03.09.20151 Training centre for Microtechnology and Nanotechnology Your experts for vocational training and further education Workshop: Introduction to

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Training centre for Microtechnology and NanotechnologyYour experts for vocational training and further education

Workshop: Introduction to Microtechnology

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Introduction to Microtechnology

The target of this key technology is to produce miniaturized structures. In recent years Microtechnology has seen a rapid development and it is established in many manufacturing processes in almost every branch of industry. You can find it

in printheads in ink-jet printers

in sensors triggering an airbag in a car

Electron Microscope figure of a micro heater rod with a wideness of 4 µm

in sensors for contact-less measuring oftemperature (e.g. fever)

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For example let us have a look at a product:

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Physical principle

Single process steps

Flow sensor

After understanding the physical principle of this sensor, we learn about the single process steps, how to manufacture this sensor with the processes of microsystem technology.

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Flow sensor

We take this example because

on one side, the manufacturing processes are simple, so the fundamental steps are very clear

on the other hand the flow sensor, as a micro component with micromechanic, fluidic and electrical functions, shows us typical process steps in micro electrics as well as in micromechanics.

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Flow sensorWe need it at many places in our daily life.

Our example comes from the automotive area.

Fuel consumption

Performance optimization

minimization of pollutants

Medical technology

Industrial process controlling Pneumatics and gas

supply plants

Process plants in Microtechnology

Test run of a cardiac catheter

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Flow sensor

The air flow rate, supplied to the motor per second.

There are two possibilities to describe the air flow rate:

per volume: volume / flow = litre/sec.

per mass: mass / flow = kg/sec.

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Flow sensor

Liquids for example have a constant density (specific weight), so it is not important which size you measure. Both dimensions can be measured by density.

It´s not the same with gas. It‘s easier to compress it, for example by the use of a turboblower which increases the air pressure of the motor. So there is much more mass flow for the same volume. So you can only measure the mass flow and the sensor is named „mass flow controller“.

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There are different measuring principles to measure flow. In our example we use thermal measurement.For a better understanding think about the situation of a bike driver and a cold day in winter. Of course he has forgotten his gloves.

When he is driving a bike his hands are very cold. It´s because cold air is flowing around his hands. So his hands are colder than when he is standing and the faster he drives the colder are his hands. So the bike driver must only measure the temperature of his hands with a thermometer and soon he has a measuring signal for the air flow.

Hands get cold because the heat is taken away by the wind.

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Wind chill factor

This device measures the mass flow directly. The feeling of the temperature is lower when at the same time an according wind velocity is given.You know this effect from skiing, when the wind blows very cold up on the hill and when the feeling of temperature on your face is much colder than in reality.

How can we transform this measuring principle into the build-up of a sensor?

First we need a sensor which is „hot“ and must be cooled down by air. Therefor it´s the best to use an electrically heated resistor. The resistor must be hotter than the measured air.This resistor must be positioned in the air flow and it must be a thin resistor, otherwise it would interfere with the air flow.

By the air flow the resistor will be cooled down

Apart of the cooling by air flow the resistor also emits heat by

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Now we must measure the temperatur of the wire!

That is very simple because the wire by itself is a good thermometer. Its resistance depends on the temperature, so we only measure the resistance to determine the temperature. Therefor we measure the applied voltage and the flowing current and by Ohms law we get the resistance .

In order we want to get a high resistance by modifiying temperature, the material of the resistor must have a high temperature coefficient., such as platinum or nickel. At least we must confirm that there is a good heat transfer to the air.

Now we have built a simple build-up of a sensor

The picture shows that the sensor consists of nothing else than a free strained platinum wire and current and voltage measuring.

An advantage is that the sensor works electrically, that means: the actual measurement dimension is an electrical one. It can be manufactured very easy for automotive applications.

heating wire

Temperature sensor

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The principle is called Cooling process and it is used in conventional sensors for many years. Apart from this measuring principle there are two other versions:

Constant-temperature process

Differential-temperature process

heating wire

heating wire

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Conventional sensors are very simple but on the other hand manufacturing and assembly is very difficult because thin wires must be exactly positioned. Also the thin resistor wires are very sensitive.

The solution are micro air flow sensors. It works by the differential-temperature-process.

The three wire resistors don‘t move free, but are positioned as a thin conduction path on a joint carrier.

Therefor we need a thin carrier instead of a thick solid carrier

Thin silicon-membrane

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You can compare this with the build-up of a circuit in electrical engineering.

You can manufacture the circuit as a „flying“ build-up (free wires bond the single devices)

or

you place the whole circuit board on the bonding conduction build-up of the circuit board carrier , which is made of polymers.

So you can solve two problems with a given build-up of a massflow sensor:

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Silicon membrane

This thin silicon membrane isn´t solid by nature because it´s much thinner than a household aluminium foil. So it must be tightened on a holder frame.This gives a plane and stable surface (perhaps you know this from foils used to close up bottled marmelade).

