Micro-Electronic Applications

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Application Bulletin n°12

- Micro-Electronic Applications
- Nano-Scratch Tester (NST) for characterizing anti-stiction coatings for Microsystems
- In-situ Integrated Circuit (IC) characterization with the Nano Hardness Tester (NHT)
- Quality control of Micro-Slit reflective coating with the Nano-Scratch Tester (NST)

”CSM Nano-Scratch Tester (NST) for characterising anti-stiction coatings for microsystems

”CSM Introduction

Due to their extremely low working force range, mechanical microsystems are sensitive to phenomena which can usually be ignored in macroscopic mechanical systems. Stiction refers to the adhesion originating from the tribological contact of moving parts, which can result in immobilisation of the device. In a typical microscopic contact, stiction forces in the nano- and micro-Newton range can prove severely detrimental to the microsystem functionality. Different force types can be responsible for stiction, although capillary forces are believed to be predominant in mechanical microsystems.

This application note describes the use of the Nano-Scratch Tester (NST) for investigating the tribological functionality of a microsystem known as a microshutter. This micromachined system consists of a shutter blade, a suspension beam, two fi xed electrodes and two stoppers at either side of each electrode to prevent short circuits. The hole under the blade is positioned so that it is either open or closed, depending on where the blade is positioned. A typical microshutter is shown in Fig. 1. With this kind of microsystem, stiction can occur between the sides of the blade and their respective stopper surfaces. Recent efforts have been made to lower the surface energy of polysilicon microstructures by depositing extremely thin coatings by plasma polymerisation. However, it is very diffi cult to deposit suffi cient material on vertical sidewall structures.


Figure 1 : SEM micrograph of a typical polysilicon microshutter.



Figure 2 : SEM micrographs showing the original polysilicon surface texture (a) of the microshutter sidewall and the Tefl on-like coated microstructure (b) which can signifi cantly reduce stiction in this type of microsystem.


The plasma polymerisation process is in this case applied with hexafl uoropropene as a precursor molecule in order to grow a Tefl on-like coating. The samples are placed under a Faraday cage (i.e., a fi eld free zone) within the reactor in order to promote a conformal growth of the polymer fi lm, this being necessary in order to cover all faces of the microstructures. The Scanning Electron Microscope (SEM) images shown in Fig. 2 confi rm that the deposition process is suffi ciently isotropic to coat the lateral surfaces of the electrodes, stoppers and blade over their whole height.

Quality control of such thin fi lms is very diffi cult in-situ owing to the complexity of the micro-machined structure and the diffi culties in positioning a probe tip onto a sidewall. For this reason, different types of Tefl on-like coatings are deposited onto wafers of mono- and polysilicon material similar to that of the microshutter itself. Subsequent NST measurements are then performed in order to characterise the adhesion and frictional properties of the coatings.

To give an idea of the dimensional limitations involved with the microshutter, some explanation of its operation are needed: electrostatic forces are only used to keep the shutter in the open or closed position, whilst the driving force to switch the shutter from one state to the other is only delivered by the spring energy stored in the suspension beam. The electrostatic voltage is only active over the narrow gap (0.5 - 1 μm) between the shutter blade and one of the electrodes. The shutters are brought to resonance by applying an alternating excitation voltage on one electrode and a direct attraction voltage on the other.


”CSM In-situ Integrated Circuit (IC) characterisation with the Nano Hardness Tester (NHT)

”CSM Introduction

Adequate quality control of integrated circuit (IC) components is fundamental during the development phase, allowing design and process engineers to evaluate the functionality of a new device before it reaches the fi nal package test, where the late realisation of design fl aws can be extremely costly in terms of time-to-market issues. Production line control is also important to maintain quality standards and check the properties of materials arriving from outside suppliers.

The Nano Hardness Tester (NHT) has already shown its value in being able to accurately measure the mechanical properties of IC aluminium bonding pads [1]. However, its combination of high positioning accuracy (< 1 μm) and automated measurement of hardness and elastic modulus at a nanometer scale, make it ideally suited to characterisation of many different kinds of IC structure. For example, a modern IC may consist of many circuit tracks which are of deposited gold, copper or aluminium. These are usually thin fi lms of thickness 0.5 - 3.0 μm and the track width may be as small as 10 μm. The substrate may be a relatively hard substrate (e.g., silicon) but in other cases may be a much softer material (e.g., polymer) such as those used for printing heads. The adhesion of the fi lm to the substrate is an important consideration, as is the hardness and elastic modulus.

