Fundamentals of Microfabrication

Fundamentals of Microfabrication

Fall 2013Prof. Marc Madou

MSTB 120

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Fundamentals of Microfabrication

Content

Definitions of ICs MEMS Why miniaturization ? Taxonomy of Microfabrication Processes Accuracy/precision Accuracy/precision and standard deviation Relative vs. absolute tolerance in manufacturing Merging of two approaches: Top-down and bottom-up machining

methodologies Biomimetics A few concluding words about manufacturing methods

From ICs to MEMS and NEMS

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From ICs to MEMS and NEMS

Today’s car differs from those of the immediate post-war years on a number of counts. But suppose for a moment that the automobile industry had developed at the same rate as computers and over the same period: how much cheaper and more efficient would current models be? Today you would be able to buy a Rolce-Royce for $ 2.15, it would do three million miles to the gallon, and it would deliver enough power to drive the Queen Elizabeth II. And if you were interested in miniaturization, you could place half a dozen of them on a pinhead.

Christopher Evans, 1979

The transistor was invented 1948 by three Bell Laboratory engineers and physicists. John Bardeen was the physicist, Walter Brattain the experimentalist, and William Shockley, who became involved later in the development, was the instigator and idea man. The team won the 1956 Nobel Prize in physics for their efforts. The transistor demonstrated for the first time that amplification in solids was possible.

Definitions of ICs

Definitions of ICs

Diodes

There are many different types of transistors, but the basic theory of their operation is all the same. The three elements of the two-junction transistor are (1) the EMITTER, which gives off, or emits," current carriers (electrons or holes); (2) the BASE, which controls the flow of current carriers; and (3) the COLLECTOR, which collects the current carriers.

Definitions of ICs

The arrow always points in the direction of hole flow, or from the P to N sections, no matter whether the P section is the emitter or base. On the other hand, electron flow is always toward or against the arrow, just like in the junction diode.

Definitions of ICs

A forward biased PN junction is comparable to a low-resistance circuit element because it passes a high current for a given voltage. In turn, a reverse-biased PN junction is comparable to a high-resistance circuit element. By using the Ohm's law formula for power (P = I2R) and assuming current is held constant, you can conclude that the power developed across a high resistance is greater than that developed across a low resistance. Thus, if a crystal were to contain two PN junctions (one forward-biased and the other reverse-biased), a low-power signal could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction. In this manner, a power gain would be obtained across the crystal. This concept is the basic theory behind how the transistor amplifies.

Definitions of ICs

Definitions of ICs

The term transistor is derived from the words TRANSfer and resISTOR. This term was adopted because it best describes the operation of the transistor - the transfer of an input signal current from a low-resistance circuit to a high-resistance circuit. Basically, the transistor is a solid-state device that amplifies by controlling the flow of current carriers through its semiconductor materials.

Definitions of ICs

Types of transistors:

– Bipolar Junction Transistor (BJT)

– MOS transistor [see Metal Oxide Semiconductor (MOS) Capacitor]

Definitions of ICs

A chip or an integrated circuit (IC) is a small electronic device made out of a semiconductor material. The integrated circuit consists of elements inseparably associated and formed on or within a single SUBSTRATE (mounting surface). In other words, the circuit components and all interconnections are formed as a unit. The first integrated circuit was developed in the 1950s by Jack Kilby of Texas Instruments and Robert Noyce of Fairchild Semiconductor.

Definitions of ICs

Integrated circuits used to be classified by the number of transistors and other electronic components they contain: – SSI (small-scale integration): Up to 100 electronic components per chip – MSI (medium-scale integration): From 100 to 3,000 electronic components per chip – LSI (large-scale integration): From 3,000 to 100,000 electronic components per chip – VLSI (very large-scale integration): From 100,000 to 1,000,000 electronic

components per chip – ULSI (ultra large-scale integration): More than 1 million electronic components per

chip------------------------------------------------------------------------------------------------------------– WSI (Wafer-scale integration): Is a system of building very-large integrated

circuits that uses an entire silicon wafer to produce a single "super-chip". – SoC or SOC( A system-on-a-chip): This is an integrated circuit in which all the

components needed for a computer or other system are included on a single chip. – 3D-IC (A three-dimensional integrated circuit): this has two or more layers of

active electronic components that are integrated both vertically and horizontally into a single circuit.

