Aravinda Kar

Microvia drilling with a green laser

Microvias drilling in polymer-copper-polymer multilayer substrate for high density electronic packaging applications is studied in this paper. Frequency doubled Nd:YAG laser (532 nm) is used for the simulation and experiment. Numerical thermal model is built to simulate the drilling process. Volumetric heating and laser self defocusing due to drilling front profile are considered in the model. Due to the low absorption coefficient of the polymer material for the green laser, the polymer-copper interface absorbs a great portion of the laser energy. Thus the drilling mechanism involves thermal phase change, chemical decomposition, and possibly explosion due to rapid thermal expansion and vaporization inside the polymer material. Experimental results show that the material is removed explosively due to high internal pressures.Microvias drilling in polymer-copper-polymer multilayer substrate for high density electronic packaging applications is studied in this paper. Frequency doubled Nd:YAG laser (532 nm) is used for the simulation and experiment. Numerical thermal model is built to simulate the drilling process. Volumetric heating and laser self defocusing due to drilling front profile are considered in the model. Due to the low absorption coefficient of the polymer material for the green laser, the polymer-copper interface absorbs a great portion of the laser energy. Thus the drilling mechanism involves thermal phase change, chemical decomposition, and possibly explosion due to rapid thermal expansion and vaporization inside the polymer material. Experimental results show that the material is removed explosively due to high internal pressures.

Laser fabrication of silicon carbide light emitting diodes

With the shrinkage in the size of the semiconductor devices, greater shift has been observed towards use of lasers, electron and x-ray beams and advanced optics for small area device fabrication. A novel laser direct-write doping and metallization technique provides a single step approach for processing wide bandgap materials for electronic and optoelectronic device applications, which are difficult to dope using conventional techniques. Laser doping opens the opportunity to use unconventional dopants for customizing emission wavelength. It effectively reduces the number of fabrication steps and allows for selective area doping and direct metallization without metal deposition. To demonstrate this technology a pulsed Nd:YAG laser (1064nm wavelength) was used to fabricate blue light emitting laser diodes in a silicon carbide (SiC) 6H:SiC (n-type) wafer substrates. A p-n junction was created by laser doping aluminum (p-type) and nitrogen (n-type) on clean SiC wafer. Pure indium wire was used to obtain good ohmic contacts. These devices were characterized by capacitance-voltage (C-V), current-voltage (I-V), and electroluminescence (EL) measurements. A narrow electroluminescence (EL) peak at 481.82 nm and broad EL peak around 498.8 nm wavelengths were observed with two different detectors, characterizing the p-n junction as a blue light emitter.With the shrinkage in the size of the semiconductor devices, greater shift has been observed towards use of lasers, electron and x-ray beams and advanced optics for small area device fabrication. A novel laser direct-write doping and metallization technique provides a single step approach for processing wide bandgap materials for electronic and optoelectronic device applications, which are difficult to dope using conventional techniques. Laser doping opens the opportunity to use unconventional dopants for customizing emission wavelength. It effectively reduces the number of fabrication steps and allows for selective area doping and direct metallization without metal deposition. To demonstrate this technology a pulsed Nd:YAG laser (1064nm wavelength) was used to fabricate blue light emitting laser diodes in a silicon carbide (SiC) 6H:SiC (n-type) wafer substrates. A p-n junction was created by laser doping aluminum (p-type) and nitrogen (n-type) on clean SiC wafer. Pure indium wire was used to obtain good ...

