Micro Electro Mechanical Systems '92 Travemunde (Germany), February 4-7,1992

MICROFABRICATION OF SUB MICRON NOZZLES IN SILICON NITRIDE

M.M.Farooqui and A.G.R.Evans

University of Southampton, Highfield, Southampton SO9 5NH, U.K.

ABSTRACT

Micro nozzles such as used in ink jet printers have been conventionally fabricated in single crystal silicon by anisotropic etching in the form of pyramidal cavities. These have fixed apex angles as determined by the intersecting 4 1 1 > planes in a silicon crystal. The exit apertures usually have square or rectangular openings of at least a few micrometres as the smallest dimension. We describe a novel microfabrication process for obtaining nanometre apertures in highly cusped nozzle like structures fabricated in silicon nitride, having apex angles of up to a few degrees. The process is based on a sacrificial etch technology using single crystal silicon as the mould and silicon nitride as the material for the nozzle. The nitride coating on the apex of the pyramid shaped mould is selectively etched off using a polymer layer as the etch mask, which leaves the tip of the silicon mould protruding from the masked nitride, thus defining the aperture of the nozzles. The silicon mould is then removed in an alkaline etchant which leaves the free standing nozzles. The process is applicable to fabrication of similar structures in a variety of other materials such as silicon dioxide, boron doped silicon, polysilicon, refractory and noble metals. The main requirement is the preferential etchability of the mould with respect to material for the nozzles. A variety of applications are envisaged.

INTRODUCTION

Microminiature apertures and nozzles are important structures in a variety of micromechanical devices, such as high resolution ink jet printing heads, microvalves, flow controllers, beam defining elements, atomizers etc. The most widely used fabrication techniques have been anisotropic etching of single crystal silicon to form pyramidal cavities with apertures at the apex ( l ) , isotropic etching of <110> silicon (2), laser or ultrasonic drilling in glass or similar materials (3-6) to form nozzles or cavities, electroless plating or electroforming or CVD deposition on suitable moulds, electron irradiation and etching of mica. The apertures obtained using these techniques are at best a few micrometres in diameter. When fabricated as nozzles, the flank geometry is either fixed as in case of the anisotropically etched <1 OO> silicon, or with minimal control of the taper angle. The aim of this paper is to describe a new method of manufacture of nozzles using silicon IC technology. The method described enables realization of nozzles with apertures in the deep sub-micron range, whose apex angles can be varied from a few degrees upwards. In addition the usual advantages of reproducibility and low unit cost associated with batch fabrication are obtained.

PROCESS TECHNOLOGY

The fabrication technology is based on the sacrificial etch process, similar to that used for producing polysilicon microstructures (7). The

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sacrificial mould in this case is made in single crystal silicon, which has been etched leaving a protrusion on its surface, using a combination of reactive ion and wet chemical etching (8). This enables the shape of the mould to be vaned from a pyramidal form with straight sides to a highly cusped one with smoothly curved flanks. The fabrication sequence, illustrated in Figs. l a to I f is as follows: A lightly doped <loo> silicon substrate is deposited with a composite masking layer of silicon nitride and oxide, 160nm and 500nm thick respectively, and patterned using dry etching, thus forming circular masked areas, typically 3 um in diameter on the substrate, as shown in Fig. l a . Using the oxide-nitride masks, the exposed silicon is reactively ion etched, semi anisotropically, to a depth of 3.5 um, resulting in undercutting the mask, and forming highly cusped structures with circular cross section, tapering down to several tens of nanometres, as shown in Figs. l b and 2. The oxide mask is removed in buffered HF, and the remaining nitride mask undercut using a short wet acid etch. This results in a very narrow tapered apex, with its diameter reduced to a few tens of nanometres. The flank angle is determined by the anisotropy of the process; if a pyramidal form of the nozzles is required, anisotropic wet alkaline etching is used in place of the reactive ion etch, which results in the crystallographically defined pyramidal form with flat sides, but still retaining a very sharp apex.

