Chapter 1 AN ENVIRONMENT OF CHALLENGES

8/14/2019 Chapter 1 AN ENVIRONMENT OF CHALLENGES

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Electronic Materials andProcessing I

Chapter 1

AN ENVIRONMENT OF CHALLENGES


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Chapter 1AN ENVIRONMENT OF CHALLENGES

1.1 Overview- Electronic applications:

Simple copper wiresHigh-performance magnetic materials for computer disks

Semiconductors for microelectronic devices (Si)- The critical properties of the materials : electronic conductivity,optical transmission, mechanical properties.- It is not reasonable to cover all aspects of electronic materials -The materials discussed here relate primarily to the mostchallenging applications, particularly with reference tomicroelectronic and optical devices.- Semiconducting materials used in active devices : sometraditional materials, such as silicon.


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1.2 A HISTORY OF MODERN ELECTRONIC DEVICES

1905 Vacuum tube diode invented by J. Ambrose Flemming1906 Triode vacuum tube invented by Lee DeForest1916 Czochralski crystal growth technique invented by Jan

Czochralski (Si ingot 300mm today)1935 First patent issued on a field-effect transistor (Oskar Heil)1938 Early reports of Si rectifiers by Hans Hollmann and JrgenRottgardt1947 Transistor (Ge) invented by Bardeen, Brattain, andShockley at the Bell Telephone Laboratories1951 First practical field effect transistor1952 Single crystal Si produced1954 SiO2 mask process developed1958 First integrated circuit invented by Jack Kilby1959 Planar processing methods, precursors of modernintegrated circuit fabrication methods, created by Noyce andMoore


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1960 First practical metal-oxide-silicon transistor1960 First patent on a light emitting diode (J.W. Allen and P.E.Gibbons)1962 Transistor-transistor logic

1962 First practical visible light emitting diode1962 First laser diode1963 Complementary metal-oxide-silicon transistors providelower power switching devices

1968 Metal-oxide-semiconductor memory circuits introduced1971 First microprocessor1978 First continuously operating laser diode at roomtemperature

1987 Polymer-based light emitters1992 Er-doped fiber amplifier1997 Introduction of Cu-based interconnects


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- In 1965, Gordon Moore observed that the number of transistorsper square inch had doubled every year since the integratedcircuit was invented in 1958. This has come to be known asMoores Law

Figure 1.2: Shows the trend known as Moores Law for microprocessorcircuit density. Theminimum feature sizehas decreased proportionate

to the square root of the circuit density. Each plotted pointcorresponds to a marketed product.

-The number of devices grew tohundreds in the 1960s,

-Thousands to tens of thousands inthe 1970s,-The current numbers of millions totens of millions.

1.3 AN ISSUE OF SCALE


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Figure 1.3: A schematic diagram of a state-of-the-artfield-effect transistorsuch as thatdiscussed here. The current densities (105 A cm-1) are sufficient to cause conductorsto fail, the fields across the gate dielectric (5x106 V cm-1 ) are barely supportable by

even the nearly perfect SiO2gate dielectric, thenumber of dopant atomsin the channellimits the practical dopant concentrations toparts per thousandtypically, and the totalnumber of electrons transferred through the device is so small that noise becomessignificant.


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-Advanced production device dimensions : 0.1 m-The challenges of producing a device of this scale : doping ofthe semiconductor and the electron current density-A transistor with a control region length and width of ~ 0.1 m(10-5 cm) and a thickness of 50 nm (5x10-6 cm) The controlvolume is 5x10-16 cm3.

-As the atom density of silicon is 5x1022 atoms cm-3, the criticalvolume of current transistors contains only 25 million atoms.-Doping the semiconductor with one part per million impurityatoms (5x1016 cm-3), the control region would contain only 25

impurity atoms.-The removal of a single dopant atom would correspond to a 4%change in doping level. Such doping levels do not provideadequate conductivity or reproducibility.

-Small devices should have doping levels closer to 1x1019 cm-3or 0.02% (at or near the solubility limit for the dopant). Thecontrol volume contains 5000 dopant atoms.


