Single crystal silicon freestanding structures for tensile and fatigue testing were treated with KrF excimer laser to improve surface roughness and accordingly mechanical performance. Sample thickness was 5 μm. Localized laser treatment was successful in eliminating the scallops developed during Bosch process and in reducing surface roughness. Harsh irradiation at laser energies up to 4 J/cm 2 was only possible due to localized treatment without significant vibrations occurring on the freestanding samples that led to fracture in preliminary experiments at energies as low as 0.16 J/cm 2. Finite element analysis was used to investigate the temperature distribution on the irradiated structures. Atomic force microscopy (AFM) and Raman spectroscopy were also used to assess surface roughness, crystallinity changes and surface stresses developing on surfaces subjected to perpendicular laser irradiation. At a high energy (3.2 J/cm 2) the top surface showed a decrease of roughness compared to fabricated samples. Raman spectroscopy showed the dominance of crystalline silicon after laser irradiation. The effects of laser energy, number of pulses
Silicon is an abundant engineering material that can be processed into high purity grade miniaturized structures. Accordingly, it has been used extensively in the field of micro-electro-mechanical systems (MEMS). Some MEMs applications like micro sensors, optical mirrors and others require that the fabricated silicon structures endure mechanical loads; static or dynamic. One of the main factors affecting mechanical performance of single crystal silicon (SCS) structures is thought to be the fabrication process [
Different techniques have been investigated for surface roughness reduction of SCS structures. Laser irradiation has been employed for having waveguides with smooth sidewalls as well as for repairing damages resulting from machine cutting using a diamond tool of silicon wafers to eliminate dislocations and reduce surface roughness [
Laser treatment of silicon wafers and supported structures has been used for decades and the effect of the treatment was studied extensively. Only recently laser treatment has been used on freestanding microstructures and was mainly used for the purpose of curvature adjustment of silicon cantilevers or for solving cantilever beam stiction problems using nanosecond and fem to second lasers [
SCS beam structures were fabricated on a (100) silicon-on-insulator (SOI) wafer while being oriented along the <110> direction. There are two types of the structures subjected to laser treatment. The first type is the tensile test sample that is shown in
samples have scallops caused by the Bosch process as shown in
Samples were irradiated using a Coherent LAEX-1000 KrF (248 nm) excimer laser source with a pulse duration of 30 ns. The laser spot has a homogenous energy distribution over the spot size within 5%. The spot size at the mask is 20 mm square and the reduction ratio used is 8 leading to a spot size of about 2.5 mm square at the sample surface. A mask is used to control the spot size to have localized laser treatment. In preliminary trials no mask was used leading to a spot covering the whole chip carrying the structures. The mask was used later on to decrease the spot size to 250 μm ×100 μm for tensile test samples to cover only the gauge length while for the fatigue samples it was reduced to 100 μm square to cover only the notch area. The energy values varied between 0.8 J/cm2 and 4 J/cm2 and the number of pulses varied between a single pulse and 1000 pulses. The pulse rate was kept constant at 1 Hz. All the experiments were done in air at 1 atm and at room temperature.
For tensile test samples perpendicular irradiation of top surfaces have been used to investigate basic effects of laser irradiation on freestanding structures at different laser energies and number of pulses. Crystallinity, stresses, roughness and surface features were investigated.
Crystallinity and resulting stresses on the top surfaces were evaluated using micro Raman spectroscopy (HORIBA Jobin Ybon, Labram HR-800) using Ar laser (488 nm) having a spot size of 1 μm. Three locations along the top surface of the laser treated gauge length of tensile test samples were used for Raman spectroscopy assessment. Curve fitting and smoothing techniques were used for the resulting Raman spectra and quantitative analysis was used to evaluate the relative significance of the amorphous and crystalline silicon phases present. A ratio was defined as
where Ia is the sum of areas under peaks representing amorphous silicon while Ic is that of crystalline silicon after area normalization. This method is suggested by Yan et al. [
For the fatigue test samples, only sidewalls including the notch area were irradiated since the cracks are supposed to initiate at the notch [
Finite element analysis (FEA) was conducted to evaluate temperature distribution due to laser irradiation using ANSYS. The FEA model developed is a 3D model that is governed by the Beer-Lambert equation
where I(z, t) is the laser intensity at a depth z from the surface and time t, R is the reflectivity, Io(t) is the laser intensity at the surface and δ is the penetration depth of KrF laser in silicon taken to be 6 nm [
Sidewalls and top surfaces of tensile and fatigue test samples were observed using field emission scanning electron microscopes (FESEM); Hitachi SU-8000 and S-4500 to assess morphology.
Observations made on tensile test samples and fatigue test samples will help come to an understanding of the effect of different irradiation parameters on the freestanding structures at hand.
Material Property | Solid Silicon | Liquid Silicon | |
---|---|---|---|
Thermal conductivity (W・m−1・K−1) | 148 | 200 | |
Density (kg・m−3) | 2320 | 2500 | |
Heat capacity (J kg−1・K−1) | 710 | 680 | |
Preliminary experiments with no mask led to fracture of resonators at laser energies as low as 0.16 J/cm2. Localized laser treatment for irradiation of released silicon structures was successful by controlling the spot size. Laser irradiation with energy values that go up to 4 J/cm2 was achieved without fracture. For energies higher than 4 J/cm2, samples fractured even for single pulse treatments.
