Large-area femtosecond laser milling of silicon employing trench analysis (2023)

Table of Contents
Article preview Abstract Introduction Section snippets Laser system Scribing of trenches Milling of cavities SOI-CMOS applications Conclusion CRediT authorship contribution statement Declaration of Competing Interest Acknowledgements References (54) Opt. Lasers Eng. Appl. Surf. Sci. Appl. Surf. Sci. Sol. Energy Mater. Sol. Cells Appl. Surf. Sci. Appl. Surf. Sci. Physics Procedia Thin Solid Films Appl. Surf. Sci. Appl. Surf. Sci. Ablation of metals by ultrashort laser pulses J Opt. Soc. Am. B, JOSAB Ultrashort-pulse laser machining of dielectric materials J. Appl. Phys. Femtosecond laser-induced periodic surface structures revisited: a comparative study on ZnO J. Appl. Phys. Femtosecond laser-induced periodic surface structure on diamond film Appl. Phys. Lett. Rapid fabrication of large-area concave microlens arrays on PDMS by a femtosecond laser ACS Appl. Mater. Interfaces Large area uniform nanostructures fabricated by direct femtosecond laser ablation Opt. Express Femtosecond laser machining of multi-depth microchannel networks onto silicon J. Micromech. Microeng. Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation Nanotechnology Ultrashort pulse laser dicing of thin Si wafers: the influence of laser-induced periodic surface structures on the backside breaking strength J. Micromech. Microeng. Ablation and cutting of planar silicon devices using femtosecond laser pulses Appl. Phys. A Femtosecond laser micromachining of crystalline silicon for ablation of deep macro-sized cavities for silicon-on-insulator applications Laser-based Micro- and Nanoprocess. XIII, SPIE Cited by (3) Recommended articles (6) Videos

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Volume 138,

June 2021

, 106866

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(Video) Alpan Bek | Behaviour of Neuro-2A Cells on Femtosecond Laser Micromachined Silicon Substrates rights and content


A femtosecond laser is a powerful tool for micromachining of silicon. In this work, large-area laser ablation of crystalline silicon is comprehensively studied using a laser source of pulse width 300fs at two wavelengths of 343nm and 1030nm. We develop a unique approach to gain insight into the laser milling process by means of detailed analysis of trenches. Laser scribed trenches and milled areas are characterized using optical profilometry to extract dimensional and roughness parameters with accuracy and repeatability. In a first step, multiple measures of the trench including the average depth, the volume of recast material, the average longitudinal profile roughness, the inner trench width and the volume removal rate are studied. This allows for delineation of ablation regimes and associated characteristics allowing to determine the impact of fluence and repetition rate on laser milling. In a second step, additional factors of debris formation and material redeposition that come into play during laser milling are further elucidated. These results are utilized for processing large-area (up to few mm2) with milling depths up to 200µm to enable the fabrication of cavities with low surface roughness at high removal rates of up to 6.9µm3 µs−1. Finally, laser processing in combination with XeF2 etching is applied on SOI-CMOS technology in the fabrication of radio-frequency (RF) functions standing on suspended membranes. Performance is considerably improved on different functions like RF switch (23dB improvement in 2nd harmonic), inductors (near doubling of Q-factor) and LNA (noise figure improvement of 0.1dB) demonstrating the applicability of milling to radio-frequency applications.


Laser micromachining is a tool which is becoming increasingly relevant for micro/nanostructuring of materials. The use of ultrashort laser processing with pulse width in femtosecond range is advantageous because it can be applied on a wide range of materials like metals, semiconductors, dielectrics, alloys, and ceramics [1], [2], [3], [4]. The range of surface processing is diverse varying from a small scale (a few nm2) [5], [6] to large scale (a few mm2) [7], [8], [9]. Heat affected zone is greatly reduced for ultrashort lasers which allows for enhanced machining quality with low thermal impact of laser radiation on material.

Femtosecond micromachining of silicon has been reported previously in different studies. It has been explored for various applications like microfluidics [10], photovoltaics [11], IC characterization [7], hydrophobic surfaces [12], and laser dicing [13], [14]. For this work, the targeted application is the fabrication of free standing membranes of SOI-CMOS RF circuits/functions on Silicon-on-Insulator (SOI) wafers. The term SOI-CMOS refers to CMOS processing technology used in semiconductor industry in combination with SOI wafer as the host substrate. SOI wafer comprises of a thick layer of handler silicon at the bottom, typically 750µm, topped with a thin isolating buried oxide (BOX) layer and finally a thin layer of active silicon on the top. Microelectronic devices and circuits are realized into the topmost silicon active layer. The buried oxide is a thin layer of silicon dioxide which provides electrical isolation between active and handler silicon. Handler silicon is present for mechanical support and thermal dissipation. Despite the presence of an isolating BOX layer, handler substrate degrades electrical performance because it offers a parasitic coupling path to RF signals. By locally removing handler silicon under the active area of circuits, they can be suspended on the BOX in the form of a membrane. The motivation for creation of such membranes is to attain better RF performance with reduced loss and improved linearity as it has been reported previously in numerous similar studies [15], [16], [17], [18], [19], [20], [21], [22].

