WO2008050354A2

WO2008050354A2 - A nanopositioner and method to nano position an object thereof

Info

Publication number: WO2008050354A2
Application number: PCT/IN2007/000503
Authority: WO
Inventors: Hilaal Alam
Original assignee: Hilaal Alam
Priority date: 2006-10-25
Filing date: 2007-10-24
Publication date: 2008-05-02
Also published as: WO2008050354A3

Abstract

The present invention is related to a nanopositioner comprising: heater to expand actuator by heating and peltier module to contract the actuator by cooling to position an object.

Description

A NANOPOSITIONER AND METHOD TO NANO POSITION AN OBJECT

THEREOF

FIELD OF INVENTION

The field of invention is related to actuation control for nanopositioning technology with nanometer resolution. Instead of using solid state actuator manipulating its domain properties, the thermal and peltier effects used to actuate the nanopositioners is the crux of the invention. Thus use of peltier effect for actuation purpose is the crux of the invention.

BACK GROUND OF INVENTION

This invention is related to positioning objects such as lenses, fibers, tools, sensors and other objects; with respect to nanometer resolution is a challenging one. With the advent of the technology in various fields such as photonics, optics, semiconductor, microscopy etc., the requirement for precise positioning with nanometer resolution is inevitable.

Solid state actuator is working based on its domain properties, which leads to hysteresis when used in open loop positioning systems. One way to eliminate the hysteresis problem is to use completely different technology where energy is not stored or stored energy is released immediately when required. Controlled heating and cooling of metal is one of actuating systems. This may not be suitable for fast, smooth and continuation operation but for step by step operations like pick and place movements.

Though the step by step displacement is not bothered in many of the applications, there are some applications such as scanning requires smooth continuous precise displacement with nanometer resolution.

Advantages: The speed of the Nanopositioner can be controlled by adjusting the temperature instead of domain properties. Larger displacement is possible with further research and development on materials. The disadvantages are addition of bulky peltier systems and unsuitable for scanning applications. Also it is very slow process comparing solid state actuator. OBJECTS OF INVENTION

The principle objective of the present invention is to develop a nanopositioner comprising: heater to expand actuator by heating. Another object of the present invention is to develop peltier module to contract the actuator by cooling to position an object.

Another main object of the present invention is to develop a method to nano position an object, said method comprises steps of: expanding actuator by heating to move a stage, and optionally cooling the actuator to contract and thereby positioning the object. Another main object of the present invention is to develop a nano nanopositioner comprising peltier module to expand or contract an actuator to nano position an object. Another main object of the present invention is to develop a method to nano position an object, comprising varying magnitude and/or changing direction of current flow in a peltier module to either expand or contract an actuator to position the object.

STATEMENT OF INVENTION

Accordingly the invention provides a nanopositioner comprising: heater to expand actuator by heating, and peltier module to contract the actuator by cooling to position an object; there is also provide a method to nano position an object, said method comprises steps of: expanding actuator by heating to move a stage, and optionally cooling the actuator to contract and thereby positioning the object; further is also provides a nano nanopositioner comprising peltier module to expand or contract an actuator to nano position an object; and a method to nano position an object, comprising varying magnitude and/or changing direction of current flow in a peltier module to either expand or contract an actuator to position the object.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1: shows thermal expansion of Aluminium

Figure 2: shows force generation on stage due to the expansion of 25mm Aluminium Figure 3: shows temperature rise with respect to change in current for aluminium.

Figure 4: shows seebeck effect

Figure 5: shows drop in temperature due to change in current for aluminium.

Figure 6: shows an experiment of peltier heat measuring Cu, Bi.

Figure 7: Usage of semiconductors of p- and n-type in thermoelectric coolers. Figure 8: Structure of a Peltier module Figure 9: Peltier module

Figure 10: An example of cascade connection of Peltier modules Figure 11: An outward appearance of a cooler with a Peltier module Figure 12: A nanopositioner assembly with heater Figure 13: A nanopositioner assembly without-heater

DETAILED DESCRIPTION OF THE INVENTION

The primary embodiment of the invention is a nanopositioner comprising: a) heater to expand actuator by heating, and b) peltier module to contract the actuator by cooling to position an object. In yet another embodiment of the present invention the actuator comprises a thermal sensor to sense temperature value of the actuator.

