalline Si $\alpha_{sc}^{Si}$ and polycrystalline Si $\alpha_{pc}^{Si}$ thin films were measured at 2.6
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Weibull diagram showing the strength of 3-point bending experiments on all die types for front- (a) and backside bending strength (b), respectively. Although both types of die have shown similar frontside bending strength, the metalized die shows a tremendous strengthening for the backside, even after etching.</figcaption></figure>

<table><caption>**Table 1**
Bending strengths and Weibull moduli of the three die types.</caption><thead><tr><th></th><th>Bare Die</th><th>Metalized Die</th><th>Metalized Die (etched)</th></tr></thead><tbody><tr><td>σ<sub>front</sub> [MPa]</td><td>335<sup>+10</sup><sub>-10</sub></td><td>343<sup>+10</sup><sub>-10</sub></td><td>267<sup>+11</sup><sub>-11</sub></td></tr><tr><td>m [-]</td><td>13.8<sup>+3.5</sup><sub>-4</sub></td><td>14<sup>+4</sup><sub>-4</sub></td><td>10.1<sup>+2.6</sup><sub>-3.1</sub></td></tr><tr><td>σ<sub>back</sub> [MPa]</td><td>425<sup>+12</sup><sub>-12</sub></td><td>957<sup>+67</sup><sub>-62</sub></td><td>861<sup>+50</sup><sub>-47</sub></td></tr><tr><td>m [-]</td><td>13.9<sup>+3.5</sup><sub>-4</sub></td><td>14<sup>+4</sup><sub>-4</sub></td><td>10.1<sup>+2.6</sup><sub>-3.1</sub></td></tr></tbody></table>

and $3.4 \times 10^{-6} \text{K}^{-1}$, respectively [38]. Therefore, the greater shrinkage during cooling of the recast layer is constrained by the interface to the Si wafer, leading to an overall in-plane tensile residual stress. This stress can be well estimated with the following equation:

$$ \sigma_{th} \approx (\alpha_{sc}^{\text{Si}} - \alpha_{pc}^{\text{Si}}) \times \Delta T \times E_{pc}^{\text{Si}} \quad (\text{Eq. 6}) $$

Investigations of Finn et al. [39] identify the heat-affected zone, i.e. the recast layer, as the point where the wafer temperature crosses the melting temperature of pure Si at 1685 K. Therefore, we assume a temperature difference $\Delta T$ of $-1392 \text{ K}$ for cooling. Finally, the in-plane equibiaxial elastic modulus of polycrystalline Si $E_{pc}^{\text{Si}}$ of 160 GPa was taken from the literature [40]. When inserted into Eq. (6), this results in a tensile residual stress of 178 MPa, which comes close to the mean observed residual stress of 200 MPa within the bare die. The difference can be attributed to the underestimation of CTE, as $3.4 \times 10^{-6} \text{K}^{-1}$ was measured for microcrystalline Si and not nanocrystalline as was present within the recast layer. Given the higher ratio of grain boundaries in nanocrystalline Si (compared to microcrystalline Si), it is reasonable to assume an even higher CTE. There is an additional thermal contribution for the metalized die, as not only the single and nanocrystalline Si differ in CTE but also the nanocrystalline metallic precipitates at the grain boundaries. For instance, a CTE of $14.5 \times 10^{-6} \text{K}^{-1}$ was reported for

TiSi₂ [41], which is significantly larger than that of polycrystalline Si. However, as only a small number of metallic particles along the grain boundaries was observed, only a trend can be estimated.

Previous studies reported stress characterization in Si(111) wafers using Raman spectroscopy [13]. The maximum tensile stress measured was 2 GPa for a nanosecond laser at a fluence of 45 J/cm², significantly greater than the maximum stresses measured in this contribution, around 600 MPa. However, it should be addressed that Raman measurements could not resolve individual components of the stress tensor but rather assume a uniaxial stress. This may be true if the wafer isn't fully separated as in Ref. [13], but the stress levels observed in Figs. 4 and 6 suggest an equibiaxial stress state. Therefore, the tensile residual stress of 2 GPa measured can't be translated to the actual residual stress within a processed die sidewall.

### 4.3. Mechanical properties

The characteristic bending strength of the dies was assessed using 3-point bending experiments. The bare die exhibited a characteristic frontside bending strength of 335 MPa, increasing to 425 MPa at the backside. This strengthening may originate in the gradient of the recast layer thickness observed in Fig. 6a. A thicker recast layer results in a higher probability of finding a defect of critical size and causing fracture.

The metalized die showed a comparable frontside bending strength of 343 MPa, but a tremendously higher backside bending strength of 957 MPa, which cannot be solely attributed to the thickness of the recast layer. One additional factor may be the additional support provided by the backside metallization. Therefore, a third sample set of etched dies was produced according to the procedure presented in Ref. [22]. The morphology of the recast layer surface was analyzed before and after the etching, revealing no alteration (cf. Suppl. Fig. 1). The etched dies exhibited bending strengths of 267 and 861 MPa for front- and back- sides, respectively. As the tremendous strengthening of the backside
bending strength remained even after removing the backside metallization, its cause has to be in the recast layer. However, the etching process resulted in an overall reduction of the bending strengths by 76 and 96 MPa for the frontside and backside, respectively. The small overall reduction of bending strength may potentially be a result of the etching process. The etching may cause a change of overall thickness not considered by the evaluation of the bending strength. Additionally, grain boundaries, which harbor weakly bonded Si atoms and metallic precipitations may serve as preferred sites to be attacked by the etching agent.