On such a membrane there are resistors like conduction paths made of copper on a printed circuit. The resistors are connected to each other by conduction paths made of aluminium and accordingly terminals.

Now wires can be bonded which integrate the sensor to an electrical analyzing unit.

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Air intake in a car motor, controlledby a senor

A sensor elementcomposed of

Measurement value

Evaluation electronics

a carrier plate

and the connections

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Planar array

Afford an opportunity of electrical integrationOn the wafer there are many sensors, produced in a parallel manner at the same time with electrical integration.

The sensors are positioned next to each other for manufacturing, without influencing each other.

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Miniaturization means

manufacturing of smaller sensors so you can apply much more sensors on the substrate.

This means the production of sensors is much cheaper.

Substrate with many single sensors

Substrate with less single sensors

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Complete waferCut and top view of a single wafer

What components are needed?

Silicon substrate to produce an electric circuit

Thermal resistor must be isolated from the substrate, so we deposit an isolating layer.

We need a thermal resistor with a platinum pattern as a thin conduction path.

We need a measuring resistor on the right and on the left side of the thermal resistor. Both can be fabricated on the substrate.

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The connection of the three resistors is realized by conduction paths made of aluminium.

We build the appropriate resistor close to the sensor. (the picture shows a transistor instead of a resistor)

The sensor isn´t packaged in a closed housing because the measuring resistors must be in the airflow. The circuits will be covered with a dense passivating layer; only the contact areas for the connections, which are made of gold, remain open.

At last we manufacture the membrane. Therefor the material beneath the resistors must be removed. Beneath the circuit and at the egde the material acts as a holding frame.

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After showing the active elements of the sensor we have a look at the manufacturing

First we have a look at the substrate. It must have following properties:

Poor thermal conductivity

Simple fabrication of membrane thickness (etchable)

Possibility to fabricate and connect semiconductor components for circuit Silicon wafer with diameter of 76 to

300 mm

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On this silicon substrate we manufacture the thermal resistor first. .

We have to isolate the resistor from the substrate. First we start with the isolation.

Suitable are typical oxides: for example aluminium oxide (mostly ceramics), however, it´s easier to transform the material on the substrate into an oxide. Silicon oxide is formed as the substrate is silicon. The substrate here is glass, a good isolator.

This process ist very simple – it´s a natural one. Aluminium is oxidized with a native oxide which prevents further oxidizing. With other processes this layer can be made thicker (anodizing). Silicon shows a similar reaction. A thin layer of native oxide is formed. Silicon is exposed to an oxygen ambient at high temperatures. Anodizing aluminium

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Now the resistor is manufactured. Here we use a common technique in microtechnology:

First the deposition of thin layers (whole area)

Then removal of the remaining material

Silicon Si

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In order to etch the correct area we have to determine the areas where material is to be removed.Lithography is used.

Silicon Si

Silicon Si

Silicon Si

Silicon Si

A photosensitive layer is deposited on the one to be etched. By exposure through a mask (having the correct pattern) the information about which areas to be etched is transformed.

After exposure the layer is changed in a way that the exposed areas can be removed in a developing solution.

On the unexposed areas the photosensitive layer remains and it protects the layer beneath it from the following etch step.

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In addition to a thermal resistor two measuring resistors at the left and right side are required.They can be manufactured in the same way as the thermal resistor.

An easier way is to enlarge the appropiate value of the conductivity of the silicon substrate in the area of the resistors (silicon is a good isolating material when unchanged) by adding atoms of other materials, such as boron. This technique is called „doping“.

In order to limit the areas to be doped a lithography step is required.

Boron atoms

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In order to connect the resistor to each other, to connect external connections and an optional measuring circuit conducting paths are required which are made of aluminumThey are manufactured the same way as the thermal resistors:

Aluminum is deposited on the whole area and patterned with lithography and etching. An isolating layer is to be deposited between containing openings (contact holes) where pre-manufactured structures (for example the thermal resistor) are positioned, in order to create an electric contact to the conducting paths.

A manifold of process steps is required for complex circuits with devices such as capacitors and transistors.

Conducting path made of aluminum (white)Contacting heat resistor (black)

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In the end the senor is deposited with a passivating layer. In contrast to the isolating layer beneath the resistors it can´t be manufactured from the substrate material (deposition techniques).

Then the substrate material is to be etched down to a thin membrane in the area of the resistors. In order to reduce thermal conductivity over the substrate in comparison to that over air.

This is a typical process step of micro mechanics.

The passivating layer protects the finished sensor structures.

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The last step is to build the sensor into a housing and to connect it with electrical contacts.

The assembly and packaging is dealt in the according module Assembly and Packaging.

Documents

03.09.20151 Training centre for Microtechnology and Nanotechnology Your experts for vocational training and further education Workshop: Introduction to