Definitions of ICs

Definition of MEMS

Micro electromechanical systems (MEMS), or micromachining (also micro-manufacturing and microfabrication), in the narrow sense, comprises the use of a set of manufacturing tools based on batch thin and thick film fabrication techniques commonly used in the integrated circuit industry or IC industry. This involved originally mainly Si based mechanical devices.

DARPA: Hybrid Insect Micro Electromechanical Systems (HI-MEMS)DARPA: Hybrid Insect Micro Electromechanical Systems (HI-MEMS)

Definition of MEMS

MEMS: Micro electro mechanical systems. In recent years, it has become obvious that Si is not always the right substrate, that batch is often not good enough and that a modular approach is sometimes better than an integrated one. This has especially become clear in the case of biomedical applications (see BIOMEMS course). The ‘science of miniaturization’ has become a much more appropriate name than MEMS and it involves a good understanding of the intended application, scaling laws, different manufacturing methods and materials .

Why miniaturization? Minimizing energy and materials use in manufacturing Redundancy and arrays Integration with electronics, simplifying systems (e.g., single point vs. multipoint

measurement) Reduction of power budget Faster devices Increased selectivity and sensitivity Wider dynamic range Exploitation of new effects through the breakdown of continuum theory in the

microdomain

Why miniaturization?

Cost/performance advantages Improved reproducibility (batch

concept) Improved accuracy and

reliability Minimal invasive ( e.g.

mosquito project) Do we have a choice? (see next

viewgraph- - the Law of Accelerating Returns)

probiscus is about 75 µm

Evolution (sophistication) of life-forms or technology speeds up because they are build on their own recorded degree of order. Ray Kurzweil calls this The Law of Accelerating Returns*

This Law of Accelerating Returns gave us ever greater order in technology which led to computation -- the essence of order.

For life-forms DNA provides the record. In the case of technology it is the ever improving methods to record information.

*Ray Kurzweil in The Age of Spiritual Machines




Moore’s law (based on a temporary methodology i.e., lithography) is only an example of the Law of Accelerating Returns. Beyond lithography we may expect further progress in miniaturization based on DNA, quantum devices, AFM lithography, nanotubes, etc.


Moore’s ‘Law’: The amounts of information storable on a given amount of silicon roughly doubled every year since the technology was invented. This relation, first mentioned in 1964 by semiconductor engineer Gordon Moore (who co-founded Intel four years later) held until the late 1970s, at which point the doubling period slowed to 18 months. The doubling period remained at that value up to late 1999. Moore's Law is apparently self-fulfilling.

Taxonomy of Microfabrication Processes

Accuracy /precision

Accuracy is the degree of correctness with which a measuring system yields the “true value” of a measured quantity (e.g. bull’s eye).

Accuracy is typically described in terms of a maximum percentage of deviation expected based on a full-scale reading.

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http://ull.chemistry.uakron.edu/analytical/animations/

http://ull.chemistry.uakron.edu/analytical/animations

http://ull.chemistry.uakron.edu/analytical/animations

Accuracy/precision

Precision is the difference between the instrument’s reported values during repeated measurements of the same quantity

Precision is typically determined by statistical analysis of repeated measurements




Accuracy/precision

Accuracy, precision and standard deviation

A measurement can be precise, but may not not be accurate.

The standard deviation (s) is a statistical measure of the precision in a series of repetitive measurements (also often given as with N the number of data, xi is each individual measurement, and x the mean of all measurements.

The value xi - is called the residual for each measurement.

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X

Relative vs. absolute tolerance in manufacturing

Lithography is excellent for achieving small absolute tolerances - - we can make much smaller devices with lithography than with mechanical machining. The relative tolerance on those dimensions though is not so good; on a 100 µm line we might perhaps achieve 1 %. In mechanical machining terms this does not even qualify as precision machining !

For a small relative tolerance, ultra-fine diamond milling is better. Can be better than 0.01 %. Of course we cannot make things as small as we can with lithography.

The above argument might decide your choice of machining approach or decide the size of the device you want to make.