In still another embodiment of the present invention a controller to maintain constant or required temperature across the actuator.

In still another embodiment of the present invention the controller controls the heater and the peltier module on the basis of inputs from the sensor.

In still another embodiment of the present invention the actuator moves a stage on which the object to be nano positioned is placed. In still another embodiment of the present invention the actuator is placed at cold junction of the peltier module.

In still another embodiment of the present invention the peltier module current is varied by the controller to cool the actuator.

In still another embodiment of the present invention the peltier module hot junction comprises a cooling system preferably an external fan to cool it.

Another main embodiment of the present invention is a method to nano position an object, said method comprises steps of: a) expanding actuator by heating to move a stage, and b) optionally cooling the actuator to contract and thereby positioning the object.

In yet another embodiment of the present invention heating or cooling of the actuator is stopped when the stage on which the object is placed reaches target place.

In still another embodiment of the present invention maintaining constant temperature across the actuator once the target place is reached. In still another embodiment of the present invention the constant temperature is maintained by sensing the actuator temperature and controlling the heater and/or peltier module to avoid undesirable contraction or expansion of the actuator.

Another main embodiment of the present invention is a nano nanopositioner comprising peltier module to expand or contract an actuator to nano position an object.

In yet another embodiment of the present invention the actuator comprises a thermal sensor to sense temperature value of the actuator.

In still another embodiment of the present invention a controller to maintain constant temperature across the actuator based on the sensors value. In still another embodiment of the present invention the controller controls direction of current flow in the peltier module on the basies of inputs from the sensor to maintain constant temperature across the actuator.

In still another embodiment of the present invention the actuator moves a stage on which the object to be nano positioned is placed. Another main embodiment of the present invention is a method to nano position an object, comprising varying magnitude and/or changing direction of current flow in a peltier module to either expand or contract an actuator to position the object.

Yet another embodiment of the present invention is sensing the temperature of the actuator with thermal sensor. Still another embodiment of the present invention is maintaining constant temperature on the actuator by a controller when the object is positioned at a predetermined position.

Still another embodiment of the present invention is varying the magnitude and/or direction of the current in the peltier module by the controller to maintain the constant temperature.

The present invention shall now be fully described with reference to the accompanying drawings in which, Figure 12 and 13 is a nanopositioner assembly showing the various parts of the assembly showing their positioners.

Actuation Expansion: The thermocouple series materials are used to control actuators here. The materials are simply heated with heating coil. For the experimental purpose, Al - Ni (or cu - Bi) combination have been chosen. The coefficient of expansion of Aluminum is 24 X 10^Λ-6 per C at 2OC. The linear expansion is (Delta) L / Lo = a.(Delta)T

(Delta) L = Lo . a . (Delta) T

The Length of the actuator is 25mm and 5mm X 5mm cross section. a = 24 X 10^Λ-6 per C at 20C

If the change in Temperature = 5 C, then the free expansion of the actuator is 3 micron. The compressive load on the actuator is 1 Kg (this is the force required to actuate the stage and hence the reaction is equal on the stage). Force applied on Stage = aTEA

= 24 X 10^A-6 X 5 X 7000 X 25 = 21 Kgf This force is sufficient to exert the force over the stage for 3 micron expansion.

Table 1 The thermal expansion is linear and hence heating the aluminum bar is quite an easy one by controlling current in the heat coil (Refer figure 1 ). However, the stage is expanded for 100 micron, the force generation is high up to 60Kgf (Figure 2). Hence the stage should withstand that. Otherwise this approach can be used for small range of expansion.

Speed of Actuation: Speed of Expansion is another factor. The speed is determined by the rate of temperature rise and co-efficient of thermal expansion of materials. In other words, it is a Joule Loss which is square of current. When current was raised from 0.1 Amp to 1 Amp the change in temperature was about 2C (refer to figure 3) and force exerted on the stage was about 5 Kgf and expansion was 1 micron.

Actuation Contraction:

The actuator that expanded is to be cooled down to the original position in order to bring the stage back. The cooling action is carried out by PELTIER MODULE. Peltier effect is nothing but reverse Seebeck effect.

