1. Field of the Invention
The present invention relates to a method of fabricating a refractive silicon microlens and, more particularly, to a method of fabricating a refractive silicon microlens that can be used in the infrared range by using micro-machining technology.
2. Description of the Prior Art
The lens is a basic element for a micro-optical system. Thus, various techniques have been used to fabricate a microlens. Microlenses can be classified by its range of use, from visible ray to infrared. Microlenses can also be classified by how it is used, i.e., from diffraction lenses to refractive lenses.
A refractive microlens usable in the visible ray range is fabricated by use of techniques such as thermal reflow using surface tension, plastic molding with isotropic etching of silicon, and selective ion exchange in a glass substrate.
A Fresnel lens is a type of diffraction lens usable in both visible ray and infrared range. Because the width between etching patterns in a Fresnel lens differs depending upon the intended wavelength for the lens to be used in, it has a disadvantage that one Fresnel lens can be used for only one wavelength.
Because refractive lenses currently usable in the infrared range are fabricated by mechanical processing of silicon or germanium, the diameter of the lenses are long to a degree of several millimeters. Therefore, it was impossible to make a two-dimensional infrared microlens array for use in infrared sensors, although it is well-known that the detectivity of infrared sensors can be increased by a degree of the refractive index of the lens if infrared lenses are used in an infrared sensor.
Therefore, it is an object of the present invention to provide a method of fabricating a refractive silicon microlens that can be used in the infrared range by using micro-machining technology that is used in processing semiconductor devices.
To this end, the method of fabricating a refractive silicon microlens of the present invention comprises the steps of forming a boron-doped region on a silicon substrate, and selectively removing regions of the substrate except for the boron-doped region to form a lens comprised of only the boron-doped region.
The step of forming the boron-doped region can be carried out by one of the following three methods.
The first method of forming the boron-doped region comprises the steps of forming on the substrate a diffusion-preventive layer pattern having an aperture exposing a front face of the substrate, forming a curved portion by diffusing boron only on an exposed region of the substrate by using the diffusion-preventive layer pattern as a diffusion mask, forming a transformed diffusion-preventive layer pattern having an aperture larger than the curved portion by patterning the diffusion-preventive layer pattern such that a surface of the substrate including the curved portion is exposed, and forming a flat portion by diffusing boron on the exposed surface of the substrate using the transformed diffusion-preventive layer pattern as the diffusion mask.
The second method of forming the boron-doped region comprises the steps of forming on the substrate a diffusion-preventive layer pattern having an aperture exposing a front face of the substrate, performing isotropic etching of an exposed region of the substrate using the diffusion-preventive layer as a diffusion mask, forming a first transformed diffusion-preventive layer pattern having an aperture larger than the isotropically-etched region by patterning the diffusion-preventive layer pattern such that a surface of the substrate including the isotropically-etched region is exposed, forming a curved portion by diffusing boron on the exposed surface of the substrate using the first transformed diffusion-preventive layer pattern as the diffusion mask, forming a second transformed diffusion-preventive layer pattern having an aperture larger than the aperture of the first transformed diffusion-preventive layer by patterning the first transformed diffusion-preventive layer pattern such that a surface of the substrate including the curved portion is exposed, and forming a flat portion by diffusing boron on the exposed surface of the substrate including the curved portion using the second transformed diffusion-preventive layer pattern as the diffusion mask.
The third method of forming the boron-doped region comprises the steps of forming a diffusion-preventive layer pattern on the substrate having a plurality of apertures exposing a front face of the substrate, forming a curved portion by diffusing boron on exposed regions of the substrate by using the diffusion-preventive layer pattern as a diffusion mask, forming a transformed diffusion-preventive layer pattern having an aperture larger than the curved portion by patterning the diffusion-preventive layer pattern such that a surface of the substrate including the curved portion is exposed, and forming a flat portion by diffusing boron on the exposed surface of the substrate including the curved portion using the transformed diffusion-preventive layer pattern as the diffusion mask.
When the boron-doped regions are formed as illustrated above, the shape of the aperture(s) on the diffusion-preventive layer pattern may be a circle, square, triangle, or star. In addition, it is preferable to form the diffusion-preventive layer pattern by forming a diffusion-preventive layer such as a thermal oxide layer on both the front and back surfaces of the substrate and patterning the diffusion-preventive layer on the front surface of the substrate to expose the substrate. The diffusion-preventive layer that is remaining on the back surface of the substrate prevents diffusion of boron into the back surface of the substrate.
The step of selectively removing regions of the substrate except for the boron-doped region comprises the step of selectively wet-etching the regions of the substrate except for the boron-doped region by using an etchant selected from the group consisting of KOH, EDP (Ethylenediamine Pyrocatechol), and TMAH (Tetramethyl Ammonium Hydroxide). In this step, the boron concentration of the boron-doped region should be at least 5xc3x971018 atoms/cm3 so that a certain degree of selectivity is obtained.
In order to prevent the surface of the boron-doped region from being etched in the step of selectively wet-etching, it is preferable to form a passivation layer such as a thermal oxide layer or a silicon nitride layer on the boron-doped region prior to the step of wet-etching.