OLED-on-CMOS based bidirectional microdisplay for near-to-eye and sensor applications

Bernd Richter, Uwe Vogel, Philipp Wartenberg, Karsten Fehse, Rigo Herold COMEDD - Center for Organic Materials and Electronic Devices Dresden

Fraunhofer IPMS, Dresden, Germany

Abstract— This paper presents a bidirectional OLED microdisplay with a monochrome 320x240 display and a nested image sensor with 160x120 pixels in 0.35µm CMOS for near-to-eye and sensor applications.

Keywords: OLED; bidirectional microdisplay; image sensor; HMD

I. INTRODUCTION Organic light-emitting diodes (OLEDs) are the first

possible opportunity to integrate highly efficient light sources with photodetectors onto CMOS substrates and therefore completely integrated optoelectronic applications based on silicon can be realized on one chip. The direct light emission from the OLED enables small devices without additional backlight making them suitable for sensor and microdisplay applications.

OLED-microdisplays achieve high optical performance with excellent contrast ratio and large dynamic range at low power consumption making them suitable for mobile near-to-eye (NTE) applications such as viewfinders or head-mounted displays (HMD). In these applications the microdisplay acts typically as a purely unidirectional output device [1-3].

With the integration of an additional image sensor, the functionality of the microdisplay can be extended to a bidirectional optical input/output device, which means images can be both displayed and received.

In see-through HMD applications the embedded image sensor captures the eye scene of the user and enables the implementation of eye-tracking capabilities to achieve gaze-based human-display-interaction.

Figure 1 OLED-on-CMOS - simplified cross-section.

II. OLED-ON-CMOS TECHNOLOGY The monolithic integration of OLED-on-CMOS is realized

by direct evaporation of the OLED stack on the silicon wafer through a shadow mask in high vacuum. At this the smallest possible OLED pattern is limited to about 100 µm because of the necessary opening in the fine metal mask.

Inside the microdisplay, the OLED stack itself is unstructured and the pixels are defined by the shape of the top metal acting as the bottom electrode of the top-emitting OLED. Due to the sensitivity of the OLED materials against humidity and oxygen, a thin film encapsulation and a cover glass are applied at the end of the OLED post processing. Figure 1 shows a cross-section of a CMOS chip with an integrated OLED and photodetector.

III. BIDIRECTIONAL OLED MICRODISPLAY This paper presents a bidirectional OLED microdisplay

(BiMi) with a monochrome 320x240 (QVGA) display and a nested image sensor with 160x120 pixels in a commercially available 0.35µm 3.3V/12V CMOS process with customized top metal. The BiMi was designed as a universal prototype for evaluating different HMD and sensor applications. Therefore the main target was to achieve a wide range of possible display luminances from 100...10000 cd/m² instead of high resolution.

The active area of the BiMi consists of nested display and image sensor pixels surrounded by driving and control circuitry. The display and image sensor systems are electrically independent of one another, simply interacting via synchronization signals. The simplified system diagram is shown in Figure 2.

Figure 2 Simplified diagram.

978-1-4577-0430-7/11/$26.00 ©2011 IEEE 1

A. Display Since the luminance of an OLED is proportional to its

current density, each display pixel comprises a current driver with analog modulation of the OLED current to realize 8b grayscale. The display pixel pitch is 36x36µm² and the pixel current ranges from pAs to µAs depending on the OLED efficiency and the desired luminance. The pixel circuit of the display and the image sensor with a simplified driving scheme is shown in Figure 3.

Figure 3 Display and image sensor pixel circuit with simplified driving/readout scheme.

The microdisplay is driven via a digital 8b video interface including synchronization signals. An integrated DAC converts the digital video data into a scaleable data and offset current to achieve a wide range of brightness and contrast. The DAC current is copied into the display data driver according to the principle of a dynamic current mirror. The display data driver is a dual set of 320 sample-and-hold (SH) stages, where the first set is being programmed by the DAC, while the second set is driving the previously stored current into the corresponding pixel cell of the selected line.

In this sample phase Md1 and Md2 are on, Md4 is off and thus Md3 is diode connected and the data current is driven through Md3. The established gate voltage is stored on Cd. In the following emit phase the switches Md1 and Md2 are turned off, Md4 is turned on and the data current flows from Md3 through the OLED. This current copy principle compensates for CMOS process variations and achieves good matching between different pixels. A screenshot is shown in Figure 4.