Seebeck Effect: when two different metals ends are subjected to different temperatures, current flow occurs along the metals (refer to figure 4). The inverse effect is called Peltier effect.

The thermoelectric series: The thermoelectric series is a set of elements that can be used to build thermocouples and which participate in the Seebeck effect and it's inverse, the Peltier effect. As you move the two elements you are using closer in the table, the thermoelectric yield goes down, as it does when you reduce the temperature difference. The following elements below are listed in the thermoelectric series in order:

Silicon, Bismuth, Nickel, Cobalt, Palladium, Platinum, Uranium, Copper, Manganese, Titanium, Mercury, Lead, Tin, Chromium, Molybdenum, Rhodinium, Iridium, Gold, Silver, Aluminium, Zinc, Tungsten, Cadmium, Iron, Arsenic, Tellurium, Germanium The above list of elements also reveals some possible wire pairings. For instance, iron or copper can be put on the positive terminal while constantan can be used for the negative terminal of a thermocouple circuit (Type J and T).

Thermoelectric Sensitivity

The Seebeck coefficients (thermoelectric sensitivities) of some common materials at 0 ⁰C (32 ⁰F) are listed in Table 1.

: Seebeck¹ Seebeck Seebeck

Material ' Material Material

Coeff. * '^" ; rCoeff. * Coeff. *

Aluminum 3.5 Gold 6.5 Rhodium 6.0

Antimony 47 Iron 19 Selenium 900

Bismuth -72 Lead 4.0 Silicon 440

Cadmium 7.5 Mercury 0.60 Silver 6.5

Carbon 3.0 Nichrome 25 Sodium -2.0

Constantan -35 Nickel -15 Tantalum 4.5

Copper 6.5 Platinum 0 Tellurium 500

Germanium 300 Potassium -9.0 Tungsten 7.5

* :. Units are mV/°C; all data provided at a temperature of 0 ⁰C (32 ⁰F)

Table 2

Peltier effect describes the temperature difference generated by EMF and is the reverse of Seebeck effect. However, not so long ago there appeared new-based cooling devices - semiconductor coolers which utilize the Peltier effect.

Peltier modules

If you put a drop of water in the hollow on the joint of 2 semiconductors Sb and Bi, and switch on the current, the drop would freeze (with the reverse direction of the current the drop would melt). This is how Peltier effect works. Unlike the Joule heat which is proportional to the current strength squared (Q=R I I t), the Peltier is proportional to the current strength and changes the sign (-/+) if the current changes the direction. The Peltier heat equals:

Qp = P ^• q Where q=H,

Hence Qp = P .1. t

P is a Peltier factor that depends on contacting materials and temperature. Peltier heat is considered positive in case of dissipation, and negative in case of absorption. Ignoring changes the sign (-/+) - current changes the direction. The temperature change is linear. See figure 5. When the current is increased from 0.5 to 1.5 amperes, the temperature rises at hot point and equally the temperature drops at cool point. This cool point is attached with the aluminum bar to cool the aluminum. The scheme of the experiment of Peltier heat measuring, Cu, Bi. Is shown in figure 6

In this case the Joule heat in both calorimeters is the same (since R = R(Cu)+R(Bi)). But the Peltier heat differs in the sign. So, this experiment allows to calculate the Peltier factor. In the table below you can see some Peltier factors for different pairs of metals.

Peltier factors for different metal pairs

Fe-constantan Cu-Ni Pb-constantan

T, K P, ιiiV T, K P, mV T, K P, mV

273 13,0 292 8,0 293 8,7

299 15,0 328 9,0 383 1 1 ,8

403 19,0 478 10,3 508 16,0

513 26,0 563 8,6 578 18,7

593 34,0 613 8,0 633 20,6

833 52,0 718 10,0 713 23,4

Table 3

Usually, a Peltier factor is calculated this way: P = μ - T

P - Peltier factor, μ - Thomson factor, T - absolute temperature.

In theory, the Peltier effect is explained the following way: electrons speed up or slow down under the influence of contact potential difference. In the first case the kinetic energy of the electrons increases, and then, turns into heat. In the second case the kinetic energy decreases and the joint temperature falls down.

