Microlens alignment procedures in CMOS image sensor design
View Patent ↗A method for aligning a microlens array in a sensor die to resolve non-symmetric brightness distribution and color balance of the image captured by the sensor die. The method includes performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor and de-centering values, calculating the error in a real product's alignment in process and image offset, performing a post simulation based on offset calculation on the real product and re-design of the microlens alignment, and repeating the steps of calculating the error and performing the post-simulation until a satisfactory brightness distribution is obtained. The sensor die has sensor pixels, each pixel comprising a photodiode and a microlens for directing incoming light rays to the photodiode, wherein optical axis of the microlens is shifted with respect to optical axis of the photodiode by a preset amount determined by at least one iteration of alignment process.
1. A method for aligning a microlens array in a sensor die, comprising:
(a) performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor;
(b) designing a new photo-mask for the microlens array based on the shrink-factor;
(c) producing a sample silicon die using the new photo-mask;
(d) capturing an image of a collimated white light source using the sample silicon die;
(e) evaluating uniformity of brightness distribution of the image; and
in case of unsatisfactory brightness distribution;
(f) calculating error in alignment of the sample silicon die and a de-centering value;
(g) performing a post-simulation based on the error to tune the shrink-factor and the de-centering value;
(h) designing a new photo-mask for the microlens array based on the shrink-factor and the de-centering value; and
(i) repeating the steps (c)-(h) until a satisfactory brightness distribution is obtained.
2. The method of claim 1 , wherein the step of performing a pre-simulation includes the step of:
calculating an optimum offset for an optical axis of each element of the microlens array with respect to a corresponding photodiode of the silicon die.
3. The method of claim 2 , wherein the step of calculating optimum offset includes:
simulating a plurality of ray acceptance angles utilizing an optical ray trace technique.
4. The method of claim 1 , wherein the step of performing a post-simulation includes the step of:
calculating an optimum offset for an optical axis of each element of the microlens array with respect to a corresponding photodiode of the silicon die.
5. The method of claim 4 , wherein the step of calculating optimum offset includes:
simulating a plurality of ray acceptance angles utilizing an optical ray trace technique.
6. The method of claim 4 , wherein the step of evaluating uniformity of brightness distribution includes the steps of:
checking an intensity distribution of the image; and
checking a color balance of the image.
7. A computer readable medium carrying one or more sequences of instructions for aligning a microlens array in a sensor die, wherein execution of one or more sequences of instructions by one or more processors causes the one or more processors to perform the steps of:
(a) performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor;
(b) designing a new photo-mask for the microlens array based on the shrink-factor;
(c) producing a sample silicon die using the new photo-mask;
(d) capturing an image of a collimated white light source using the sample silicon die;
(e) evaluating uniformity of brightness distribution of the image; and
in case of unsatisfactory brightness distribution;
(f) calculating error in alignment of the sample silicon die and an image de-centering value;
(g) performing a post-simulation based on the error to tune the shrink-factor;
(h) designing a new photo-mask for the microlens array based on the shrink-factor and the de-centering value; and
(i) repeating the steps (c)-(h) until a satisfactory brightness distribution is obtained.
8. A system for aligning a microlens array in a sensor die, comprising:
means for performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor;
means for designing a new photo-mask for the microlens array based on the shrink-factor;
means for producing a sample silicon die using the new photo-mask;
means for capturing an image of a collimated white light source using the sample silicon die;
means for evaluating uniformity of brightness distribution of the image; and
in case of unsatisfactory brightness distribution;
means for calculating error in alignment of the sample silicon die and an image brightness center offset;
means for performing a post-simulation based on the error to tune the shrink-factor;
means for designing a new photo-mask for the microlens array based on the shrink-factor and the image brightness center offset; and
means for repeating the steps of producing a sample silicon die to the step of designing a new photo-mask until a satisfactory brightness distribution is obtained.