Components for optoelectronic and photonic integrated circuits — design, modelling, manufacturing and monolithic integration on silicon

Materials Science and Engineering B74 (2000) 89 – 95 www.elsevier.com/locate/mseb

Components for optoelectronic and photonic integrated circuits — design, modelling, manufacturing and monolithic integration on silicon D. Cristea a,*, F. Craciunoiu a, M. Caldararu b a

National Institute For R&D in Microtechnologies, Bucharest R-72225, Romania b ECOSEN Limited, Bucharest R-73101, Romania

Abstract This paper presents our results in the field of silicon optoelectronic and photonic integrated circuits. We integrated photodetectors, linear or logic electronic circuits, waveguides, coupling elements, interferometers, transducing layers and micromechanical structures on the same silicon chip. Design, modelling and experimental manufacture of these components are presented, underlining the original approaches and results. Special structures for photodetectors were designed in order to allow monolithic integration with electronic circuits and waveguides. The attention was focused on the matching of all the involved technologies, to allow the monolithic integration of all components. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Optoelectronic integrated circuits; Integrated optics; Microphotonic devices; Microsensors

1. Introduction Optoelectronic and photonic integrated circuits bring both the advantages of the classic optics: insensitivity to electromagnetic noise, noncontact measurements (which allows optical sensor to be used in harsh environments) and the advantage of integration: miniaturization, better reliability, low cost. Silicon is a very good substrate for such circuits and microsystems due to its good mechanical and thermal properties and due to the possibility of integration on the same chip of electronic circuits, photonic circuits and micromechanical structures using low cost and high reproducibility technologies [1 – 3]. That is why in recent years silicon optoelectronic and photonic integrated circuits saw an important development (Refs. [4–7] and [8 –13], respectively). We realized different types of optoelectronic and photonic circuits by integrating on the same silicon chip: photodetectors, linear or logic electronic circuits,

* Corresponding author. E-mail addresses: [email protected] (D. Cristea), [email protected] (M. Caldararu)

waveguides, coupling elements, interferometers, transducing layers and micromechanical structures. This paper presents the design, modelling and experimental realization of these components, underlining the original approaches and results. Special structures for photodetectors were designed to allow monolithic integration with electronic circuits and waveguides. Original models for these photodetectors were developed. The electronic circuits we realized, unlike those reported in the literature [4–7], can operate at very low input currents ( B 10 nA). Also new materials and processes were studied and experimented in order to improve the component performance. Attention was focused on the matching of all the involved technologies to allow for the monolithic integration of all the components. Thus, specific technologies for optoelectronic and photonic circuits were established. The development of these new technologies increases the efficiency of the circuits, achieving ruggedness, stability, reduced size, weight and cost and assures the technical conditions for use even in harsh environment. The aim of our research was to realize different types of microsenzors: optoelectronic, chemo-optical, mechano-optical.

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D. Cristea et al. / Materials Science and Engineering B74 (2000) 89–95

Fig. 1. Spectral characteristics of photodiodes realized using: (a) bipolar compatible processes and (b) MOS compatible processes.

2. Components and submodules — design and modelling

2.1. Photodetectors Photodiodes possess excellent linearity, speed, stability, temperature coefficient and dynamic range. They also have a very simple structure, so they can be made using either bipolar or CMOS compatible processes. As the structure and the technological process have to be matched with the other components (electronic circuits and waveguides) technology, the photodiodes responsivity is rather low. The values of the quantum effi-

ciency, h, and responsivities, R, we obtained using a bipolar or CMOS compatible process are: “ bipolar compatible technology — junction depth xj = 2.2–2.4 mm, epitaxial wafers: h =0.58 (R=0.4 A W − 1) at l= 850 nm h= 0.85 (R=0.5 A W − 1) at l= 725 nm “ MOS compatible technology (xj = 1.4–1.6 mm) h =0.65 R= 0.45 A W − 1 at l= 850 nm. These values are similar or even greater than those reported in the literature [4–7] and can be increased at a certain wavelength as required by a specific application using an antireflection coating. The spectral characteristics of the two types of photodiodes are given in Fig. 1a and b. The two curves are quite different due the different technological parameters. One can see that the photodiodes obtained using bipolar technology achieve the maximum responsivity around 725 nm. This shift towards the visible range is due to the epitaxial layer that has low thickness (approximately 10 mm) and low resistivity (2–10 Vcm). The main drawback of the photodiode is the low value of the responsivity. To increase the optical sensitivity of the integrated circuit a preamplifier can be introduced in the circuit, or a high gain photodetector can be used. The first solution is suitable for high-speed applications, and the second one is indicated for applications that require a high optical sensitivity and a simple structure. We designed and realized two types of high gain photodetectors: a photo-FET and a NPN phototransistor. The structures of the photo-FET and NPN phototransistor for optoelectronic integrated systems are given in Fig. 2a and b. Both devices have modified structures that allow the optical coupling with the waveguides and can be fabricated using CMOS processes. The photo-FET has only one gate (Fig. 2a). As the upper gate has been removed the device can be top illuminated with radiation propagating along the yaxis, or can be coupled to a waveguide (using a leakywave coupling [14]). The phototransistor base in the illuminated region and the photo-FET channel are shallow (B 0.3 mm) and low doped (NA/ND B 10), in order to improve the

Fig. 2. Structures of: (a) photo-FET structure, and (b) NPN phototransistor.