In case of usage of semiconductors of p- and n- types the effect becomes more vivid. In figure 7 you can see how it works. Combination of many pairs of p- and n- semiconductors allows to create cooling units - Peltier modules of relatively high power (see the scheme shown in figure 8). A Peltier module as shown in figure 9 consists of semiconductors mounted successively, which form p-n- and n-p-junctions. Each junction has a thermal contact with radiators. When switching on the current of the definite polarity, there forms a temperature difference between the radiators: one of them warms up and works as a heat sink, the other works as a refrigerator. A typical module provides a temperature difference of several tens degrees Celsius. With forced cooling of the hot radiator, the second one can reach the temperatures below 0 Celsius. For more temperature difference the cascade connection as shown in figure 10 is used.

Peltier module's power depends on its size. The modules of low power might not be efficient enough. But the usage of the modules of too high power might cause moisture condensation, what is dangerous for electronic circuits. The distance between conductors on the modern printed circuit boards constitutes parts of a millimeter. Nevertheless, they were powerful Peltier modules and additional cooling systems which helped KryoTech and AMD companies to overclock AMD processors up to 1 GHz. We should notice here, that the systems work was stable and reliable enough. Similar experiments were made with Intel Celeron, Pentium II, Pentium 111, which achieved tremendous performance growth.

We should point out that Peltier modules dissipates a lot of heat. That's why it's necessary to use not only a powerful fan in the cooler, but also other different fans inside the case, (refer to figure 1 1 ) The present invention uses thermal properties of materials for actuation of nanopositioners. "Seebeck" Materials having higher thermal expansion co - efficient are heated uniformly for actuating the nanopositioner.

Concept of Using Peltier Effect

Approach 1: Heating the actuator (Aluminum) with external sources to actuate and Peltier Module for cooling the actuator is shown in figure 12. Heater is on by closing the switch and this heats the actuator (aluminum bar). As the bar expands, the stage is moved forward. If the stage is desired to be moved backward, the heater is switched off and peltier module is on. The cold junction is made such a way that it touches the hot actuator (aluminum bar). The hot junction is fitted with an external fan as microprocessor is cooled off. By increasing the current, the the temperature is made drop to the desired level to contract the actuator. Once the stage reaches target place change of current is withheld. Now due to natural cooling or heating of bar to the room temperature, the bar may contract or expand respectively. This variation is adjusted using the temperature sensors.

For example, the bar actuator is heated to 50C using an external source and cooled down to 4OC using the peltier module. Now at 4OC the peltier module is stopped.

However, since the room temperature is 25C, the bar will tend to contract further till the temperature reaches 25C. To prevent this undesirable contraction of bar and to maintain the bar at 45C, the temperature sensor is used to record the temperature (45C here) at the point when Peltier module is stopped operation. The heater is switched on for 45C.

The nanopositioner here uses a couple of Seebeck materials for actuation purposes. Two metals having higher thermal expansion co-efficient are used in this set up. One end is called cold junction and the other end is called hot junction. These two junctions are closed by power supply. The cold junction acts as an actuator and hence it is placed inside the nanopositioners. This actuator is connected with heating coil and temperature sensors. The temperature sensor is used to maintain the desired temperature at the metal so that the metal never contracts or expands. The heater is ON so that temperature increases in the actuator region which results in expansion of the metal. During this process the hot junction can be disconnected to prevent overheating of the hot junction. When the desired displacement is achieved, the temperature is fixed so that the sensor maintains the same condition.

When the reverse displacement is required, the heater should be OFF and peltier systems should be ON. By increasing the current flow along the circuit, the cold junction becomes cold and thus contract to the original state. In case any intermediate position is desired, the peltier systems should be switched OFF simply and activate temperature sensor to remain heating the metal with heater.

Approach 2: Changing the direction of Peltier Module to heat and cool the actuator (aluminum) (Thus eliminating the need of heating coil) is shown in figure 13. In this method, by reversing the current flow of peltier module, the cold junction acts as hot junction. When the bar is to be maintained at certain temperature, the peltier module changes its current direction accordingly. When the bar (actuator) needs to be heated, the current flow direction is reversed and hence the bar gets heated. When the bar (actuator) needs to be cooled, the current flow direction is brought forward and thus the bar becomes cold. The thermal sensor senses the temperature and sends signal to the controller in order to maintain the temp. The controller decides the current flow direction to heat or cool the bar (actuator).

