- So far, only two approaches are feasible: optical imaging by CIS and ultrasonic echoing.
- Panel makers such as BOE and AUO are developing TFT-based fingerprint solutions.
With the development of full-screen displays and the emergence of rigid OLED and flexible OLED smartphone displays, display-based fingerprint technologies are increasingly under discussion. As discussed in “Display-based fingerprint sensor structures,” building the fingerprint sensor into the display helps drive the 18:9 display trend and is a strong selling point.
Due to technological limitations, only under-display solutions will use CMOS image sensor (CIS) in the second half of 2017 and first half of 2018. CIS is not exactly built into the display, but fingerprint sensing can be achieved within the display’s active area by sacrificing some thickness. Display-based fingerprint sensing has not completely matured, but smartphone brands and panel makers are looking forward to offering it to convince consumers to replace old models.
Vivo was the earliest to adopt a display-based fingerprint solution with its X20 and X21 smartphones. The company is using a CIS solution, as shown below. The CIS supplier for the X20 is Synaptics; X21 suppliers are Synaptics and Goodix. The teardown revealed that the CIS is rotated 45 degrees and placed under the OLED display. Interestingly, with its tilted angle, the CIS is visible under the display. The Vivo X20 and X21 have the same solution we analyzed in the article “Fingerprint recognition for AMOLED may arrive in 2018.”
Questions and Answers
Below are our answers to some questions we receive about display-based fingerprint technologies.
Question: How close are under-display fingerprint sensor solutions to being commercialized?
Answer: Two approaches are feasible so far, optical imaging by CIS and ultrasonic echoing. With a CIS, there are two solutions available from IC makers—1D imaging from FPC and 2D imaging from Qualcomm. A CIS solution can be applied to rigid and flexible AMOLED displays. However, Qualcomm’s Sense ID2.1 is better and is applicable only to flexible AMOLED displays. We have not seen FPC’s solution applied by a smartphone maker.
For LCD, there are more challenges due to the LCD panel structure. Other technologies are still under development. One is optical imaging by transmitting and receiving, and makers are searching for feasible materials with the necessary photosensitivity and transmittance. Another is optical imaging by Fourier transform infrared (FTIR) spectrometer, which is still questionable.
True in-display solutions will likely not be available until 2019. In 2018, the major solution will be the CIS attached beneath the AMOLED display. Under-display is not necessarily in-display, but it can offer in-display active area fingerprint sensing.
Question: Is the fingerprint under-display solution something that will become part of the display/touchscreen module?
Answer: Over the long term, panel makers such as BOE and AUO are developing TFT-based fingerprint solutions. See “Display-based fingerprint sensor structures.” BOE adopted capacitive type and focuses on smartphones. AUO adopted optical imaging and focuses on commercial applications. The TFT process replaces the silicon semiconductor process. A glass substrate replaces the silicon wafer. The analog front end (AFE) converts the analog signals into fingerprint images for the back-end algorithm. Regardless, it may take a while for their solutions to be applicable.
An in-display solution will come later because panel makers have prioritized improving AMOLED yield, rather than adding complicated, new circuits such as in-cell touch or fingerprint sensors. True in-display solutions will likely not be available until 2019.
For display makers, 2018 should be the year for fine-tuning these CMOS solutions. In Q1 2018, we saw Vivo X20, X21, and the Huawei Mate RS Porsche Design with CIS under-display fingerprint solutions from Synaptics (FS9500) and Goodix (GF8508). Regardless, there are minor issues to be optimized, such as module assembly, visibility of the sensor (through the cover glass), and the wet/dry finger problem (a wet finger unable to activate the CIS).
Samsung is considering display-based fingerprint (DFP) sensors for its next flagship smartphone, the Galaxy Note 9. It should make a final decision in Q2 2018. We believe the candidate suppliers are Egis and Samsung LSI.
Based on panel makers’ abilities and priorities, display integration solutions (in-display or on-display) will not arrive for a while. CIS and ultrasound solutions that are combined with the display (located under the display so that less modification of the display is required) will take the lead for two years.
When DFP becomes the mainstream solution, panel makers will consider integration. This is like what occurred with touch as it moved from an add-on to embedded structures, first on-cell, and then in-cell. The display-based fingerprint market will move from CIS or ultrasonic modules to display fingerprint technology (on-display), and then finally to an in-cell structure (in-display) See “Replacing silicon wafers with TFT circuitry for fingerprint sensor.” The issues with in-display solutions that must be resolved are the light source (micro-LED level) and aperture competition.
We believe Samsung’s long-term strategy is to integrate the cover glass, AMOLED display, and DFP module into a whole assembly and become a one-stop shop to increase revenue. This may be the reason Samsung is waiting for its own Samsung LSI to offer its CIS solution, and is not applying DFP to its Galaxy S9 series—instead, using a traditional silicon-based fingerprint solution.
Question: Which smartphones will be the first to have a fingerprint under the display solution?
Answer: It is already on the Vivo X20, X21, and Huawei Mate RS Porsche Design. We believe Xiaomi is planning to use it, most likely on the Mi7. We also believe Samsung will adopt DFP on the Samsung Galaxy Note 9.
Question: Will Apple return with Touch ID under the glass?
Answer: No, Apple will not return to Touch ID under the glass. Apple already applied for many DFP patents. The silicon Touch ID conflicts with 18:9 displays (or wider aspect ratios like 19.5:9) because there is no space for the silicon fingerprint sensor on the screen. We believe Apple will continue to use Face ID, which is on the current iPhone X, for two to three years until it develops its own DFP solution, possibly ultrasonic. Apple’s evolutionary steps will be as follows:
- Use structured light for Face ID to solve its biometric ID needs. Meanwhile, build a mature supply chain for the next steps—vertical-cavity surface-emitting laser (VCSEL) light source and diffractive optics element (DOE) by wafer-level optics.
- Use the artificial reality (AR) kit only to support the 2D camera, with no 3D depth sensing. Make AR well-known to end users.
- Develop a new display fingerprint (DFP) technology to replace Face ID for biometric ID on the front side of the smartphone.
- Enhance its structured light solution—or develop time-of-flight (ToF) or active stereo vision—to deliver a 3D sensing upgrade to the AR kit.
- Develop an optical wafer-level package to solve the camera issue on the rear side of the phone.