Laser drilling of single crystal silicon carbide substrates

Silicon carbide (SiC) is a wide bandgap compound semiconductor suitable for high temperature and high voltage power electronics applications due to its high electrical breakdown strength and high thermal conductivity. It also exhibits excellent metallurgical properties such as high hardness and resistance to chemical degradation. These properties make SiC processing difficult with conventional machining methods. Laser cutting, drilling and etching are promising technologies for SiC machining in advanced device fabrication. An analytic transient thermal model is developed to analyze laser drilling of SiC. The model is based on volumetric heating to account for the semi-transparent optical properties of doped SiC at the Nd:YAG laser wavelength of 1.06 µm. The results of the mathematical model are compared with experimental data pertaining to the drilling speed, hole size and the taper of the hole under different laser parameters. Laser Microfabrication ConferenceSilicon carbide (SiC) is a wide bandgap compound semiconductor suitable for high temperature and high voltage power electronics applications due to its high electrical breakdown strength and high thermal conductivity. It also exhibits excellent metallurgical properties such as high hardness and resistance to chemical degradation. These properties make SiC processing difficult with conventional machining methods. Laser cutting, drilling and etching are promising technologies for SiC machining in advanced device fabrication. An analytic transient thermal model is developed to analyze laser drilling of SiC. The model is based on volumetric heating to account for the semi-transparent optical properties of doped SiC at the Nd:YAG laser wavelength of 1.06 µm. The results of the mathematical model are compared with experimental data pertaining to the drilling speed, hole size and the taper of the hole under different laser parameters. Laser Microfabrication Conference

Effect of large deflection angle on the laser intensity profile produced by AOD scanners in high precision manufacturing

Laser beam scanners have found wide applications in a variety of laser-assisted advanced microprocessing technologies, such as printing, patterning and doping. Traditional galvo-scanners affect the accuracy of beam positioning and repeatability in high precision manufacturing due to mechanical motion of the mirrors and backlash errors. An Acousto-optic Deflector (AOD), which is made of a transparent photoelastic medium bonded to a piezoelectric transducer, is a promising device to overcome these limitations. AODs are commonly used in laser direct writing systems to provide flexible and high-speed beam scanning with high precision and accuracy.We have developed an analytic model based on Bessel functions, which will be referred to as Bessel model, to calculate the strain tensor, stress tensor and stress-induced birefringence, and the change in the refraction index of the crystal. This refraction index variation produces the volume phase grating and provides a mechanism for diffraction, and consequently, deflection of the laser beam as it propagates through the AOD crystal. Various laser parameters, such as the diffraction efficiency and the laser intensity of the diffraction pattern at different deflection angles are studied in this paper.Laser beam scanners have found wide applications in a variety of laser-assisted advanced microprocessing technologies, such as printing, patterning and doping. Traditional galvo-scanners affect the accuracy of beam positioning and repeatability in high precision manufacturing due to mechanical motion of the mirrors and backlash errors. An Acousto-optic Deflector (AOD), which is made of a transparent photoelastic medium bonded to a piezoelectric transducer, is a promising device to overcome these limitations. AODs are commonly used in laser direct writing systems to provide flexible and high-speed beam scanning with high precision and accuracy.We have developed an analytic model based on Bessel functions, which will be referred to as Bessel model, to calculate the strain tensor, stress tensor and stress-induced birefringence, and the change in the refraction index of the crystal. This refraction index variation produces the volume phase grating and provides a mechanism for diffraction, and consequently, de...

