Polymer anchored microelectromechanical system (MEMS) cantilever and method of fabricating the same

Abstract

A microelectromechanical system (MEMS) cantilever includes a base and a cantilever beam projecting from the base. The cantilever beam includes a piezo layer sandwiched between an inorganic material structural layer and an inorganic material encapsulating and immobilizing layer. A pair of electrical contacts are formed in the encapsulating and immobilizing layer in contact with the piezo layer. The base consists of polymer. A method includes depositing a sacrificial layer on a substrate; forming a MEMS cantilever beam on the sacrificial layer by depositing an inorganic material structural layer thereon; depositing a piezo layer on the structural layer; and depositing an inorganic material encapsulating and immobilizing layer on the piezo layer; forming a pair of electrical contacts in the encapsulating and immobilizing layer in contact with the piezo layer; forming a polymer base for the cantilever beam; and etching the sacrificial layer to release the MEMS cantilever beam from the substrate.

Claims

1. A polymer anchored microelectromechanical system (MEMS) cantilever, comprising: a base; and a cantilever beam projecting from the base, consisting of: an inorganic material structural layer; an inorganic material encapsulating and immobilising layer; and a piezo layer sandwiched between the inorganic material structural layer and the inorganic material encapsulating and immobilising layer; and a pair of electrical contacts formed in the inorganic material encapsulating and immobilising layer and in contact with the piezo layer, wherein the base consists of polymer, and each of the inorganic material structural layer and the inorganic material encapsulating and immobilising layer is formed of a material that is selected from silicon nitride, silicon oxide, silicon carbide or diamond; and wherein the cantilever beam is released from a substrate by etching a sacrificial layer between the cantilever beam and the substrate using a chrome etchant to form the cantilever devoid of the substrate; and wherein the sacrificial layer comprises an adhesion layer consisting of a first chromium layer deposited on the substrate, and a fast etching layer consisting of a gold layer deposited on the first chromium layer and a second chromium layer deposited on the gold layer.

2. The cantilever as claimed in claim 1, wherein the piezo layer is selected from polysilicon, graphene, doped zinc oxide, zinc oxide nanowires, silicon nanowires, carbon nanotubes, platinum, gold, indium tin oxide, polyvinylidene difluoride or Dy modified BiFe03.

3. The cantilever as claimed in claim 1, wherein the electrical contacts consist of titanium-gold or titanium-platinum.

4. The cantilever as claimed in claim 1, wherein the polymer base consists of SU-8 polymer or polymethyl methacrylate polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A of the drawings accompanying this specification is an isometric view of the cantilever of the invention.

(2) FIG. 1B of the drawings accompanying this specification is a sectional view of the cantilever of FIG. 1A along line A-A.

(3) FIG. 2 of the drawings accompanying this specification is a graph showing the relationship between force vs deflection of the cantilever of the invention.

(4) FIG. 3 of the drawings accompanying this specification is a graph representing the natural resonant frequency of the cantilever of the invention.

(5) FIG. 4 is a graph representing relationship between delta R/R vs deflection of the cantilever of the invention.

(6) The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1

(7) Silicon wafer substrate was surface cleaned according to RCA (Radio Corporation of America) protocol and a sacrificial layer consisting of a bottom adhesion layer of chromium (10 nm thick) and a top galvanic layer or fast etching layer of gold (50 nm thick) at the bottom and chromium (50 nm thick) at the top was deposited on the substrate by sputtering. A structural nitride layer (650 nm thick) was deposited on the sacrificial layer by CVD (chemical vapour deposition) technique using silane and ammonia at a flow rate ratio of 1:20 sccm (standard cubic centimeters per minute), respectively for 60 minutes and at a filament temperature of 1900° C. substrate temperature of 160° C. and chamber pressure of 1.1×10.sup.−1 mbar. Structural nitride layer was patterned using positive photoresist S1813 (Shipley 1813, Microchem, USA) and etched using 5:1 BHF (buffered hydrofluoric acid) followed by stripping the positive photoresist with acetone.

