Peeping into Pixel – A micrograph of CMOS sensor
February 6, 2013 13 Comments
Macro-photography are done at 1x ~ 2x magnification. Microscope on the other hand could easily deliver a 40x magnification without eyepiece. In this post, we are peeping into the basic element that captures the image in digital photograph – a pixel on CMOS sensor. I had obtained a Nikon JFET LBCAST sensor from a broken D2H imaging board. LBCAST is still based on CMOS fabrication technology and it’s an Active Pixel Sensor.
Photographing an opaque sample compared to biological slice is extremely difficult, since ordinary trans-illumination will not work. An epi-illumination, de facto illuminating through the objective, should be used instead. Basically a half mirror is in place of the optical path to direct light towards the objective, then back in to the eyepiece and camera. Epi-fluorescence will use a dichroic mirror and a pair of filters.
The D2H sensor die is sitting inside a robust 38 pin ceramic dual-in-line package. But the bonding wire is shielded by a metal frame underneath the cover glass, thus made it impossible to see the die marking. These’s no package marking on the backside except a tape indicating its serial number (or could be color correction information used for calibration). The cover glass is rather thick, roughly 0.7mm.
The corner has clearly shown the active pixel region covered by optical black and non-microlensed region. This image is taken by a 10x objective on a stereo microscope. Now we peep in using 40x objective!
The effective pixel array (The pixel array which responds to light normally). Note that active array discards the periphery of the effective array due to color interpolation.
Unfortunately the camera is B/W. The brighter ones are green pixels while darker ones are red and blue. With this resolution, we can actually calculate the optical fill factor, it’s well below 60% given such a big lens gap! Even though a square microlens seemed to be employed, not all light is directed into the window. It seems the microlens array are not fabricated in one cycle, as you notice the lenslet on blue and red pixel are slightly larger then green lenslet.
Now comes the optical black (OB) region, and the edge of active pixels. The optical black pixels have a metal shielding in the photodiode window. By blocking light, it will only output dark current and bias level, which will be used as black reference for active pixel region. Nikon subtract the average value of OB from the intensity value in active pixel region, which transforms the black level to 0. This is not good for astrophotography and Canon will add a 1024 ADU to it. From OB pixels, it becomes even clear that only a partial region of the microlens is illuminated, roughly 40%. I believe that’s the reason for low QE, and as a result, low 18% SNR in D2H.
Finally comes to the border of lens array, now you have bare Color Filter Array (CFA) above pixel. You can clearly see the metal lines (column lines; the sensor is oriented 90°) running in between, which occupies a lot of space and is also the reason in need of microlens.
The very bottom right corner of the total pixel array
Each row has a pair of control lines. The upper one in the pair is for JFET select/reset, while the bottom controls photodiode transfer gate. The pair is made of poly-silicon on the substrate. The 2 small black dots in between are likely vias or contacts. The column line (metal layer 1) connectes to the source of JFET transistor according to this paper, which relay the pixel signal. Notice the line is really thick to reduce electroresistance! The reset drain is the metal layer 2 above the row and also serves as a photo shield for the transistor below. It is not visible here.
Another interesting observation is the lack of blurring function of this cover glass. Canon has integrated its second anti-aliasing layer of OLPF as sensor cover glass in full frame and new generation APS-C DSLRs. Apparently the OLPF stack is standalone in D2H.
Moving the view to the bottom long edge reveals the column circuit, possibly the buffer, CDS and column scanning driver that latches to the output amplifier. Note a letter “4” is photo-lithographed on the die, this indicates the column 40. The die also has a "+" mark every 5 columns in between.
Somewhere along the line between column 385 to 405, there’s a recess in the long edge. I’m not sure what this for. (Image in 10x)
Top left corner on the opposite long edge of the sensor. (Image in 10x) Even though some of the non-microlensed pixels are hiding beneath the metal frame, we can still see the column number from 3, 4, 5…
Top right corner, the last connected line is 256, which indicates total of 2560 columns.
The ceramic package viewed through a X-Ray scanner showing the bonding wires linking the lead and die. The metal frame is also visible.
Very interesting right, huh? Now we can compare it against a Micron CMOS sensor (Now Aptina) with 5.2um pixel.
Die marking MI-1300 from year 2002, MT9M001.
Now the microlens itself. We can clearly see a much narrower lens gap and higher fill factor. This sensor boost a 55% peak QE, but still less than the Sony Exmor sensors. I hope someone can donate me one for dissection.
The contributor behind the scene – Olympus LUC PL FLN 40x objective. This objective is designed for inverted microscope and has a collar to set the glass slide thickness, allowing the compensation of chromatic and spherical aberration.
Image orientation corrected.