Image Sensor Microlens Fabrication via Two-Stage Exposure
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Solution Overview
Problem
Conventional CMOS image sensor fabrication methods result in large gaps between microlenses, reducing light collection area and color saturation due to reliance on a single exposure process in the photolithography stage.
Innovation Solution
A two-stage exposure process using light sources of different wavelengths, with a defocus method and adjusted exposure times to form tapered and straight foot patterns, followed by a developing process to create funnel-shaped gaps between photosensitive blocks, which are then transformed into microlenses through a reflow process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single exposure process is used to form microlenses, then the fabrication process is simple, but large gaps are produced between microlenses reducing light collection area and color saturation
Solution Approach 1:
The single exposure process is segmented into two separate exposure processes: a first exposure process that forms an initial microlens pattern, and a second exposure process that forms a refined microlens pattern with reduced gaps. This segmentation allows each exposure to be optimized for its specific purpose, achieving both process simplicity and precision.
Solution Approach 2:
The first exposure process performs a preliminary action by forming an initial microlens pattern that establishes the basic structure. The second exposure process then performs the final refinement to achieve the desired gap reduction. This preliminary action approach allows the fabrication process to build complexity progressively while maintaining overall simplicity.
2Manufacturing precision
If the photolithography resolution is increased to reduce gaps between photosensitive blocks, then manufacturing precision improves, but the fabrication complexity and cost increase
Solution Approach 1:
The photolithography process is segmented into two separate exposure processes with different parameters. The first exposure uses one set of conditions to form an initial pattern, while the second exposure uses different conditions to refine the pattern and reduce gaps. This segmentation avoids the need for a single high-resolution exposure that would increase device complexity.
Solution Approach 2:
The invention changes exposure parameters between the two exposure processes, including exposure time, light intensity, and possibly wavelength. By adjusting these parameters, the process achieves better gap control without requiring consistently high-resolution settings throughout, thereby reducing overall fabrication complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces microlens gaps, enhancing light collection area and color saturation by forming microlenses with a more circular surface, thereby improving image sensor resolution and performance.
Implementation Method 1
a photosensitive material (not shown) composed of resin is coated over the surface of the planarizing layer 112. The photosensitive material is composed of I-line photoresist adapted for wavelength of 365 nm. Next, an exposure process is conducted with a 365 nm UV light on the photosensitive material
Implementation Method 2
a reflow process is performed by exposing the image sensor 140 to high temperature for 5-10 minutes. The high temperature utilized during the process transforms the photosensitive blocks 130 to a plurality of microlenses 134, in which each of the microlenses 134 has a semi-circular surface
Data Source
AI summary
A method for fabricating an image sensor is disclosed. First, a semiconductor substrate is provided, in which a photosensitive region is defined on the semiconductor substrate. At least one photosensitive material is then formed on the semiconductor substrate, and a first exposure process is performed to form a tapered pattern in the photosensitive material. A second exposure process is performed to form a straight foot pattern in the photosensitive material, and a developing process is performed to remove the tapered pattern and straight foot pattern to form the photosensitive material into a plurality of photosensitive blocks. A reflow process is conducted thereafter to form the photosensitive blocks into a plurality of microlenses.