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-Reduction in scale of five in the lateral dimensions (0.02 m)with no reduction of thickness, the control volume wouldcontain only 1000 dopant atoms at the higher doping level.-A variation of only 10 dopant atoms corresponds to a 1%change in doping level.Such changes can affect the resulting performance.

-Typical devices now operate at 109 Hz (1 GHz)-Current of 1 amp flowing for the corresponding cycle time 1ns(10-9 s) transfers only ~2x109 electrons.

- Current of 1 nA transfers only one or two electrons in ananosecond.-A reasonable number of electrons to activate a device is of theorder of a few thousand, corresponding to a required current of

a few microamps in each cycle.-The 0.1 micron device : a current of 10 A (10-5 A) wouldcorrespond to a current density of 2x105 A cm-2.


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-Current densities of this magnitude produce an electron wind

with sufficient momentum to push atoms along the conductor.This phenomenon of electromigration has been one of thelong-standing causes of device failures limiting the lifetime ofoperating integrated circuits.-Current densities are very high, threatening to melt devicesduring operation, insulators are beginning to fail.-Insulator SiO2 grown by thermal oxidation of Si wafers.

Such oxides can support fields of up to 10 million voltsper cm. These fields mean that a 1 V potential requires aminimum of 1 nm (10-7 cm) of oxide if there are no defects orthickness changes present.

- At this voltage, oxide of 2 nm are required.-Shrinking the overall device dimensions has required shrinkingthe oxide dielectrics. Oxides are approaching 1 nm thick-This has required reduction of the voltages, which producedramatic changes in the design of switching transistors.- The development of new dielectric materials with higher

dielectric constants.


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1.4 DEFINING ELECTRONIC MATERIALS

Figure 1.4: Shows the primary application of the various elements in theperiodic table in microelectronics. Elements left blank are used only invery rare applications or are not used at all. Many elements such as Al

have a number of applications. In the case of Al, these include as acontact/metallization, a semiconductor component, a dopant, and ininsulators. Element symbols shown in gray are very rarely used.


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The most common elements used in microelectronics are ingroups IIIa to Va.Group Va elements are largely used in compoundsemiconductor form, and as dopants in the group IVsemiconductors.Group IVa elements include the common semiconductors Si

and Ge as well as C.The Group IIIa elements include the excellent electricalconductor Al, elements found in semiconducting compoundswith group Va elements, and as dopants in group IVa

semiconductors.Group IIb elements form compound semiconductors with thechalcogenides (group VIb elements) and have a variety ofother uses.


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-The most common conductors : group Ib elements Cu and Au

with Ag also occasionally used.-Transition metals are commonly used in compound formeither as silicides or nitrides, primarily as stable contactmaterials bridging between Si and a highly conductive metal,

-The rare earths are not used extensively (Erbium, GadoliniumEtc.).-The IIa elements such as calcium are used as conductors orcontacts,

-Finally, the group Ia alkali metals are rarely used because oftheir reactivity and rapid diffusion rates in many materialsBoth group Ia and IIa elements are increasingly used, forexample, in organic electronic devices.


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1.5 PURITY

The fundamental property concerning electronic materials ispurity.In many cases extreme measures are taken to preventcontamination.

- The contamination problem is dealt with in microelectronicapplications.-Aluminum oxide has often been used as the packagingmaterial for military-specification computer chips because of

its outstanding resistance to penetration by contaminants.-The original packages were made from standard aluminumoxide powder produced directly from bauxite ore.-There is typically a very low level of uranium oxide

contamination in this material.-This tiny amount uranium led to false data in the informationstored in the chips.


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- To avoid this, the aluminum oxide must be decomposed

electrochemically to aluminum and oxygen.-The aluminum is then converted to a vapor compound andpurified by fractional distillation.-Finally, the compound is reacted with purified oxygen to

produce electronic-grade aluminum oxide.-It is a lot of trouble to go through to get rid of a few uraniumatoms, but it turns out to be necessary.


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Chapter 1 AN ENVIRONMENT OF CHALLENGES