Monolithic Integration of a Novel Microfluidic Device with Silicon LightEmitting Diode-Antifuse and Photodetector

P. LeMinh, J. Holleman,J.W. Berenschot, N.R. Tas, A. van den Berg

MESA+ Research Institute, University of Twente, the NetherlandsE-mail P.LeMinh@el.utwente.nl

Abstract

Light emitting diode antifuse has been integrated intoa microfluidic device that is realized with extendedstandard CMOS technological steps. The devicecomprises of a microchannel sandwiched between aphotodiode detector and a nanometer-scale diodeantifuse light emitter. Within this contribution, thedevice fabrication process, working principle andproperties will be discussed. Change in the interferencefringe of the antifuse spectra has been measured due tothe filling of the channel. Preliminary applications areelectroosmotic flow speed measurement, detection ofabsorptivity of liquids in the channel…

1. Introduction

Silicon is well-known as an electronic and mechanicalmaterial but hardly ever used as an active optoelectronicelement, due to its inefficient radiative recombinationover an indirect bandgap. Integrated silicon opticalsensors themselves encounter that critical obstacle.Recently, silicon nanometer-scale diode antifuse hasemerged as a reasonable choice with improvedefficiency, small size, and CMOS compatiblefabrication[1][2].

With the rapid growth of microfluidic andmicrochemical systems in research as well as inapplications, specialized devices for sensing andmeasuring parameters are becoming more and moreimportant [6][7]. The integration of the light emitter anddetector with a microfluid channel provides such auniversal tool for that purpose. This device structure hasalso been greatly desired in µTAS, biochemical sensorsand actuators[4][5]. Furthermore, the approach can alsogive birth to new types of sensors.

The technology used in the creation of this device is acombination of standard CMOS technology with siliconsensor micromachining technology. The antifuse andphotodiode were made by standard CMOS steps, but themicrochannel required a sacrificial poly-Si etch inpotassium hydroxide (KOH). However, we are now

developing an CMOS compatible etching recipe usingTetra Methyl Ammonium Hydroxide (TMAH).

In the next sections of the paper, fabrication process,some critical technological design parameters, electricaland optical properties, few results on first applicationswill be described respectively.

2. Device realization

The realization of the device (figure 1) is a 10-maskprocess. The starting material is a 3-in silicon n-typewafer with standard doping. First of all, junctionphotodiode areas were fabricated by implantation ofboron. So as not to make any step height difference at thejunction between n-type and p-type region, thick resistwas used as masking layer for the implantation. This isimportant to the evenness of the channel. The shape ofthe channel was then produced by shaping a layer of500nm thick LPCVD polysilicon deposited on top of thefirst silicon rich nitride (SiRN) layer. Next another SiRNfilm is deposited to completely cover the polysilicon.These two SiRN layers acts as walls of the channelopened later by sacrificial polysilicon etching in KOH.They also electrically isolate the detector and integratedantifuse from the channel. On top of the second SiRNlayer, a capacitor structure with size of 5×5µm2 isfabricated. The first electrode is 300nm phosphorusimplanted polysilicon. The capacitor dielectric layer is8nm LPCVD deposited silicon dioxide. The secondpolysilicon electrode was implanted with BF2 with a doseof 3.1015 cm-2 at 70 keV (figure 2,3). The activation ofall dopants were simultaneously done in the next step asa protection layer of SiRN was deposited at 850°C forapproximately 90min. Next, for the opening of thechannel, windows at two channel ends were patternedand etched until reaching the sacrificial polysilicon. Theetching in KOH etches polysilicon at a rate of roughly1µm per minute. That means a channel of 2mm wouldtake almost 1.5 days of etching (figure 4) [3]. The siliconwafer is, however, well protected because the SiRN isetched extremely slow in KOH. SiRN was also chosenfor the process due to its low mechanical stress comparedto stoichiometric silicon nitride. As the channel was

etched through, the processed wafer then needed astandard treatment to dissolve alkali ions and othercontaminations. The inner surfaces of the channel neededhydrophilic “Piranha” treatments to be able to conductliquid. The next step is to make electrical contacts to thedetector and the antifuse by SiRN etching and metaldeposition. To fill the channel, confinement of liquid atthe channel ends is necessary. This is done by plasmadeposition of a hydrophobic FC-layer in the form of ringsonto the wafer using a shadow mask. The process wasfinished with the etching and metalization of the backside.

3. Device properties and discussion

The completion of the device showed that amicrofluidic component could be integrated into standardCMOS device structure (figure 1, 2, 3, 4). The devicestrategy is vertical instead of lateral [7]. The novelty ofthis structure proves that waveguide is not alwaysnecessary. By the geometry, it is clear that coupling lossand loss within the fibre and can be avoided.

In figure 5, the pre-breakdown current voltagecharacteristics of the antifuse capacitor are shown in bothbiasing polarities. It shows well known Fowler-Nordheimtunnelling at the applied electrical field of approximately10 MVcm−1. At lower field, the leakage current showspattern as has been commonly observed in standard thinoxide MOS capacitors.