The technique of laser milling is used in this work to create cavities underneath RF functions implemented in SOI-CMOS technology. The laser beam is raster scanned over the area to be milled. The starting thickness of SOI wafer is usually high (~750µm) in order to prevent wafer warping during fabrication. As a preliminary step, the wafer is thinned down using processes like grinding and chemical mechanical polishing (CMP). The final thickness of silicon is much smaller and depends on the application requirements. With this in mind, the laser milling process is studied for depths up to 200µm. The process is adaptable for geometries of side length ranging from ~100µm to several mm by appropriately defining the laser milling trajectory.

The objective of this study is the development of a milling methodology that enables the control of surface quality and roughness while keeping the volume removal rate high. During micromachining of silicon, a commonly identified problem is the appearance of surface corrugations [13], [23], [24], [25], [26], [27] which renders the surface cross section comb like as shown in Fig. 1. In the top view (Fig. 1c), these corrugations appear as microholes which can penetrate deep into the material depending on the process parameters used for milling. These corrugations are also observed in other materials like germanium [28]. They are not to be confused with the Laser Induced Periodic Surface Structures (LIPSS) which are ripples of the order of the wavelength of laser radiation [29], [30]. In this study, this problem is tackled by choosing appropriate process parameters to minimize formation of corrugations in order to keep the roughness low while still having sufficiently high rates of removal.

In this work, a systematic approach is employed whereby step-by-step insight is gained towards best parameters for laser milling. In the first step, trenches (grooves) are scribed in silicon by linear displacement of the laser beam. Scribing allows quick estimation of impact of different process parameters on milling without performing two-dimensional surface machining. Section 3 reports different trench measures that are going to be studied and quantified by using optical profilometer measurements. The understanding of these parameters lays the foundation for high quality and high removal rate milling. Section 4 subsequently proposes the study of the surface laser milling to further understand the aspects of changing surface morphology, effect of debris, surface roughness and sidewall quality. The application of laser milling is presented in Section 5 where membranes of RF circuits are fabricated. RF characterization of such circuits is performed in order to demonstrate the superior performance of the circuits on SOI membranes.

Section snippets

Laser system

The experiments are performed under ambient conditions using a Diode Pumped Solid State (DPSS) ultrafast fiber laser from Amplitude Systèmes (Tangerine) with a fundamental output at 1030nm and pulse width (FWHM) of 300fs. Additional wavelength output of 343nm is generated using a harmonic box (Amplitude-Systèmes). The 343nm wavelength is obtained by frequency tripling realized by cascading non-linear crystals, beginning with frequency doubling of the fundamental input beam at 1030nm and

Scribing of trenches

This section presents the discussion regarding the different experimental observations made in the scribing of trenches utilizing the parameters detailed in Section 2.2.

Milling of cavities

Section 3 provided an overview of trench scribing with different parameters and the optimal values have been highlighted. In this section, further analysis is presented for milling over a surface area using the chosen parameters and how process parameters are further refined in order to avoid high surface roughness.

Laser milling is performed by raster scanning the laser beam with line to line spacing (pitch) of 10µm. This value is chosen because it is close to the beam waist of 8.2μm. In a

SOI-CMOS applications

The previous sections have dealt with the fabrication of cavities. For application in SOI-CMOS technology, an etch-protect layer is first laminated onto the handler side of the SOI die. Milling is then performed in the area where membranes need to be fabricated. In order to speed up the process, a two-step milling strategy has been used in the applications. In the first step, a cavity with larger area is created at a higher removal rate (fast step). In the second step, the smaller cavity of


Large-area femtosecond laser milling of silicon has been reported which employs trench analysis. Five measures have been analysed for the trenches: i) average depth ii) recast layer volume (iii) average roughness (iv) trench width and (v) volume removal rate. Optical profilometry has been systematically used to obtain accurate and reliable measurements of all the parameters under study. Using these analyses, fluence and repetition rate conditions have been found suitable for laser milling.

CRediT authorship contribution statement

Arun Bhaskar: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Justine Philippe: Resources, Validation. Flavie Braud: Resources, Validation. Etienne Okada: Resources, Validation. Vanessa Avramovic: Resources, Validation. Jean-François Robillard: Conceptualization, Supervision. Cédric Durand: Investigation, Supervision, Project administration. Daniel Gloria: Investigation, Supervision,

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.


This work was supported by the French government through the National Research Agency (ANR) under program PIA EQUIPEX LEAF ANR-11-EQPX-0025, the STMicroelectronics-IEMN common laboratory and the French RENATECH network on micro and nanotechnologies. The authors also thank Oxford Lasers Ltd for helpful discussions on instrumental developments and laser processing techniques.

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  • Cited by (3)

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      Laser milling of silicon by IR picosecond pulses (λ=1030nm) has been investigated with a special emphasis on influence of laser fluence, spot size and spot overlapping on the surface roughening and ablation productivity. The detailed recipe for optimization of the silicon milling process has been formulated: reduction of the maximum laser fluence down to about ∼1J/cm2 by means of corresponding laser spot enlargement, while the ratio of the laser spot displacement to the spot radius is maintained below the level of 0.3. Fulfillment of these conditions provides minimal possible surface roughness at moderate reduction of the ablation productivity. The silicon ablation process under multi-spot irradiation was numerically simulated to specify which of two factors plays a decisive role in the surface roughening at different processing conditions: spatial non-uniformity of laser exposure or ablation-induced surface instabilities.

    • Substrate-Induced Dissipative and Non-Linear Effects in RF Switches: Probing Ultimate Performance Based on Laser-Machined Membrane Suspension

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    • Localized Backside Etching Structure of SOI Substrates on Total Ionizing Dose Effect Hardening for RF Applications

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