Product Embedded Surface Sensors

Integrated Under Wear Resistant Coatings

Introduction

The industry is continuously demanding more actively responding machine parts

capable of communicating their current state of operation. Today you can buy

many different types of discrete thin film sensors off the shelf, but many

environments cannot tolerate insertion of discrete components. With surface

embedded sensors under wear resistant coatings we can create intelligent

machine parts with active-adaptive protection through online control of the

current physical status. The fabrication of surface embedded sensors on three-

dimensional (3D) machine parts and other high-tech products require new

technologically demanding approaches. The present trans-Nordic COSMOS

project has successfully merged clean room based sensor design and fabrication

with wear-resistant hard coatings, fabricating a prototype of a surface embedded

temperature sensor on a steel substrate.

Insulating Layer

For standard clean room processed structures it would be straight forward to either

grow a thermal silicon oxide or deposit an insulating layer by PECVD or LPCVD to

get a good electrical isolation between the sensor structure and the substrate.

Since the substrates relevant for the COSMOS project have geometries far beyond

flat silicon wafers and in some cases also have a limited process temperature

window, these techniques cannot be used. Instead we have been investigating

other deposition techniques such as DC Sputtering, Magnetron Sputtering, Arc

Diamond Like Carbon (Arc-DLC), Ion Beam Assisted Deposition of DLC (IBAD-

DLC) and Atomic Layer Deposition (ALD).

Sensor Fabrication

Thin film sensors are traditionally fabricated by deposition of metal and isolation

layers with subtractive or additive patterning by conventional photolithography

and clean room processing techniques. However, these processes require flat

and smooth surfaces which is not valid for industrially machined parts. Therefore

our sensor structures were fabricated using shadow mask technology where the

sensor materials are directly patterned during deposition through a plate with

openings, e.g. a patterned metal foil. We used a magnetic metal called 7C27Mo2

with openings down to 100 µm in size. The shadow mask was held in place

using several strong magnets on the back side of the substrate.

y [ohm] = 0,74 * Temp + 511,4

R2 = 0,99

500

525

550

575

600

625

650

0 50 100 150 200

Temperature (C)

res

ista

nc

e /

oh

m

AcknowledgementsWe want to thank the Nordic Innovation Center, NICe, for the financial support.

Tommy Schönberg1, Magnus Svensson1, Lars Pleth Nielsen2, Helena Ronkainen3, Niels Peter Østbø4

1Acreo AB (Sweden), 2DTI (Denmark), 3VTT (Finland), 4SINTEF ICT (Norway)

COSMOSComponents and Smart Machines with

Micro-Nano Surface Embedded Sensors

Substrate

AlN

Cu

Substrate

AlN

Cu

Steel

ChromiaAlumina

Chromia or CrN

Alumina

Alumina

Alumina

Chromia

Chromia

Chromia

Steel

ChromiaAlumina

Chromia or CrN

Alumina

Alumina

Alumina

Chromia

Chromia

Chromia

In the cases of DLC coatings it was necessary to increase the adhesion of the sensor layer to the underlying DLC-layer by pre-treating the substrate with 70 keV oxygen implantation.

The best results for the isolating layer were obtained for multilayer insulating

coating system such as: (i) an Atomic Layer Deposition of Al2O3 followed by Dual

Magnetron Deposition of Al2O3, or (ii) several layers of Diamond Like Carbon

(DLC, Ta:C), or (iii) a multilayer system composed of several layers of alternating

Al2O3 / Cr2O3.

Temperature Measurement

A prototype Pt100-like temperature sensor was buried under Diamond Like Carbon

revealing promising properties for a wide range of industrial applications. The

resistive sensor response gives a close to linear signal as a function of the

temperature.

Close-up image of the sensor structures revealing the fingerprint of the underlying Pt structures

Temperature response for a surface embedded sensor sandwiched between multilayer Cr2O3 / Al2O3 and a non-conducting IBAD DLC coating.

Several different types of sensor structures were fabricated on a steel substrate using a shadow mask. A 1000 Å thick Pt layer with an underlying adhesion layer of 50 Å Ti was deposited using E-beam evaporation.

Substrate

CrN-CrC

DLC

Embedded

sensor

There are several difficulties that need to be addressed and subsequently overcome in order to fabricate a working sensor on a traditionally 3D machined metal part. First the metal needs to be electrically isolated, second, the sensor structure needs to be fabricated on the surface and third, the sensor needs to be embedded under a wear resistant coating to protect it from mechanical damage.

The coatings were tested on several different parameters. Here we see the results from a 150 kg Rockwell C indentation of a buried sensor under DLC. No delamination of the coating means good adhesion and pass on the test.

The 100 µm thin shadow mask is formed as a 4” wafer to fit the processing equipment without any need for special holders. The industrial steel substrates are 100 mm in diameter for the same reason.

AlN was one of the tested coatings that showed poor insulating qualities. This could be due to pin holes in the coating or in poor step coverage of the rough substrate surface.

Al2O3 is generally a good insulating material, but the final results are very much dependent of the deposition technique used to cover the substrate. Dual magnetron deposition showed poor insulation, but with an underlying ALD layer of Al2O3 the insulation was perfect.

The insulating properties can also be improved by alternating the coating material in several steps and thereby creating a multilayer coating. Here we see a dual magnetron multilayer stack of Al2O3 / Cr2O3.