Laser doping of GaN for advanced optoelectronic applications

GaN is an important compound semiconductor for optoelectronic applications including light-emitting diodes, ultraviolet lasers and photodectectors. p-type and n-type semiconductors and ultimately p-n junctions are necessary to fabricate these types devices. GaN can be doped with n-type dopant atoms, such as silicon, relatively easily; however, doping GaN with a p-type dopant such as Mg is challenging. For example, ion implantation can be used to increase the concentration of electrically active impurities in the source and drain regions of the device, but this process requires high temperatures for electrical activation, along with capping layers to prevent GaN decomposition. Additionally, ion implantation creates lattice damage that is difficult to remove via annealing and acts to compensate the dopants. Two laser doping techniques, based on 1) gas immersion and 2) molten precursor laser doping are modified to improve p-type doping in GaN. Bis-magnesium dihydrate [Mg(TMHD)2] is the precursor used in both cases to supply Mg atoms. In the gas immersion method, the precursor powder is heated to 300°C in a bubbler and then a carrier gas, nitrogen, is introduced to the bubbler to transport the Mg-containing vapor to a laser doping chamber that was originally under a vacuum of 0.1 mTorr. In the molten precursor technique, on the other hand, a bed of [Mg(TMHD)2] powder is sandwiched between the GaN wafer and a soda lime glass slide at the wafer surface. The soda lime glass slide allows the incident laser (532 nm wavelength) irradiation to reach the GaN wafer surface, restrains the [Mg(TMHD)2] powder at the GaN surface and confines the molten precursors to the surface. Various laser processing parameters and the dopant concentration are discussed in this paper.GaN is an important compound semiconductor for optoelectronic applications including light-emitting diodes, ultraviolet lasers and photodectectors. p-type and n-type semiconductors and ultimately p-n junctions are necessary to fabricate these types devices. GaN can be doped with n-type dopant atoms, such as silicon, relatively easily; however, doping GaN with a p-type dopant such as Mg is challenging. For example, ion implantation can be used to increase the concentration of electrically active impurities in the source and drain regions of the device, but this process requires high temperatures for electrical activation, along with capping layers to prevent GaN decomposition. Additionally, ion implantation creates lattice damage that is difficult to remove via annealing and acts to compensate the dopants. Two laser doping techniques, based on 1) gas immersion and 2) molten precursor laser doping are modified to improve p-type doping in GaN. Bis-magnesium dihydrate [Mg(TMHD)2] is the precursor used in both...

Large angle of Bragg diffraction using interfrence of acoustic wave inside acousto-optic deflector

The laser beam is often steered in different directions in a variety of laser-advanced microprocessing technologies, such as laser micromachining, laser patterning and laser selective-area doping. Scanners are traditionally used to direct the beam to different locations on the workpiece. The mechanical motion of x-y mirrors in such scanners affects the accuracy of beam positioning and repeatability in high precision manufacturing. Beam steering without any moving optical component is necessary to overcome the limitations of current scanning technology. Stationary optical devices such as acousto-optic modulators (AOM), acousto-optic deflectors (AOD) and acousto-optic tunable filters (AOTF) have widespread applications in the field of laser microfabrication for intensity modulation and laser beam steering. AODs are commonly used with continuous-wave lasers in laser direct writing systems to provide flexible and high-speed beam scanning with high precision and accuracy.A pair of AODs is mounted orthogonally to provide dual axis (x-y) laser scanning. The deflection scan angle, however, is small due to the finite distance between the two separate deflectors. This optical configuration can be simplified by realizing 2D AO interaction in a single AO crystal. This paper discusses an integrated AOD to allow compact construction and efficient operation with large deflection scan angle. This type of AODs would be useful for large-area processing with high throughput.The laser beam is often steered in different directions in a variety of laser-advanced microprocessing technologies, such as laser micromachining, laser patterning and laser selective-area doping. Scanners are traditionally used to direct the beam to different locations on the workpiece. The mechanical motion of x-y mirrors in such scanners affects the accuracy of beam positioning and repeatability in high precision manufacturing. Beam steering without any moving optical component is necessary to overcome the limitations of current scanning technology. Stationary optical devices such as acousto-optic modulators (AOM), acousto-optic deflectors (AOD) and acousto-optic tunable filters (AOTF) have widespread applications in the field of laser microfabrication for intensity modulation and laser beam steering. AODs are commonly used with continuous-wave lasers in laser direct writing systems to provide flexible and high-speed beam scanning with high precision and accuracy.A pair of AODs is mounted orthogonally ...