In order to investigate the effect of number of laser pulses during perpendicular irradiation, a single pulse of 2 J/cm2 sample was compared with a 10 pulses sample.
In order to investigate the effect of laser energy on morphology, topography, crystallinity and developing surface stresses, different samples were treated at different laser energies with the number of pulses kept constant at 10. FESEM images for top surfaces are shown in
In order to investigate the effect tilt angle on sidewall morphology, two samples were irradiated at 45˚ and 65˚ while laser irradiated using 10 pulses of laser at 4 J/cm2. The sample irradiated at 65˚ also had perpendicular irradiation of the top surface at the same laser conditions to evaluate irradiation at 0˚ as well.
curvature adjustment while
the crystal silicon peak of as fabricated samples. The dominant peak has shifted to the left for all irradiated samples indicating tensile stress development on the surface. The shift showed a general trend of decrease with laser energy especially for energies above 1.2 J/cm2 at which a maximum shift of 1.14 cm−1 was observed corresponding to tensile stresses of about 570 MPa [
A white line is also observed across the notch area and sidewalls of certain samples up to 2 J/cm2. The line however moves further downwards, fades away and completely disappears with higher laser energies and pulses. The tilt angle causes the development of a temperature gradient along the sidewalls with the top corner having the highest temperature as can be seen from
Laser irradiation on free standing SCS microstructures was successful in improving roughness of the sidewalls
and top surfaces. No out-of-plane remnant deformation was observed which indicates that no stress gradient was present across the whole thickness of the specimen. According to the knowledge of the authors, this is the first successful laser irradiation procedure with thin freestanding structures at energies beyond melting (0.5 J/cm2) and even ablation (1.3 J/cm2) thresholds [
Localized treatment was the reason behind efficient exposure even at high laser energies reaching 4 J/cm2. As mentioned earlier, preliminary trials with a laser spot that covered the whole chip carrying the resonators led to fracture of the fatigue test structures at much lower energies (0.16 J/cm2). Fracture always occurred at the notch. It is thought that fracture occurred due to out of plane bending by the mechanical shock caused by the energies absorbed on the resonator mass. Although the expected temperature rise along the top surface due to 0.16 J/cm2 of laser energy is about 200 K, the sudden contraction and expansion can lead to vibrations. Although the calculated static deformation caused by the temperature rise is within nanometers, the expected acceleration of the top surface is within 106 - 107 m/s2 due to the pulse duration being 20 ns [
Trends observed in surface morphologies of tensile and fatigue samples as functions of the energies and number of pulses are summarized in
Less than 1.6 J/cm2, no improvement in sidewall roughness is observed. At a high number of pulses (>100), sidewalls develop columnar structures. Since the ablation threshold is around 1.3 J/cm2, such structures are thought to occur due to evaporation and deposition and they increase in length as the number of pulses increase. In case of the top surfaces of tensile test samples subjected to perpendicular irradiation, there is an increase in residual surface stresses and roughness.
From 1.6 J/cm2 to 2.4 J/cm2, a good compromise between notch distortion and sidewall roughness was achieved for fatigue test samples. Surface roughness and residual stresses for tensile samples surfaces subjected to perpendicular irradiation also improved.
From 2.4 J/cm2 to 3.2 J/cm2, scallops disappear totally from sidewalls however the notch radius increases significantly with laser energy. For perpendicular surface irradiation, the highest increase in surface roughness was observed within that domain due to particle deposition.
From 3.2 J/cm2 to 4 J/cm2, an improvement in surface roughness of tensile samples top surfaces subjected to perpendicular irradiation compared to as fabricated samples was achieved. Further improvement in residual sur-
face stresses was also noticed.
Samples subjected to more than 4 J/cm2 fractured.
Since the melting time is supposed to be within the nanosecond regime and that the time interval between each pulse is 1 second, it is suggested that the effect of the number of pulses is an accumulation in melting time with no heat accumulation or change in the maximum temperature with each pulse. The laser energy on the other hand would be effective to increase the maximum temperature, the melting time, and depth, which decreases viscosity and enhances the flow of the molten silicon. This explains why the laser energy is more significant in liquid flow than the number of pulses as was indicated by the flow of liquid front indicated by the white line on sidewalls as seen in
In total, the surface morphology of irradiated surfaces is that of the liquid which gets solidified after a melting time depending on the laser energy and number of pulses. Laser energy enhances the flow more significantly than the number of pulses because of the higher maximum temperature, which causes a decrease in viscosity and increase in molten region, but the main contribution of the number of pulses would be to increase the total melting time. So for surfaces that need no precise control on dimensions or profiles, an irradiation with higher laser energy could be efficient for improving surface roughness but for surfaces requiring preservation of their original shape, multiple shots at lower laser energies might be a better approach.
Localized laser treatment with energies up to 4 J/cm2 was successful in improving surface roughness of sidewalls and top surfaces of freestanding structures without fracture in order to improve mechanical performance. Laser energies between 1.6 J/cm2 and 2.4 J/cm2 could be used for sidewall irradiation of fatigue test samples since the notch profile needs to be preserved while achieving acceptable roughness. Higher values of energies between 2.4 J/cm2 and 4 J/cm2 could be used for sidewall irradiation of tensile test samples since scallops totally disappear even with single pulses. Energy values between 3.2 J/cm2 and 4 J/cm2 could be considered for perpendicular irradiation of top surfaces of tensile test samples since an improvement in roughness was achieved compared to fabricated samples together with an improvement in crystallinity and developed surface stresses.