Previous investigations by Marks et al. [22] provide additional examples of such strengthening. 3-point bending experiments performed on 20 µm thick dies with a similar backside metallization exhibited 450 MPa and 1.014 MPa for front- and backside bending strength, respectively. This is in good agreement with the values obtained in this work, considering that the lower die thickness reduces the probability of large defects and enhances the bending strength. The authors explained the strengthening between frontside and backside bending strength by the differing thickness of the recast layer. However, as outlined before, the comparison to the bending strength of the bare die suggests additional factors.

## 4.4. Microstructure, residual stress and bending strength correlations

This work was conducted on the hypothesis that residual stresses influence primarily the bending strength of laser-diced Si dies. Therefore, gradual microstructures characterized using SEM (Fig. 2) and TEM (Fig. 3), were correlated with the gradients of residual stresses (Figs. 4–9). In the front geometry, areas of overlapping grooves exhibited an increase in $\rho^{\text{Si 220}}$ and $\sigma_{yy}^{\text{Si 220}}$, indicating the presence of smaller Si grains and/or a high density of crystalline defects. In the edge geometry, the metalized die exhibited an additional gradient of increased $\rho^{\text{Si 220}}$ and $\sigma_{xx}^{\text{Si 220}}$ aligned with the increased Ag peak intensity towards the backside of the recast layer. On close observation the largest span of $\sigma_{xx}^{\text{Si 220}}$ and $\rho^{\text{Si 220}}$ are in the area of 25–30 µm and the area of 40–45 µm, respectively (cf. Figs. 8c and 9b). This correlates to intensity maxima of the Ag 111 DS ring of 3 % and 10 % (of the maximum intensity observed at the backside metallization) in the former and the latter, respectively (Fig. 7). It further indicates that the metallic content of the recast layer between 25 and 30 µm is not high enough to cause renucleation and columnar grain growth is favored. Nevertheless, since these small amounts of Ag are still insoluble in the Si matrix, the elevated tensile residual stress is induced. The great distribution of $\rho^{\text{Si 220}}$ in the area between 40 and 45 µm, of almost bimodal nature might indicate the metallic elements might be sufficient to cause renucleation of the columnar Si grains. Renucleation was caused by nanoscale metallic precipitates (Fig. 3), which decorate grain boundaries of the recast layers, influencing not only the microstructure and residual stresses (Figs. 7–9) but also potentially impacting the bending strength of the die (Fig. 10, Table 1). Indeed, the greatest difference in die bending strength was measured between the backside breaking strength of the bare and metalized dies (Fig. 10), where the latter exhibited a characteristic fracture stress increased by a factor greater than two. Therefore, it is assumed that the drawbacks associated with the heightened tensile residual stress are offset by the advantages stemming from the altered microstructure. The initial hypothesis that residual stresses influence the die bending stress must be therefore refined, and particular attention should be given to the role played by the recast layers’ microstructure and the chemical composition of the recast layers’ grain boundaries.

First, as the open facets of the recast layer indicate intergranular fracture (Fig. 2), the reduction of Si grain size also reduces the critical defect size within the recast layer. Furthermore, the presence of nanoscale metallic precipitates at these grain boundaries appears to be another crucial factor in explaining the fracture strength increase. In

literature, similar phenomena have been addressed before for bulk materials [42–44] and thin films [45] and often referred to as *grain boundary segregation engineering*. For example, Meindlhumer et al. [45] have shown that CrN precipitates located at the grain boundaries of AlCrN coatings can benefit the mechanical properties and increase the fracture strength by altering the fracture mode. It is argued, that additionally to the size of the critical defect the energy necessary to create the new fully separated surfaces also plays a vital role [46]. Therefore, precipitates like $\text{TiSi}_2$ along the grain boundaries might beneficially influence the interfacial energy and increase the fracture stress of the recast layer. Further investigations could take one of two different approaches. First, Si dies without any metallization and of simpler architecture should be investigated to draw clear correlations between the laser dicing process and the residual stresses. Second, the influence of the backside metallization should be further investigated using simulations to compare the interfacial energy between two individual Si grains and a Si grain and the neighboring metallic precipitates. However, these investigations are outside the scope of this work.

## 5. Conclusions

Nanoscale cross-sectional and lateral residual stress gradients were observed within the recast layers of laser-diced Si dies. Two types of dies differing only in an additional backside metallization consisting of Ti, Ni, Ag, and V were investigated. The recast layers exhibited an overall tensile residual stress state with mean values of 200 and 295 MPa for the bare and metalized dies, respectively. Close observations of the metalized device’s recast layer using TEM and CSnanoXRD revealed an intricate microstructure of metallic precipitates along the grain boundaries. They led to a refinement of the surrounding Si grains at the cost of an enhanced tensile residual stress. As the backside bending strength more than doubled from 425 to 957 MPa, from bare to metalized die, the benefits of the microstructure and chemical composition of the recast layer’s grain boundaries outweigh the cost of increased tensile residual stress.

### CRediT authorship contribution statement

**T. Ziegelwanger:** Writing – original draft, Investigation, Conceptualization. **M. Reisinger:** Project administration, Conceptualization. **K. Matoy:** Validation, Resources, Conceptualization. **A.A. Medjahed:** Resources, Methodology. **J. Zalesak:** Investigation. **M. Gruber:** Methodology, Investigation. **M. Meindlhumer:** Supervision, Conceptualization. **J. Keckes:** Supervision, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

The provision of synchrotron facilities by the ESRF (European Synchrotron Radiation Facility) is gratefully acknowledged by T.Z., M. M. and J. K. CzechNanoLab project LM2023051 funded by MEYS CR is gratefully acknowledged for the financial support of the measurements/sample fabrication at CEITEC Nano Research Infrastructure. This work was funded by the Austrian Research Promotion Agency (FFG, Project No. 905107).
# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.mssp.2024.108579.

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