(8) A polysilicon piezo layer (300 nm thick) was deposited on the structural nitride layer by CVD technique using silane, diborane and H.sub.2 gases at a flow rate ratio 1:7:10 sccm (standard cubic centimeter per minute) respectively for 30 minutes at a filament temperature of 1900° C., substrate temperature of 170° C. and chamber pressure of 1.1×10.sup.−1 mbar. The polysilicon piezo layer was patterned using S1813 (Shipley S1813, Microchem, USA) and etched using HNA, (a mixture of hydrofluoric acid and nitric acid in deionized water).

(9) An encapsulating and immobilizing nitride layer (125 nm thick) was deposited on the piezo layer by CVD technique using silane and ammonia at a flow rate ratio 1:20 sccm (standard cubic centimeters per minute) respectively for 12 minutes at a filament temperature of 1900° C., substrate temperature of 160° C. and chamber pressure of 1.1×10.sup.−1 mbar. The encapsulating and immobilizing nitride layer was patterned and contact windows were opened in the nitride layer using lithography and the nitride layer was etched with 5:1 BHF (buffered hydrofluoric acid).

(10) Contacts of titanium (20 nm thick) at the bottom and gold (200 nm thick) at the top were deposited in contact with the piezo layer by making contact windows in the encapsulating and immobilising nitride layer by sputtering and lift off techniques. A base or support layer of SU-8 polymer (about 100 μm thick) was spin coated on the nitride-polysilicon-nitride cantilever beam in cycles of 300 rpm for the first 15 seconds followed by 500 rpm for the next 10 seconds, 3000 rpm for next 45 seconds and 500 rpm for the next 10 seconds. The SU8 polymer support layer was prebaked, UV exposed and post baked to cross link the polymer.

(11) The cantilever beam was released from the substrate by keeping the cantilever beam with the substrate in a chrome etchant consisting of a solution of ceric ammonium nitrate, acetic acid and deionized water to obtain a microcantilever device 1 as shown in FIG. 1A and FIG. 1B, in which the polymer base and cantilever beam projecting from the base are marked 2 and 3, respectively. The structural layer, piezolayer and encapsulating and immobilising layer of the cantilever beam are marked 4, 5 and 6 respectively. Electrical contacts of the cantilever are marked 7. The cantilever beam had a thickness of 1075 nm as measured by profilometer.

Example 2

(12) Mechanical and electromechanical properties of the cantilever of Example 1 were measured as follows:

(13) i) Spring Constant

(14) Spring constant of the cantilever (ratio of force to deflection) was measured by the beam-bending technique using Berkovich Indenter and Hysitron Triboscope. Tip of the Indenter was placed at the apex of the cantilever and force was applied at the tip to vary the displacement of the tip from 100 nm to 1200 nm. At various displacements of the tip, load or force versus displacement was plotted to obtain a graph as shown in FIG. 2 of the drawings accompanying this specification. Spring constant of the cantilever as deduced from FIG. 2 is 0.9 N/m.

(15) ii) Resonant Frequency

(16) Resonant frequency measurements of the cantilever were performed in air using Polytec Laser Doppler Vibrometer (LDV). Die was mounted on a piezo buffer to provide actuation to the cantilever. Measurement results were as shown in FIG. 3 of the drawings accompanying this specification. Measured resonant or natural frequency of the first mode of vibration as given by LDV was 23.5 KHz and quality factor Q as estimated from the plot was 28. FIG. 3 clearly shows that the cantilever of the invention has high resonant frequency so as not to be susceptible to external noise which falls in the range of 5 KHz and below.

(17) iii) Deflection Sensitivity

(18) Deflection sensitivity measurement of the cantilever was carried out by deflecting the tip of the cantilever with a precalibrated micromanipulator needle and simultaneously measuring the voltage and current using a Keithley 4200 source measuring unit. One probe of the measuring unit was placed at the tip of the cantilever and the two other probes of the measuring unit were placed at the two contacts of the cantilever. Needle at the tip of the cantilever was moved in the z direction in steps of 10 micrometer with the help of the manipulator. Current and voltage measurements were done at the same time. Relative change in resistance as a function of deflection was plotted as shown in FIG. 4 of the drawings accompanying this specification. Slope of the graph gave the deflection sensitivity of the cantilever. Deflection sensitivity of the cantilever as deduced from the graph in FIG. 4 is 0.3×10.sup.−6 nm.sup.−1. Deflection sensitivity is an important measure for cantilever based sensor applications.