Lithography (e.g. Si-micromachining) is excellent for small absolute tolerances

For relative tolerances, ultra-fine diamond milling is better

In some cases we might want to keep our micromachine somewhat larger to optimize relative tolerances (see Mass Spectrometer example)

10 km1 km100 m10 m1 m10 cm1 cm1 mm100 µm10 µm1 µm0.1 µm0.01 µm1 nm1 ÅAbsolute size Absolute tolerancePrecision Machining Application DomainLinear dimensionLinear dimension0.01 %Relative ToleranceCityHouseArmOpticalfiber

VirusAtomRelative tolerances for buildinga house and a lithography based micromachine

Bacteria100 m1 m1 cm100 µm1 µm0.01 µmPrecision Machining1%100 %10 %0.1 %0.0001 %0.01 %


Lawrence Livermore National Laboratories (LLNL), at one point used LIGA to make the next generation mass spectrometer

The picture below shows an array of holes in PMMA to electroplate Ni posts (poles)

The diameter of each hole is 40 µm !!

A larger mass spectrometer is machined with ‘traditional’ ultra fine diamond milling at JPL

Relative tolerance is better than with the LIGA machined one, so its performance is better


Merging of two approaches: Top-down and bottom-up machining methodologies Most human manufacturing methods of small

devices involve top-down approaches. Starting from larger blocks of material we make smaller and smaller things. Nature works the other way, i.e., from the bottom-up. All living things are made atom by atom , molecule by molecule; from the small to the large. As manufacturing of very small things with top-down techniques (NEMS or nano mechanical devices) become too expensive or hit other barriers we are looking at nature for guidance (biomimetics).

Nature and mankind have developed competitive manufacturing methods on the macro level (e.g., steel versus bone). Biomimetics mostly failed in the larger world (see Icarus). Background reading: Cats’ Paws and Catapults by Steven Vogel (Efficiency of mechanical systems in biology and human engineering in the macro-world).

Merging of two approaches: Top-down and bottom-up machining methodologies

On the nanoscale nature is outperforming us by far (perhaps because nature has had more time working towards biological molecules/ cells than towards making larger organisms such as trees and us).

Further miniaturization might be inspired by biology but will most likely be different again from nature -- the drivers for human and natural manufacturing techniques are very different.

Merging of two approaches: Top-down and bottom-up machining methodologies

Seeman

Eigler

Montemagno

Merging of two approaches: Top-down and bottom-up machining methodologies --NEMS

MEMS’ little brother is NEMS, the top-down approach to nano devices. This biomimetic approach to nano devices I like to call nanochemistry. To succeed in the latter we will need :– self-assembly and directed assembly

(e.,g, using electrical fields -see next viewgraph)

– massive parallelism– understanding of molecular

mechanisms -- chemomechanics– engineers/scientists who understand

‘wet’ and ‘dry’ disciplines

Merging of two approaches: Top-down and bottom-up machining methodologies --NEMS

Example nano chemistry approaches: – Natural polymers: e.g., NAs and proteins not

only as sensors but also as actuators and building blocks (Genetic engineer NA’s and proteins-rely on extremophiles for guidance)

– Mechanosynthesis– NEMS/biology hybrids --to learn only

Biomimetics

Bimimetics: Many examples in nature provide hints

for future manufacturing methods but as stated earlier the purpose for their development is different from the reasons for human manufacturing methods (e.g., teeth and sea shells might be excellent strong building materials but their growth is typically way too slow to be attractive for human manufacturing)

A few concluding words about manufacturing methods

Serial versus batch versus continuous manufacturing methods

Projected versus truly 3D Additive process versus

subtractive process Top-down versus bottom-

up

Something to think about

Looking back at the worst times, it always seems that they were times in which there were people who believed with absolute faith and absolute dogmatism in something. And they were so serious in this matter that they insisted that the rest of the world agree with them. And then they would do things that were directly inconsistent with their own beliefs in order to maintain that what they said was true.

From Richard P. Feynman in The Meaning of it All.

If in the course of these lectures I can make you doubt most of the things you have come to believe then I probably put you on the path of becoming a true scientist/engineer.

Something to think about

Homework

Describe to a 12 year old, in the shortest and clearest fashion how a transistor works and why it is so important in applications all around us (figure is ok but words are required).

Characterize using the following criteria:– projected versus 3D, – serial, batch or continuous– top-down versus bottom-upLaser machiningMechanical machiningE-beam machining and plastic molding.

Calculate the number atoms in a 100 µm long Ag line (1 µm wide and 1 µm heigh). If we put one atom down per second (e.g., using an STM) how long will it take to finish this Ag line ?

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Fundamentals of Microfabrication