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similar photodetectors, such as Si-MESFET, and GaAsOPFET have been reported [15–17], but they cannot be applied at our device because they neglect the photovoltaic effect at the channel-gate junction. The model takes into account the dependence of channel depth on channel voltage (V) and two effects of the incident illumination: (1) the variation of channel conductivity; (2) the variation of channel depth (y1), due to the photovoltaic effect across the channel-gate junction. Thus, the drain current is: IDS = qmp Fig. 3. Channel depth of the photo-FET in dark and illuminated conditions for uniform illumination.

Table 1 The drain current of the photo-FET in dark and illuminated conditions, function on the gate voltage VGS (V)

IDS (dark) (mA) IDS (DPOPT = 0.1 W cm−2) (mA)

DIDS (mA)

0 −0.35 −0.4 −0.45 −0.5 −0.55 −0.6

0 0.2 3 5 12 20 30

20 24.8 30 38 50 65 80

20 25 33 43 62 75 110

responsivity at short wavelengths and the leaky-wave coupling to the waveguide. The depth and the doping of the photo-FET channel have to be further decreased to minimize the depth of the channel neutral region y1, and thus to obtain a very low dark current. Because the photo-FET structure is different from that of the conventional field-effect-phototransistors, we had to develop a model for this device. Models for

& & VDS

0

y1(V)

NA(y) dy dV

0

+ q(mp + mn) Z L

Fig. 4. Drain current IDS vs. drain voltage VDS in dark and illuminated conditions, with optical power as parameter, for uniform illumination.

Z L

& & VDS

0

y1(V)

0

DPopt at(1− exp (− ay) dy dV hn

where Z is the device width (600 Vm), L is the channel length (20 mm), DPopt is the incident optical power density, a is the absorption coefficient in silicon, and t is the photogenerated carriers lifetime. The dependence of y1 on the channel voltage V(x) and on the photovoltage VOP was obtained using a one-dimensional Poisson equation with the boundary conditions: V%(y1)= V%(y2)= 0, V(y2)− V(y1) =VGS + F0 − V(x)− VOP (F0 is the built-in voltage of the implanted junction). The model shows that, at low input optical power, the drain current increases due to the photovoltage developed across the channel-gate and thus the optical radiation controls the drain current by changing the channel depth rather than its conductivity (Fig. 3). That is why, the dependence of drain current on input illumination is logarithmic (Fig. 4). The model was verified on a test device with a totally depleted channel under dark condition [18]. As the channel is totally depleted, the dark current is IDS(dark) : 0. At DPopt = 0.1 W cm − 2, l= 670 nm, the drain current strongly increases (IDS = 20 mA) due to the photovoltaic effect. A similar value for IDS was obtained at DPopt = 0 but with a negative voltage applied on the gate, VGS = − VOP = − 0.55 V and with RG \50 kV (Table 1). The measured values of the drain current upon illumination are greater than those calculated, due to the carriers generated outside the channel region. Taking into account the lateral collection, the real value of the device is approximately 20 A W − 1. The responsivity increases if a negative voltage VGS is applied (RG \ 50 kV). Higher responsivities can be achieved if the device channel doping is increased so that the device can operate at lower input optical power [19]. A bipolar phototransistor was realized on the same chip (Fig. 2b) using the same technological processes. The collector current of the bipolar phototransistor under dark and illuminated conditions depending on the base current is given in Table 2. The light current of the collector-base photodiode, at the same input optical

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Table 2 The collector current of the bipolar phototransistor in dark and illuminated conditions, function on the base current IB (mA)

IC (dark) (mA)

ICL (DPOPT = 11 mW cm−2) (mA)

DIC (mA)

0 0.5 1 4 6 8 10

0 25 60 320 500 600 800

120 160 220 500 720 850 1100

120 135 160 180 220 250 300

Fig. 5. Structure of the waveguide integrated on silicon.

power, is ILPD = 1 mA. The phototransistor gain is thus ICL/ILPD =120. The responsivity at l =670 nm is R= 36 A W − 1. The gain increases when a current is injected into the transistor base, but responsivities greater than 100A W − 1 cannot be achieved. In conclusion, the photo-FET can achieve very high responsivities (104 A W − 1) at low input optical power. Its disadvantage is the strong dependence of the responsivity on the input illumination and on the channel doping profile. The NPN phototransistor has a lower responsivity ( B100 A W − 1), but its performances are less dependent on the technological process and on input illumination. Due to their high responsivities, these photodetectors an replace both a photodiode and a preamplifier

2.2. Wa6eguides The waveguides consist of a SiON or Si3N4 LPCVD layer, 0.3–0.5 mm thick, deposited on top of a 2-mm SiO2 layer (Fig. 5). The SiON layer is preferred, as its refractive index can be varied in the range 1.5–2, depending on deposition conditions.

