Kitty Kumar

Femtosecond laser direct hard mask writing for selective facile micron-scale inverted-pyramid patterning of silicon

We report on the fabrication of high-fidelity inverted-pyramids in crystalline silicon (c-Si) at the 1 μm scale through the selective removal of a silicon nitride (SiNx) hard-mask with a 522 nm femtosecond (fs) laser and subsequent alkaline potassium hydroxide (KOH) etching. Through a series of systematic experiments on a range of hard-mask thicknesses, the use of 20 nm thick SiNx film yielded a 0.6 μm diameter laser-ejected aperture in the hard-mask at a single pulse fluence of 0.45 J cm−2, resulting in 1 μm wide inverted-pyramid structure in c-Si after KOH etching. Anisotropic KOH etching of the partially amorphized c-Si underlying the fs-laser patterned hard mask was found to render clean (111) planes of c-Si. An array of inverted-pyramids on c-Si surfaces as large as 4 cm2 was produced with a defect density of less than 1 in 104. This facile, non-contact, and cleanroom-independent technique serves a variety of applications including anti-reflective texturing of thin c-Si for photovoltaics, wafer marking, labeling, and fabrication of microfluidic and optical devices or laboratories on silicon wafers.We report on the fabrication of high-fidelity inverted-pyramids in crystalline silicon (c-Si) at the 1 μm scale through the selective removal of a silicon nitride (SiNx) hard-mask with a 522 nm femtosecond (fs) laser and subsequent alkaline potassium hydroxide (KOH) etching. Through a series of systematic experiments on a range of hard-mask thicknesses, the use of 20 nm thick SiNx film yielded a 0.6 μm diameter laser-ejected aperture in the hard-mask at a single pulse fluence of 0.45 J cm−2, resulting in 1 μm wide inverted-pyramid structure in c-Si after KOH etching. Anisotropic KOH etching of the partially amorphized c-Si underlying the fs-laser patterned hard mask was found to render clean (111) planes of c-Si. An array of inverted-pyramids on c-Si surfaces as large as 4 cm2 was produced with a defect density of less than 1 in 104. This facile, non-contact, and cleanroom-independent technique serves a variety of applications including anti-reflective texturing of thin c-Si for photovoltaics, wafer marki...

Ultrafast laser direct hard-mask writing for high performance inverted-pyramidal texturing of silicon

We demonstrate a simple and versatile laser assisted technique to produce an inverted pyramid texture in monocrystalline silicon to reduce surface reflectance. With this method the inverted pyramid size, distribution, spacing, and hence the spectral reflection of the textured surface can be tailored. The process removes minimal amounts of silicon during the texturing process, thus making it compatible with ultra-thin silicon photovoltaics. Surfaces with the fabricated inverted pyramid texture and an antireflection coating reflect 4% of incident light.

Optimizing inverted pyramidal grating texture for maximum photoabsorption in thick to thin crystalline silicon photovoltaics

We use the wave optical approach to optimize the front surface inverted pyramidal grating texture on 2 to 400 μm thick crystalline silicon in order to derive the maximum photocurrent density from the cell. We identify a “one size fits all” front grating periodicity of 1000 nm for c-Si absorbing layer configured with a back surface reflector that maximizes the absorption of normally incident AM1.5g solar spectrum irrespective of the layer thickness. With the identification of such universal inverted pyramidal grating texture, a common texturing process can be developed for high-efficiency devices on thick to thin c-si. Furthermore, our studies show that the photocurrent decreases by 0.02 mA/cm2 with every nanometer increase in the width of the flat region (mesa) between inverted pyramids in the optimum texture. The decrease in photocurrent due to reflection from the mesas can be recovered with the addition of an antireflective coating of optimum thickness of 80 nm and refractive index ~ 2.1.

Interferometric femtosecond laser processing for nanostructuring inside thin film

Abstract Femtosecond laser interactions inside transparent dielectric films of refractive index, nfilm, with tight focusing presents strong nonlinear interactions that can be preferentially confined at the fringe maxima as formed by Fabry-Perot interference, to generate thin nanoscale plasma disks separated on half-wavelength, λ/2nfilm. The nano-thin disk explosions can be controlled inside the film to cleave open subwavelength internal cavities at single or multiple periodic depths at low laser exposure, while higher exposure will eject a quantised number of film segments with segment thickness defined by the laser wavelength. This new method enables high-resolution film patterning for ejecting nanodisks at quantised film depth for colouring and three-dimensional (3D) surface structuring, as well as for fabrication of free-standing nanofilms.

Novel method for fabricating high efficiency 10 μm thick c-Si solar cells

The most straightforward route to reducing the cost of c-Si photovoltaics is to make cells thinner. However, as cell thickness is reduced, light absorption is compromised. This necessitates the use of light trapping mechanisms to mitigate reduction in absorption. In this paper, we present a novel method to fabricate a high efficiency solar cell on a 10 μm thick silicon membrane. The method involves in-house fabrication of 10 μm thick membranes and integration of optically efficacious inverted pyramid texture with feature size and pitch on the order of the solar wavelength via a resist-free laser-writing method.

Laser ablation inside transparent thin films

The laser interactions and processing of thin films is a rapidly growing area of research that serves broadly in microelectronic, display, photovoltaic, sensing and biological applications. Traditional concepts of laser marking, machining and scribing in opaque materials have been widely extended into transparent substrates or films to enable new approaches in printing or manufacturing by laser induced forward transfer (LIFT) [1] and other novel directions for catapulting single cells [2] and forming blisters [3] or microfluidic structures [4]. These approaches typically rely on inducing strong laser-plasma interactions internally at the interface of two media. In contrast, nonlinear optical interactions can be more flexibly positioned within the focal volume of short-pulsed lasers to drive new approaches for welding, writing optical circuits, or shaping three-dimensional opto-fluidic circuits inside transparent materials.