Nanoindentation can reveal significant information regarding the structural integrity of a certain coating. For the case of aluminium bonding pads, which must serve the dual function of a probe-testing and bonding platform, mechanical properties need to be accurately controlled. Insuffi cient hardness of the fi lm results in deep scrub marks (during probe-testing) which then prevent a good bonding between the pad and the gold connecting wire. In addition, if the pad is too soft then substantial debris may be produced when the probe tip comes into contact with it, this being a very important consideration in such a particle sensitive environment. Surface topographical observation (e.g., scanning force microscopy (SFM)) is also useful in measuring the surface roughness of IC contacting parts as a high roughness may induce premature wear, or prove detrimental to the functionality of the specifi c device.


Figure 1 : Optical micrograph of a 200 mN nanoindentation performed on a circular bonding pad consisting of a 1 μm aluminium fi lm deposited onto a Si substrate.


”CSM Application

The optical micrograph shown in Fig. 1 shows a 200 mN indentation placed in the centre of a circular bonding pad (pad diameter = 40 μm). In this case, the indentation depth is slightly greater than the pad thickness in order to investigate the deformation which is produced. This can be used to simulate the effect of a probe tip contacting the pad during the device testing procedure. Indenting through the coating can also cause cracking or delamination of certain coatings from the substrate. This allows fracture toughness to be investigated in addition to hardness and modulus. Fig. 2 shows a typical gold conducting track onto which a nanoindentation has been placed. Highly precise position control is required in order to measure the properties of the gold independently to those of the surrounding Si structure. The size of the indentation is also important because if it is too large then the measured mechanical properties may not be representative of the track material alone.

[1] N. X. Randall, E. Holländer and C. Julia-Schmutz, Characterisation of integrated circuit aluminium bonding pads by nanoindentation and scanning force microscopy, Surface and Coatings Technology 99 (1998) 111-117


Figure 2 : Optical micrograph of nanoindentations placed in the Si structure of an IC device (left) and on a gold conducting track (right). Note that the positioning accuracy of the NHT allows the indent to be accurately positioned within a track of width 15 μm. The track is produced by lithographic etching, after which gold is deposited by sputtering into the prefabricated channel.


”CSM Quality control of Micro-Slit refl ective coating with the Nano-Scratch Tester (NST)

The variable-entrance slit system (or Micro-slit) is now commonly used as a critical diaphragm component in many spectrophotometers whose principal function is to analyse the molecular fi ngerprint of liquid samples. This micromachined structure (Fig. 1) consists of a central aperture plate supported by a pair of fl exible beams which allow light of different wavelengths to pass through the slit. The aperture plate is coated with a thin (500 nm) aluminium coating which serves the function of masking any light around the aperture.

Characterisation of the adhesion of this Al coating to its Si substrate is difficult owing to the small size of the aperture plate. The Nano-Scratch Tester (NST) has been used to accurately measure the scratch resistance by making progressive load scratches over the load range 0 - 10 mN with a 5 μm diamond tip. Fig. 1 shows two such scratches made on each sid of the central slit. Subsequent optical microscopy along the scratch paths allows the critical failure points to be observed: fi rst failure consists of cracking at the sides of the path (Fig. 2(a)), whereas fi nal failure is seen as delamination of the coating from the substrate (Fig. 2(b)). Such measurements confi rm the use of the NST as a useful tool for characterising coatings in-situ on ultra-small devices where low loads and high positioning accuracy are indispensible.


Figure 1 : Optical micrograph of a typical Microslit structure showing
the central aperture plate supported by a pair of fl exible beams of
thickness 80 μm. The zoomed image shows two scratches made on each        
side of the central slit (scratch direction from left to right).


Figure 2 : Optical micrographs of fi rst failure (a) where initial cracking
occurs and fi nal failure (b) where the aluminium coating completely
delaminates from the Si substrate. These images correspond to one of
the scratches shown in Fig. 1.



This Applications Bulletin is published quarterly and features
interesting studies, new developments and other applications
for our full range of mechanical surface testing instruments.


Dr. Nicholas Randall

Should you require further information, then please contact:

CSM Instruments
Rue de la Gare 4
CH-2034 Peseux

Tel: + 41 32 557 5600
Fax: +41 32 557 5610

[file] AB_12.pdf

Download the file:

[file] AB_12.pdf