Effect of laser irradiation passes for fabricating mid-wave infrared silicon carbide detectors

A Mid-Wave Infra-Red (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. Doping is one of the challenges for silicon carbide (SiC) device fabrication due to its hardness, chemical inertness and the low diffusion coefficient of most impurities. A laser doping technique is chosen to dope 4H-SiC by gallium which is incorporated into the semiconductor as p-type dopants. The substrate is simultaneously heated with a continuous wave Nd:YAG laser of wavelength 1064u2005nm under the laser power, focal length, laser beam diameter and micro-stage speed were 10.5u2005W, 150u2005mm, 200u2005µm and 0.8u2005mm/sec respectively using metal-organic dopant precursor. The gallium dopant profile in the laser-doped SiC wafer is obtained by secondary ion mass spectroscopy and the maximum concentration of Ga in SiC wafer surface with 4 time of laser irradiation passes is found to be 6.251×1020 cm−3, which is two orders of magnitude higher than the reported value (6.0×1018 cm−3) and the dopant depth is approximately 360u2005nm. The data revealed enhanced solid solubility exceeding the equilibrium solubility limit. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the doped SiC. This process modifies the electron density, which changes the refractive index and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam such as a He-Ne laser. The variation of refractive index was calculated as a function of absorbed irradiance based on the reflectance data for the as- received and doped samples. A distinct change was observed for the refractive index of the doped sample, indicating that the detector is suitable for applications at 4.21u2005µm wavelength.A Mid-Wave Infra-Red (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. Doping is one of the challenges for silicon carbide (SiC) device fabrication due to its hardness, chemical inertness and the low diffusion coefficient of most impurities. A laser doping technique is chosen to dope 4H-SiC by gallium which is incorporated into the semiconductor as p-type dopants. The substrate is simultaneously heated with a continuous wave Nd:YAG laser of wavelength 1064u2005nm under the laser power, focal length, laser beam diameter and micro-stage speed were 10.5u2005W, 150u2005mm, 200u2005µm and 0.8u2005mm/sec respectively using metal-organic dopant precursor. The gallium dopant profile in the laser-doped SiC wafer is obtained by secondary ion mass spectroscopy and the maximum concentration of Ga in SiC wafer surface with 4 time of laser irradiation passes is found to be 6.251×1020 cm−3, which is two orders of magnitude higher than the reported value (6.0×1018 cm−3) and the dopant depth is a...

Laser doping for bandgap engineering

Laser doping is used to modify the reflectivity and refractive index of embedded regions in wide bandgap semiconductors for selective detection of gaseous chemical species. Each of the four quadrants of a 1u2005cm × 1u2005cm × 300u2005µm silicon carbide (SiC) sensor are laser doped with a different element; gallium, aluminum, scandium and phosphorus, respectively; to create energy levels that selectively absorb photon emissions from a specific gas molecule chemical composition. For example, the energy level EV + 0.29 created in SiC by the gallium dopant detects only CO2 gas while the energy level EV + 0.23 created in SiC by the aluminum dopant detects only NO. Changes in refractive index, remotely interrogated by a helium neon laser, are correlated to the concentration of the select chemical species. A 1064u2005nm wavelength Nd:YAG laser source was typically operated at 10-15u2005W power, 65-200u2005µm beam diameter and 0.5-0.8u2005mm/s scan speed using gas, metal-organic or powder dopant precursors. This wireless chemical sensor technology is an advance over interferometers since embedded active regions and a high melting/dissociation point of the sensor, 2730°C for silicon carbide, allow operation in extremely harsh environments.Laser doping is used to modify the reflectivity and refractive index of embedded regions in wide bandgap semiconductors for selective detection of gaseous chemical species. Each of the four quadrants of a 1u2005cm × 1u2005cm × 300u2005µm silicon carbide (SiC) sensor are laser doped with a different element; gallium, aluminum, scandium and phosphorus, respectively; to create energy levels that selectively absorb photon emissions from a specific gas molecule chemical composition. For example, the energy level EV + 0.29 created in SiC by the gallium dopant detects only CO2 gas while the energy level EV + 0.23 created in SiC by the aluminum dopant detects only NO. Changes in refractive index, remotely interrogated by a helium neon laser, are correlated to the concentration of the select chemical species. A 1064u2005nm wavelength Nd:YAG laser source was typically operated at 10-15u2005W power, 65-200u2005µm beam diameter and 0.5-0.8u2005mm/s scan speed using gas, metal-organic or powder dopant precursors. This wireless chemical sensor te...