In order to create the antifuse, a constant current of100 nA was used to stress the capacitor until breakdown(known as charge to breakdown method) in electroninjection mode. Further programming currents up to5mA were then applied. The diode antifuse emits visiblelight in reverse breakdown condition. Current voltagecurve of the antifuse is plotted in figure 6 showing abreakdown voltage at 3V. The mechanism of thebreakdown is supposed to be Zener breakdown due to thehighly-doped level of both electrodes. The seriesresistance, has not been improved from the devicereported in [1], is largely caused by the columnarstructure of the polysilicon (high sheet resistance). For abetter power conversion efficiency of the emitter, thefuture device should be fabricated with small grainpolysilicon.

In figure 7, the spectra at two emitting currents aredisplayed showing the interference fringes of the emittedlight waves due to the multi-layer structure of the device.The fringes have been previously observed in simplepoly/mono antifuse structure. In this case, the pattern ismore complicated with more pronounced peaks andvalleys. Nevertheless, the position of these extremescould be altered by filling the channel with othermaterials instead of air. The displacement of theextremes is dependent on the complex refractive index ofthe filled material and can be theoretically calculated bymulti-layer physical optics formulations.

We have filled the channel with deionised water andaqueous fluorescine solution, measured and comparedthe spectra at 1mA to the one of unfilled channel. Theshift of the extremes is clearly seen as shown in figure 8.The measurements on the unfilled channel was done bothbefore and after the liquid filling, showing consistentshape and slope. It is clear that this device has potentialapplications in sensing metrology for microfluidics.

The mechanism of flow speed measurement andabsorptivity can be discussed based on figure 9 wherebyshowing the photocurrents dependence on the regulatedantifuse currents [1]. At a fixed light emitting current, thechange in the medium of the channel can be sensed onthe detector by the alteration in the magnitude of thephotocurrent. This alteration is caused by the absorptionof photon in the liquid that fills the channel, and the newrefractive index difference at the two interfaces of thechannel walls.

4. Conclusion

The success of the novel device indicates that asystem of light emitter, photodetector and a reactionchamber, a microchannel in this case, can be fabricatedon silicon with CMOS technology. For the first time,nanometer-scale light emitting diode antifuse has beenapplied to a microfluidic device. The electrical andoptical properties of the components are signs of a newlyproven technological combination and realistic deviceworking principle.

Acknowledgement

We acknowledge the assistance of M. de Boer and A.Aarnink. This work is supported by MiCS, a strategicresearch orientation at University of Twente.

References

[1]. P. LeMinh et al., “A Novel Silicon Electro-OpticalDevice for Sensor Applications”, Proc. IEEE/ LEOS 2000,Puerto Rico, 2000, pp. 523-524.

[2]. P. LeMinh et al., “Photochemical Reaction ByNanometer-scale Light Emitting Diode-Antifuses”, Proc.Transducer ’01, Munich 2001, pp. 544-547.

[3]. J.W. Berenschot et al., “Advanced Sacrificial Poly-SiTechnology for Fluidic Systems”, Proc. Transducer ’01,Munich 2001, pp. 624-627.

[4]. A. van den Berg et al., “Micro Total Analysis Systems:Microfluidic Aspects, Integration Concept and Applications”Topics Cur. Chem., 194, (1997), pp. 21-50.

[5]. Yu-Chong Tai, Presentation “MEMS Sensors forBiomedical Applications” NSF/ERC Industry Day, May 16-17,2001, California Institute of Technology.

[6]. N. R. Tas et al., “Nanofluidic bubble pump usingsurface tension directed gas injection” accepted for AnalyticalChemistry, 2002.

[7]. M. Foquet et al., “DNA fragment sizing by singlemolecule detection in submicrometer-sized closed fluidicchannels” accepted for Analytical Chemistry, 2002.

Photodetector & 1 SiRNst

Poly-Si channel & 2 SiRNnd

Antifuse structure on 2 SiRNnd

3 SiRN rd cover & channel openning

Metal contactsFigure 1. Device fabrication process

Figure 2. Channel under the antifuse window, twopolysilicon electrodes are clearly visible

Figure 3. SEM image of integrated device on top ofthe channel

Figure 4. SEM image of channel entrance

0 2 4 6 8 101E-14

1E-12

1E-10

1E-8

p electrode +p electrode -

Cur

rent

[A]

Voltage [V]

Figure 5. Pre-breakdown current voltage curve ofthe antifuse capacitor in both biasing directions

-10 -5 0 5 10

-2

-1

0

1

2

3

Cur

rent

[mA

]

Voltage [V]

Figure 6. Current voltage characteristics of thediode antifuse

800 700 600 5001E-4

1E-3

0.01

0.1Ir=0.5[mA]

Ir=1.0[mA]

Irra

dian

ce[µ

W/c

m2 /n

m]

Wavelenght [nm]

Figure 7. Irradiance spectra of the integratedantifuses

Figure 8. Filled channels shifting the interferencepattern of the antifuse spectra at I = 1mA

0.1 0.0 -0.1 -0.2 -0.3

2.0n

0.0

-2.0n

-4.0n

(IV)(III)

(II) (I)At antifuse currentsIr=0.0mA

Ir=0.5mA

Ir=1.0mA

Ir=1.5mA

Ir=2.0mA

Ir=2.5mA

Pho

tode

tect

orcu

rren

ts[A

]

Detector Voltage [V]

Figure 9. Photocurrents on the detector

800 700 600 5001E-4

1E-3

0.01

0.1

airwaterfluorescineIr

radi

ance

[µW

/cm

2/nm

]

Wavelength [nm]