The SiO2 layer, with refractive index nSiO2 =1.46 is used for optical isolation between waveguide (n=1.5– 2) and silicon substrate (nSi  3.85). For light confinement, the core or the upper cladding is structured to ribs by plasma etching. The upper cladding of the waveguide is air or a CVD SiO2 layer. The waveguides were fabricated using only an MOS compatible process.

2.3. Coupling elements The optical couplage between waveguides and photodiodes was achieved: 1. By a smooth transition of the SiON or Si3N4 waveguide from the field oxide to the active silicon area (Fig. 6a). On the top of the field oxide the electromagnetic wave is completely separated from bulk silicon. In the active area of the photodetector the wave partly couples into silicon and generates charge carriers. 2. Using a grating coupler (Fig. 6b). In both types of couplers leaky-wave coupling is involved. The incoming waveguide mode varying as exp (− jbmz) is transformed, in the coupling region, into a leaky-wave varying as exp (−jkzz) with kz =b− jag, and the optical power is transferred to the detector. In the case of grating coupler, optical power can be transferred both in substrate and in air. The power carried in the waveguide is P(z)=P0 exp (−agz), and the power density transferred in photodetector is DPinc(z)= P0/L × ag × exp (− agz). We have calculated the leakage parameter ag function on waveguide parameters (refractive index, and thickness and on wavelength). To calculate ag, we developed models for the two types of couplers [18,19]. A very interesting result was obtained for the second type of coupling between the photo-FET and the waveguide. For this coupler (Fig. 6b) the model takes into account the channel depth variation along the channel length (due to the potential variation) and along the channel width (due to the exponential decrease of optical power). Fig. 7 shows the channel depth variation for two channel

Fig. 6. Leaky-wave coupling of waveguide to photodetector: (a) using a grating coupler, and (b) by removing the isolation layer between waveguide and photodetector.

D. Cristea et al. / Materials Science and Engineering B74 (2000) 89–95

IDS =

&& & && & L

qmp L ×

VDS

y1(V,z)

NA(y) dy dV dz +

0

0

0

L

VDS

y1(V,z)

0

0

0

93



q(me +mn) L



DPopt(z) y at 1− exp −a hn sinu

n

dy dV dz The most interesting conclusion of the model is that, in case of the leaky-wave coupling of the opto-FET to a waveguide, the dependence IDS(P0) is practically linear (Fig. 8), although in the case of uniform illumination it was logarithmic (Fig. 4).

3. Integration and experimental results

3.1. Optoelectronic integrated circuits

Fig. 7. Channel depth under dark and illuminated conditions for two channel impurity profiles.

Fig. 8. Drain current of photo-FET IDS vs. drain voltage VDS with optical power as the parameter and for leaky-wave coupling with a waveguide.

profiles, with the following parameters: net charge implanted Q1 =1.9×1011 cm − 2, Q2 =2.3 ×1011 cm − 2, implant straggle parameters s1 =0.04 mm, s2 = 0.05 mm, effective ion-implant range parameters Rp1 = 0.02 mm, and Rp2 = 0.05 mm. As the channel depth, y1, is a function both of channel voltage, V = V(x), and of z, thus the drain current becomes:

Our aim was to obtain optoelectronic integrated circuits (OEICs) for low input optical power. Thus the preamplifiers included in these circuits should operate at low input currents: 10 nA–1 mA. The circuits reported in the literature can operate at high frequency (100 MHz–1 GHz), but the amplifier current has to be higher than 1 mA [4,6,20,21]. Moreover, some of these amplifiers are not monolithically integrated with photodiodes [20,21], or the photodiodes dark current is rather high (\ 10nA, Ref. [4]). We studied different types of preamplifiers and concluded that for high speed, high dynamic range and high responsivity OEICs, the best choice was the transimpedance type [22]. To achieve good performance for both the photodiode and electronic circuits, the design should be carefully optimized. Table 3 shows the relationship between the technological parameters (epitaxial layer thickness and resistivity — xepi, repi, base diffusion depth and sheet resistance — xjB, RB) and circuit characteristics (photodiode responsivity and bandwidth — RPD, BPD; amplifier transimpedance, bandwidth and input noise density — Az, BA, In; OEIC responsivity and bandwidth — RT, BT). The arrows indicate the way of the parameter variation. One can see that the requirements are contradictory, so a trade-off has is needed. Moreover, some requirements, such as high thickness and resistivity epi layer cannot be achieved by bipolar technology. That is why we optimized the technological parameters taking into account the main characteristics required for a specific application (Table 4). We developed three types of OEICS: “ high speed photodetector for optical interconnects (photodiode+ two-stage transimpedance preamplifier) “ photo-trigger (photodiode+ preamplifier+Schmitt trigger)