Figure 4 Die photograph and a photo taken directly from a bidirectional Mikrodisplay with orange OLED on top.

A disadvantage of current programmed pixels is the different settling behavior over the entire signal range because of large parasitics on the data line. For low brightness levels and highly efficient OLEDs, the data current would be too small for sufficient settling in the sample phase. Since the current driver Md3 is working in the sub-threshold region, the programmed data current can be much larger in the sample phase and scaled down by shifting the reference voltage of the sampling capacitor Cd in the emit phase [1].

The complete display circuit operates on a 3.3V supply with an additional negative OLED cathode voltage. For evaluation purposes of different OLED stacks combined with high brightness, switch Md4 is implemented with a 12V transistor with rather large area consumption. The optical characterization of the display using a green OLED showed good linearity for peak luminances from 100 up to 24000 cd/m² and a high contrast ratio of 3000:1@1000 cd/m². The measured display transfer characteristic is presented in Figure 5.

Figure 5 Measured display transfer characteristic for peak luminances from 160...24000 cd/m².

B. Image sensor The integrated image sensor is composed of 160x120 pixels

with 8x8µm² photodiodes in a pitch of 72x72µm². Thereby the photodiodes are placed between the OLED pixels and the active pixel circuitry is located underneath the OLED anode, see Figure 6. The work function is global shutter with synchronous start and end of the exposure for all pixels to avoid moving artifacts during frame capture.

Figure 6 Setup of photodiode and display electrode.

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Before the exposure phase the gate of Mc3 is connected to vdd by turning Mc1 and Mc2 on. The exposure starts by turning the reset transistor Mc1 off, allowing the integration of the photocurrent from the photodiode, PD, on the gate of Mc3 and on Cc. The end of the exposure phase is determined by turning the transfer transistor Mc2 off and thus separating the PD from the integrating node. Subsequently, the stored picture information is read out line by line by turning Mc4 on and biasing Mc3 with a current via the data line. Now Mc3 works as a source follower and drives the stored gate voltage to the readout circuit.

To eliminate the influence of the threshold voltage of Mc3, the readout circuit performs correlated double sampling (CDS) by subtracting the reset value from the exposure value at each column. The offset corrected pixel values are latched on capacitors, get selected by the column addressing and are then transferred to an external ADC by an output driver.

IV. SYSTEM With the exception of the external ADC for image readout,

all control, configuration (I2C) and driving circuitry is integrated in the BiMi to allow the direct operation on a small mobile PDA sized host system.

Figure 7 shows the setup of a realized see-through NTE-application with bidirectional optic. Inside the optic the paths for display and camera are separated by filters for NIR and VIS spectrum to realize the different focal lengths between retina and eye scene. The realized display exhibits bright crisp images without motion artifacts and the captured images from the eye scene are suitable for basic eye tracking.

Furthermore the BiMi can be used in different sensor applications as combined light source and detector on a single chip. Figure 8 shows an optical feedback demonstration whereby the detected image is directly displayed.

Figure 7 Setup of the realized bidirectional optic with separated optical paths for display (green) and camera (red).

Figure 8 Feedback demonstration.

V. SUMMARY

ACKNOWLEDGMENT This work was partly funded by grants from the Federal

Ministry for Education and Research of the German government (Bundesministerium für Bildung und Forschung, BMBF, 16SV2283, 16SV3682).

REFERENCES [1] Haiqing Lin, Eric Naviasky, John Ebner, William Evans, Patrick Farrell,

Mike Hufford, Gary Levy, David Wheeler, Bryan Allison, Olivier Prache1, "An 852x600 Pixel OLED-on-Silicon Color Microdisplay Chip using CMOS Sub-Threshold-Voltage-Scaling Current Driver", ISSCC 2002, SESSION 26, MEMS AND DISPLAYS, 26.6

[2] Uwe Vogel, Ian Underwood, Gunther Notni, Christian Zilstorff, Klaus Meerholz, Gunther Haas, "HYPOLED - VGA OLED Microdisplay for HMD and Micro-projection", IMID2009

[3] Kelly, G.; Woodburn, R.; Underwood, I.; Bums, D.; Monteith, R., "A Full-Color QVGA Microdisplay using Light-Emitting-Polymer on CMOS", ICECS '06, pp.760-763

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