Two-dimensional transient modeling of CO2 laser drilling of microvias in high density flip chip substrates

Thermal modeling is essential to understand the laser-materials interactions and to control laser drilling of blind micro holes through polymeric dielectrics in multilayer electronic substrates. In order to understand the profile of the drilling front irradiated with different laser beam profiles, a two-dimensional transient heat conduction model including vaporization parameters is constructed. The absorption length in the dielectric is also considered in this model. Therefore, the volumetric heating source criteria are applied in the model and the equations are solved analytically. The drilling speed, temperature distribution in the dielectric and the thickness of the residue, termed smear, along the microvia walls and at via bottom/copper pad interface are studied under different laser parameters. An overheated metastable state of material is found to exist in the workpiece. The overheating parameters are attained for various laser drilling parameters to control the thermal damage and to minimize smear residue on the micro hole wall/bottom diameter.Thermal modeling is essential to understand the laser-materials interactions and to control laser drilling of blind micro holes through polymeric dielectrics in multilayer electronic substrates. In order to understand the profile of the drilling front irradiated with different laser beam profiles, a two-dimensional transient heat conduction model including vaporization parameters is constructed. The absorption length in the dielectric is also considered in this model. Therefore, the volumetric heating source criteria are applied in the model and the equations are solved analytically. The drilling speed, temperature distribution in the dielectric and the thickness of the residue, termed smear, along the microvia walls and at via bottom/copper pad interface are studied under different laser parameters. An overheated metastable state of material is found to exist in the workpiece. The overheating parameters are attained for various laser drilling parameters to control the thermal damage and to minimize smear...

Laser thin film deposition on plastic substrates using silicon nanoparticles for flexible electronics

The melting temperature of silicon nanoparticles decreases significantly compared to the melting temperature of bulk silicon when the particle size is less than 5u2005nm. This concept is utilized for reducing the processing temperatures to deposit thin silicon films on plastic substrates. An aqueous dispersion of 5u2005nm silicon nanoparticles was used as precursor to produce polycrystalline silicon (c-Si) thin films on plastic substrates. A Nd:YAG (1064u2005nm wavelength) laser in continuous mode (CW) was used for deposition, annealing and recrystallization. Experiments were carried out in air as well as in argon atmosphere for obtaining a continuous recrystallized c-Si thin film on nickel substrates. Optimized parameters were utilized for obtaining c-Si films on plastic substrates. The substrate was not preheated and the entire film deposition process was conducted maintaining the substrate at room temperature while the film was heated with the laser beam. The process involved two steps: (i) a film forming step in which the aqueous medium was evaporated and the silicon particles were fused simultaneously to obtain a continuous film, and (ii) a recrystallization step in which the films were annealed and recrystallized by laser heating. The films were characterized by optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. Doping of these films with nitrogen and boron was also carried out successfully which was confirmed by the current voltage (I-V) measurements. The oxygen content of the laser air-treated film decreased with increasing both the laser power and the irradiation time. Laser argon-treated films showed comparatively lower amounts of oxygen. Raman spectroscopy showed a shift from amorphous to more crystalline phase with increasing the laser power and irradiation time. The increasing number of silicon crystallites observed using SEM confirmed this transformation.The melting temperature of silicon nanoparticles decreases significantly compared to the melting temperature of bulk silicon when the particle size is less than 5u2005nm. This concept is utilized for reducing the processing temperatures to deposit thin silicon films on plastic substrates. An aqueous dispersion of 5u2005nm silicon nanoparticles was used as precursor to produce polycrystalline silicon (c-Si) thin films on plastic substrates. A Nd:YAG (1064u2005nm wavelength) laser in continuous mode (CW) was used for deposition, annealing and recrystallization. Experiments were carried out in air as well as in argon atmosphere for obtaining a continuous recrystallized c-Si thin film on nickel substrates. Optimized parameters were utilized for obtaining c-Si films on plastic substrates. The substrate was not preheated and the entire film deposition process was conducted maintaining the substrate at room temperature while the film was heated with the laser beam. The process involved two steps: (i) a film forming step in ...