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Sensing & Measurement

Optoelectronic chips integrate emitters and sensors

Electronic circuits and optical devices can be integrated on a single silicon chip using low-cost fabrication processes and simple packaging.
16 June 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.1151

Organic light-emitting diodes (OLEDs) permit the monolithic integration of integrated circuits and light-emitting devices on the same silicon chip. Using integrated photodetectors, low-cost CMOS processes, and simple packaging, economical optoelectronic integrated circuits (OEICs) with combined sensors and actuating elements can now be realized.

Highly efficient OLEDs are well suited for integration onto CMOS substrates, as evidenced by their use in microdisplays.1 OLEDs are directly deposited by evaporation onto the top metal layer of CMOS substrates. They can also be deposited over a large area and be patterned during post-processing using a vertical in-line fabrication system, such as the VES400 from Applied Materials (formerly Applied Films), or other tools.

The top metal layer of integrated circuits provides the interface to the OLED. Figure 1 shows a cross-section of a CMOS chip with an OLED sandwiched between the top metal layer of the electronics (which acts as the OLED's bottom electrode) and a thin metal layer (OLED top electrode). Before the organic layers are deposited, the top metal layer is passivated and then patterned. Subsequently, we deposited both of the organic layers as well as a few-nanometer-thick metal layer onto the surface. (The OLED stacks were provided by NOVALED AG.) To achieve reasonable lifetimes, the OLED must be protected against water vapor and oxygen. For orange OLEDs on CMOS test substrates, a lifetime of 10k hours was reached.2

Figure 1. Organic LEDs (OLEDs) can be monolithically integrated onto CMOS chips. (Left) An OLED sandwiched between metal electrodes consists of a p-doped hole transport layer (HTL), electron blocking layer (EBL), emitter-doped emission layer (EML), hole blocking layer (HBL), and an n-doped electron transport layer (ETL). A 15nm Ag layer is the semi-transparent top electrode. (Right) Cross-section of a CMOS chip and monolithic integrated OLED.

Figure 2. An application-specific integrated circuit (ASIC) includes optical emitters and detectors, as well as other sensors. (Left) Detailed view of an OLED electrode with integrated circuits below, and (right) larger view of the chip.

We developed application-specific integrated circuits (ASICs) manufactured using commercial 1μm CMOS technology with a high-voltage option. Among other things, 5V and 12V metal-oxide-semiconductor (MOS) transistors, as well as three metal layers, were used in the circuit design. The ASIC combines three different types of sensors: a reflective sensor, a color sensor, and a flow sensor. Each uses a different arrangement and geometry for the photodetectors and light emitters (see Figure 2). The detail on the left shows an OLED electrode with integrated circuits below the electrode and a photodiode at the top. Photodiodes used n-well/p-substrate diodes. The OLED electrode dimensions vary. The color sensor's electrode is 1mm×1mm and the flow sensor's is 6mm×0.5mm. The reflective sensor includes six OLED electrodes: two electrodes have dimensions of 1.5mm×1.5mm while the others are narrower, with dimensions of 1.5mm×0.2mm. The chip's dimensions are 18.2mm×8.5mm. The ASIC contains a control unit, a bias and reference network, sensory signal conditioning, and an interface output. Each of the sensors contains a dynamic OLED driver and logarithmic amplifier to evaluate photocurrents.

Figure 3. (a) Radiance and current-density and (b) the spectral radiance for monolithic integrated OLEDs.

Orange and red phosphorescent positive-intrinsic-negative (PIN) OLEDs were integrated on the ASIC (see Figure 3). At 7V, we measured a radiance of 4.8W/m2sr for the orange and 3.2W/m2sr for the red emitter. These operating voltages are inferior to the ones we reported last year.3 One reason might be the roughness of the top metal layer.

Figure 4. The optoelectronic integrated circuit mounted on a printed circuit board.

We have shown that PIN-type OLEDs can be used for monolithic integration onto CMOS integrated circuits. Furthermore, on-chip electronics can also implement highly complex sensor signal processing functions and thus contribute to the realization of powerful sensor systems. Cost-related advantages argue for monolithic integration of CMOS and OLEDs. Compared to III-V-based OEICs or sensor systems, reduced costs allow a commercial breakthrough for certain optical sensor applications. Figure 4 shows our OEICs mounted on a printed circuit board.

Sven Reckziegel, Daniel Kreye, Tino Puegner, Christiane Grillberger, Michael Toerker, Uwe Vogel 
Fraunhofer Institute for Photonic Microsystems (IPMS)
Dresden, Germany

Sven Reckziegel received his diploma in electrical engineering from the University of Applied Science, Lübeck, Germany, in 2006. He is currently a PhD student at IPMS in the Sensor and Actuator Systems business unit, engaged in developing optical sensors and their integrated circuits.