Fig. 2 Cusped etched silicon with mask in place

FABRICATION PROCESS

Fig. l a Mould etch mask

v

Fig. l b RIE mould etch

Fig. IC Pad oxide, nitride

Fig. I d Polymer reflow etch bac k

Fig. l e Free standing nitride nozzle

Fig. If Back etched nozzle support

U m I I I m Silicon nitride oxide polymer

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The next step is to replicate the silicon moulds in silicon nitride to form the nozzles. LPCVD nitride

layers deposited on silicon have a high tensile stress, and if more than a few hundred nanometres thick, are prone to cracking. This tensile Stress

can be reduced by annealing, or making the film non stoichiometric by making it silicon rich, or by the use of low temperature plasma enhanced

deposition. Prior to the deposition of the silicon

nitride to form the nozzles, a layer of padding oxide, few tens of nanometres, is interposed, which serves to relieve the stress, followed by LPCVD or PECVD nitride. The thicknesses of the nitride layers used have been between 350 and 550 nm. The deposited layers are highly conformal, replicating even the highly cusped moulds (Fig. IC). Finally a second layer of oxide is deposited over the nitride. This serves as a protective layer for the fine mould tips, as well as acting as an etch mask in the following step.

In order to define the nozzle aperture, the nitride has to be etched back to expose the required portion of the silicon mould. This is accomplished by coating the surface with a thick polymer layer. This is planarized by a thermal treatment so that the tips are nearly submerged in the polymer film. The coating is then reactively etched back in an oxygen plasma for a predetermined time to expose the required height of the covered tip, giving the required nozzle diameter. The etch rate is well characterized, so that the process is reproducible. This is followed by etching of the oxide and the nitride exposed layers (Fig. 1 d). A SEM photograph of a nozzle with the silicon mould still in position is shown in Fig. 3. The nozzles are made free standing (Fig. l e ) by etching away the silicon mould and part of the substrate in KOH. The remaining substrate forms a base for the nozzle. Optionally, back masked anisotropic etching can be used to etch the silicon pedestal, as depicted in Fig. I f . Finished sub micron nozzles are shown in the SEM photographs Figs. 4a and b.

Fig. 3 Nitride nozzle with silicon mould

Fig. 4a SEM of nitride nozzle Aperture 585 nanometres

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REFERENCES Fig. 4b SEM of nitride nozzle

Aperture 150 nanometres 1. E.Bassous and E.F.Baran, Fabrication of High Precision Nozzles by the Anisotropic Etching of (100) Silicon, J.ECS., 175, 1978,

2. K.E.Petersen, Fabrication of an Integrated, Planar Silicon Ink-Jet Structure, IEEE-ED, 26,

3. J.Olschimke et al., Fabrication of 15 um Thick Silicon Hole Masks For Demagnifying Projection Systems For lon- Or Electron- Beams, Microel. Eng., 6, 1987, 547-552,. 4. H.Siedel and L.Csepregi, Advanced Methods for the Micromachining of Sensor, Tech. Dig. 7th. Sensor Symp., 1988, 1-6. 5. M.Esashi, The Fabrication of a Micro Valve by Means of Micromachining, Proc. 6th. Sensor Symp. (May, 1986), 269-272. 6. T.Ohnstein et al., Micromachined Silicon Microvalve, IEEE CH2832, 1990, 90-95. 7. M.M.Farooqui and A.G.R.Evans, Polysilicon Microstructures,Proc. IEEE Workshop on MEMS

8. M.Farooqui, A.G.R.Evans, M.Stedman and J.Haycocks, Micromachined Sensors for Atomic Force Microscopy, Sensors: Technology, Systems and Applications, Ed. K.T.V.Grattan, A.Hilger, 1991, 373-378.

1321 -1 327.

1979, 1918-1 920.

1991, 187-191.

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