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Sunday, April 27, 2014

#286 Silicon and sodium

Considering sodium (Na) ubiquity, its presence in semiconductor manufacturing environment, in this case as an unwanted contaminant, should not come as a surprise. The good news is that sodium will not harm silicon as silicon is essentially impenetrable by sodium. The bad news, however, is the ease with which sodium can penetrate SiO2 when this last is formed on the Si surface in the course of thermal oxidation.


Even worst news is that Na in SiO2 can readily move around under the influence of an electric field causing severe instabilities of the device characteristics. In fact, it was due to the uncontrolled effect of sodium on transistor operation that the introduction of MOSFET into mass production was significantly delayed some 40 years ago. Not until extraordinary measures in terms of the cleanliness of process environment were implemented that the debilitating impact of sodium in semiconductor manufacturing environments was brought under control.

Posted by Jerzy Ruzyllo at 07:44 PM | Semiconductors | Link

Saturday, April 19, 2014

#285 Silicon and iron

Due to the way chemicals used in silicon processing are manufactured and handled, trace contamination of Si wafers with iron (Fe) is essentially unavoidable. Also, iron finds its way into Si wafer during conventional single-crystal growth process. More serious of the problem in terms of the potential harm are occasional malfunctions/degradation of the stainless steel gas-delivery systems which result in wafer contamination with Fe above acceptable limits.


Just like in the case of most metallic contaminants, the adverse effect of iron is triggered by the elevated temperature treatments of Si wafer during device processing. Unlike copper, iron is not a fast diffusant in silicon. Once in Si, however, iron will have an adverse impact on device performance by forming defects acting as carrier recombination centers. If allowed on the wafer surface during thermal oxidation for instance, Fe will promote formation of interface traps at the Si-SiO2 interface.


Either way, there is nothing good that results from Fe interactions with Si wafer. Fortunately, iron contamination of silicon and silicon device manufacturing environment is much more effectively prevented now than it used to be in the past.

Posted by Jerzy Ruzyllo at 09:39 AM | Semiconductors | Link

Sunday, April 13, 2014

#284 Silicon and copper

Copper is not a friend of silicon. In fact, it should be considered one of its worst enemies. Unfortunately, copper contamination is fairly common in silicon processing. Copper can contaminate the surface during wet cleaning operations and penetrate beneath the surface as a component of the polishing slurries during wafer manufacturing.


The problem is that copper features extremely high diffusivity and high solubility in Si. With any thermal step, copper will spread around Si lattice almost instantaneously. During thermal processes, combined with strain in the lattice and/or concentration gradient, copper will precipitate in silicon, decorate other defects and overall act as a major killer of the carrier lifetime, and hence, ruin the performance of any device. Furthermore, if allowed into the gate oxide on Si surface, copper will thoroughly destroy dielectric integrity of such oxide.


Considering all of the above it is no wonder that introduction of copper as interconnect metal replacing aluminum in advanced silicon integrated circuits was delayed by the industry as much as possible. Eventually, it did happen but only because copper as interconnect line never comes in contact with Si from which it is carefully separated by barrier layers as well as interlayer insulators.

Posted by Jerzy Ruzyllo at 07:58 PM | Semiconductors | Link

Sunday, April 6, 2014

#283 Silicon and oxygen

Among all elements silicon is interacting with in the course of device manufacturing, its strong affinity to oxygen is particularly distinct. First and foremost it manifests itself in the ease with which silicon reacts chemically with oxygen forming its native oxide SiO2 which happens to be an excellent insulator. No other semiconductor forms on its surface such a high quality native oxide by merely being exposed to elevated temperature in the presence of oxygen. What is equally important from the device manufacturing point of view is that silicon dioxide can be very easily etched off using HF based chemistry whether in liquid or in vapor form.


This ease of silicon oxidation combined with highly favorable etch characteristics and advantageous electronic properties of silicon dioxide, made technology driving silicon MOSFETs, and hence, CMOS, and hence, entire cutting-edge micro and nanoelectronics possible. The easy of silicon oxidation has also its drawbacks as the ultra-thin, typically not thicker than 1 nm, oxide spontaneously grown on Si surface in ambient air or during wet cleaning/rinsing operations, may adversely interfere with some follow up processes, most notably contact metallization and epitaxial deposition. Thus, it is imperative that such native/chemical needs to be controlled and/or removed.