Operating features

Peltier modules are very reliable; they haven't got any moving parts, unlike refrigerators constructed according to the traditional technology. But despite all the mentioned advantages, Peltier modules have some specific features, which must be taken into account when using as a part of a cooling unit. The most important characteristics are:

• The modules, dissipating much heat, require the relative fans and heat sinks which would manage to carry off the heat effectively. We should notice, that the thermoelectric modules have a quite low performance factor and they are themselves a powerful source of heat. The usage of these modules might cause overheating of the other components inside the system block. That's why it's necessary to install additional cooling systems inside the block. Besides, the modules consume a lot of energy, and in this connection a power supply unit capacity shouldn't be less that 250 W. Though there are sometimes Peltier refrigerators with its own power supply unit.

In case of the module's failure the cooler becomes isolated from the cooled element. It might lead to fast overheating of the latter.

Low temperatures might cause moisture condensation. This might lead to short circuits between the elements. That's why you should use the modules of the

I O optimal power. Moisture condensation depends on the temperature inside the system block, the temperature of the cooled device and air moisture. The warmer air is and the more moisture is, the condensation is more probable. The table below shows the dependence between a condensation temperature on a cooled element and a temperature and moisture of the surrounding air.

15 Advantages of the Invention

• The expensive feedback systems, complex algorithm can be avoided.

• Long range displacement is possible

• No friction is present as in case of Omicron

20 • Entire systems forms a monolithic structure and hence manufacturing process is relatively simple.

Applications of the invention

Claims

We claim:

I . A nanopositioner comprising: a) heater to expand actuator by heating, and b) peltier module to contract the actuator by cooling to position an object.

2. The nanopositioner as claimed in claim 1, wherein the actuator comprises a thermal sensor to sense temperature value of the actuator.

3. The nanopositioner as claimed in claim 1 comprises a controller to maintain constant or required temperature across the actuator.

4. The nanopositioner as claimed in claims 1 , 2 and 3, wherein the controller controls the heater and the peltier module on the basis of inputs from the sensor.

5. The nanopositioner as claimed in claim 1, wherein the actuator moves a stage on which the object to be nano positioned is placed.

6. The nanopositioner as claimed in claim 1, the actuator is placed at cold junction of the peltier module.

7. The nanopositioner as claimed in claims 1 and 3, wherein the peltier module current is varied by the controller to cool the actuator.

8. The nanopositioner as claimed in claim 1 , wherein the peltier module hot junction comprises a cooling system preferably an external fan to cool it.

9. A method to nano position an object, said method comprises steps of: a) expanding actuator by heating to move a stage, and b) optionally cooling the actuator to contract and thereby positioning the object.

10. The method as claimed in claim 9, wherein heating or cooling of the actuator is stopped when the stage on which the object is placed reaches target place.

1 1. The method as claimed in claim 9, wherein maintaining constant temperature across the actuator once the target place is reached.

12. The method as claimed in claim I I , wherein the constant temperature is maintained by sensing the actuator temperature and controlling the heater and/or peltier module to avoid undesirable contraction or expansion of the actuator.

13. A nano nanopositioner comprising peltier module to expand or contract an actuator to nano position an object.

14. The nanopositioner as claimed in claim 13, wherein the actuator comprises a thermal sensor to sense temperature value of the actuator.

15. The nanopositioner as claimed in claims 13 and 14 comprises a controller to maintain constant temperature across the actuator based on the sensors value.

16. The nanopositioner as claimed in claims 13, 14 and 15, wherein the controller controls direction of current flow in the peltier module on the basies of inputs from the sensor to maintain constant temperature across the actuator.

17. The nanopositioner as claimed in claim 16, wherein the actuator moves a stage on which the object to be nano positioned is placed.

18. A method to nano position an object, comprising varying magnitude and/or changing direction of current flow in a peltier module to either expand or contract an actuator to position the object.

19. The method as claimed in claim 18, wherein sensing the temperature of the actuator with thermal sensor.

20. The method as claimed in claim 18, wherein maintaining constant temperature on the actuator by a controller when the object is positioned at a predetermined position.

21. The method as claimed in claims 18 and 20, wherein varying the magnitude and/or direction of the current in the peltier module by the controller to maintain the constant temperature.