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Table 3 The relationship between the technological parameters and circuit characteristics Technological parameter

Influence on photodiode characteristics

Influence on amplifier characteristics

Influence on integrated circuit characteristics

Pxepi Prepi PxjB PRB

oBPD PRPD PBPD PRPD oRPD

oBA, PAz PBA, PAz

PBT PRT PBT PRT oRT oBT PRT

oBA, PAz, oIn

Table 4 Optimal technological parameters function for different applications Main characteristics

Optimal technological parameters

High speed, high optical sensitivity, linearity Very high optical sensitivity, TTL compatible output High optical sensitivity, TTL compatible output, high speed

xepi (mm)

repi (Vcm)

xjB (mm)

wB (mm)

RB (V)

12.3 8.5 10

6.5 1.7 6.5

2.56 2.5 2.2

0.64 0.6 0.75

200 170 220

Table 5 Optoelectric characteristics of the integrated photodetectors Characteristics

Linear OEIC

Photo-trigger

Logic gate photodetector

Photodiode responsivity

0.4 A W−1 at l =850 nm

0.5 A W−1 at l = 725nm

0.4 A W−1 at l = 850 nm

Amplifier gain Preamplifier dynamic range Amplifier input noise density Circuit responsivity Circuit illumination threshold Input illumination range

20 kV 5 nA–20 mA 2.5 pA Hz−2 6 mV mW−1 – 5 lx–20 klx 2mW cm−2–10 mW cm−2 \25 MHz 500–900

2 nA–200 mA

5 nA–20 mA 2.5 pA Hz−2 – 0.3mW cm−2 at l = 850 nm 20 mW cm−2– 10 mW cm−2 \25 MHz 500–900

Circuit bandwidth Circuit spectral bandwidth — Dl0.5 (nm) “

logic gate photodetector (photodiode + two-stage transimpedance preamplifier+logic gate) The main characteristics of these photodetectors are given in Table 5 and Fig. 9 shows the block diagram and the layout of the logic gate photodetector.

3.2. Photonic integrated circuits We made photonic integrated circuits for sensor applications. To obtain a chemo-optical sensor, three different technologies were used and matched: CMOS technology for photodetectors and amplifying circuits, LPCVD technology for waveguides and sol-gel technology for transducing layers [22]. In the first step we integrated on the same chip waveguides coupled to different types of photodetectors: photodiodes, NPN phototransistors, and field effect phototransistors (Fig. 10). The optical coupling between the photodetectors and waveguides was obtained by tapering the isolation oxide (Fig. 11).

– 5 lx 2 lx–2 klx 150 kHz 500–900

The second step was the integration of the transducing layers [22,23]. We made, in cooperation with ECOSEN Limited, absorptive sensors for ammonia and refractive sensors for proteins. The refractive structure includes a Mach Zehnder interferometer coupled to the reference arm through a Y branch. (Fig. 12). The transducing layer of the absorptive sensor was doped with bromcresol purple in order to detect low ppm ammonia. This sensor was included in a measurement system that can work independently from the computer

Fig. 9. Block diagram (a) and lay-out (b) of the logic gate photodetector.

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monolithic integration of all the components. High sensibility optical sensors and chemo-optical sensors for ammonia integrated to silicon have been obtained. Acknowledgements

Fig. 10. Waveguides coupled with different types of photodetectors.

This work was supported by the National Agency for Science, Technology and Innovation — Romania. The authors would like to thank V. Avramescu for circuits images obtained with a computer aided image capturing system. References

Fig. 11. Waveguide to photodetector coupling via a ramp of tapered isolation oxide.

Fig. 12. Microphotograph of the refractive chemo-optical sensor.

and has its own power supply. This system can be used to measure ammonia concentration in the range 50– 1000 ppm.

4. Conclusions We demonstrate that sensors with optical detection can be made using only processes compatible with silicon technology. We integrated photodetectors, linear or logic electronic circuits, waveguides, coupling elements, interferometers, transducing layers and micromechanical structures on the same silicon chip. Special structures for photodetectors were designed in order to allow monolithic integration with electronic circuits and waveguides. Attention was focused on the matching of all the technologies involved, to allow the

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