While surface reactions of silicon and oxygen are well known, what is less obvious is that oxygen, similarly to carbon, finds its way into bulk Si during single-crystal fabrication process. Presence of oxygen in the bulk of Si wafers has its bad and a good side. Bad because oxygen in silicon may precipitate at high temperatures forming carrier recombination defects. Good, because oxygen present in silicon in moderate amounts actually strengthens silicon making wafer breakage less likely. All in all, there is no doubt that if not for the distinct, very favorable nature of silicon interactions with oxygen, electronic revolution of the last 50 years would not proceed the way it did.

Posted by Jerzy Ruzyllo at 06:26 PM | Semiconductors | Link

Sunday, March 30, 2014

#282 Silicon and carbon

In addition to hydrogen (see below) carbon is another element which plays a special role in silicon device technology. Unlike in the case of hydrogen, however, the effect of carbon in silicon on material and device characteristics is unambiguously deleterious.


Once allowed into Si crystal, carbon, in contrast to hydrogen, cannot be removed from Si structure where its harmful role is triggered by the high temperature processes employed during device manufacturing. But what the mechanisms that allow carbon penetration of silicon?  For starters, carbon impurities end up in single crystal Si as a result of crystal growth process. If not contained within acceptable limits, carbon will lead to the formation of swirl defects which may be transformed into precipitates and stacking faults during wafer exposure to high-temperature. Furthermore, during the device processing, particularly during dry etch operations, carbon involved in etch chemistries can get “implanted” into a very shallow near surface region of Si substrate. Once there, it can cause all sorts of harms during subsequent high-temperature processes such as thermal oxidation or epitaxial deposition.


Yet another story is a control of carbon contamination of Si surfaces with carbon compounds resulting from resist stripping as well as interactions with hydrocarbons from the ambient air. Prevention of carbon penetration into Si lattice through very thorough cleaning of Si surface every step of the way is a key to the successful control of potentially very harmful impact of carbon contamination of Si surfaces.

In the light of all of the above it is interesting to take note of the other side of Si-C interactions. This other side is that the homogenous, stoichiometric bonding between Si and C forms silicon carbide (SiC) which is an excellent semiconductor. The point is to see a fine line between carbon acting as a major impurity in silicon and the same carbon being an integral part of silicon compound featuring very attractive characteristics.

Posted by Jerzy Ruzyllo at 11:48 AM | Semiconductors | Link

Sunday, March 23, 2014

#281 Silicon and hydrogen

Intrinsic physical properties of any given semiconductor material are not the only factor determining its suitability for commercial device manufacturing. What also counts is the nature and the extent of interactions with process ambient and its components. From this point of view, interactions (mostly unintentional, by the way) of silicon with hydrogen play a very special role both in terms of pros and cons.


Hydrogen is plentiful in semiconductor process environment and can readily interact with silicon and silicon dioxide at each and every stage of the process. Wet etching and cleaning operations including water rinses, dry etch processes and wafer polishing operations all result in the hydrogen penetration of silicon and result, between others, in de-activation of p-type boron dopants near the wafer surface as well as formation of recombination sites in silicon.


On the other hand, however, hydrogen fulfills several critically important positive functions in silicon device technology. For instance, if not for hydrogen ability to passivate defects in amorphous silicon, thin-film silicon solar cells would be hardly possible. Also, hydrogen termination of Si surfaces provides a solid barrier against uncontrolled interaction of silicon with oxygen, moisture and organic contaminants. The good news in all this is that hydrogen interactions with silicon are well enough understood to minimize their potential negative effects and to take full advantage of those playing highly positive role.

Posted by Jerzy Ruzyllo at 09:01 PM | Semiconductors | Link

Sunday, March 16, 2014

#280 Lattice strain

Among less than fully versed "semiconductorers" there seems to be a bit of a confusion regarding the purpose of introducing strain in the crystallographic structure in the channel of the MOSFET (being part of the CMOS cell in high end logic ICs). Well, it's all physics. In the strained lattice, effective mass of an electron is smaller than in a relaxed lattice (again, check your physics). Smaller effective mass means higher mobility of electron which means electrons in the strained channel can move from the source to the drain of the transistor faster, i.e. you get faster operating transistor.


In other words, to shorten transition time between points A and B (gate length) you can shorten the distance (which requires major improvements in the resolution of pattern definition technology which is synonymous with major changes in photolithography technology) or make carriers move faster. Strain in the crystal lattice of the channel of the transistor accomplishes the latter.

Posted by Jerzy Ruzyllo at 08:25 PM | Semiconductors | Link

Saturday, March 8, 2014

#279 2D fascination

Interest in ultimate 2D, i.e. one atom/molecule thick, semiconductor material systems is expanding rapidly. In terms of attention, there is no doubt that graphene leads the field. In fact, in this very blog I commented on graphene on five different occasions starting six years ago in entry #30 followed by #31, 116, 138, and 154. More recently, 2-dimensional molybdenum disulfide, MoS2, also attracts a great deal of attention in semiconductor community because, unlike graphene, it actually features an energy gap, and hence, allows implementation of a switching transistor essential in logic ICs.


The most recent addition to the field is a 2D silicon known as silicene (see blog #229). Assuming adequate progress in the fabrication of silicene will be accomplished, one atom thick Si sheets could become 2D material system of choice in next, next, next ... generation transistor technology. You may wonder why this fascination with 2D material systems for device application? Reasons are many, but the most important is electron mobility being orders of magnitude higher in 2D confined materials as compared to their 3D equivalents.

Posted by Jerzy Ruzyllo at 07:59 PM | Semiconductors | Link

Sunday, February 23, 2014

#278 Semiconductors everywhere

It is unfortunate, but a blatant lack of recognition of the role semiconductor devices play in our lives seems to be pretty widespread. Well, there seem to be a loose association of semiconductors with computational and information processing electronics among the general public. Other than that, however, there seem to be little understanding of the function of semiconductors in essentially everything in our surrounding equipped with an “on-off” switch. Take modern cars, for instance. On more than one occasion I was strike by the lack of appreciation of the fact that modern cars these days are as much mechanical instruments, as they are electronic products controlled by semiconductors. The fact is that there is no single function modern cars are designed to perform these days, not a single vehicle operation monitoring system that do not controlled by semiconductor devices. The conclusion that I draw from the above is that the members of semiconductor community should be more vocal in spreading a word about increasingly broad penetration of our daily lives by semiconductors. It will serve well both academia, by attracting more students to the related classes, as well as semiconductor industry by attracting more investments.

Posted by Jerzy Ruzyllo at 05:17 PM | Semiconductors | Link

Sunday, February 16, 2014

#277 More on flexibility

Mechanical flexibility of semiconductor electronic and photonic systems is currently a very big deal in semiconductor science and engineering. Flexible displays, flexible sensors, flexible solar cell panels, flexible LED-based lighting panels, flexible bioelectronic devices and integrated circuits are in the process of moving sizable segments of semiconductor industry into the entirely new territory. As you tell from my past blogs (e.g. #258 and 259) I found opening of this new avenue for applications of semiconductors truly exciting.

The interest in flexibility of semiconductor devices was sparked by the progress in making organic semiconductors a commercial reality. As a reminder, organic semiconductors maintain their physical properties even when extremely flexed, thus, making flexible electronics and photonics feasible. Problem is that organic compounds are really not the best semiconductors in terms of physical properties. Wouldn't it be nice if for instance normally rigid and brittle crystalline silicon could be made into a flexible substrate? Well, it turns out that it is entirely feasible. I recommend you take a look at this paper to learn more about flexible silicon. Very interesting...


Posted by Jerzy Ruzyllo at 09:58 PM | Semiconductors | Link

‹‹ ›› is the personal blog of Jerzy Ruzyllo. With over 35 years of experience in academic research and teaching in the area of semiconductor engineering (currently holding position of a Distinguished Professor of Electrical Engineering and Professor of Materials Science and Engineering at Penn State University), he has a unique perspective on the developments in this progress driving technical domain and enjoys blogging about it.

With over 2000 terms defined and explained, Semiconductor Glossary is the most complete reference in the field of semiconductors on the market today.

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