Photolithography Processing.

Photolithography Processing.

Photolithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer.

Photolithography Processing: Basic concepts for photolithography, process overview, negative and positive lithography, critical dimension generations, light spectrum, resolution and process latitude, Eight basic steps of photolithography process.

In spite of advances in photolithography, processor clockspeeds remained largely constant from 2000 to 2006, because clock speed of memory was maximum 666 MHz. Hence the industry has adopted multitasking and multi core architectures.

Types of lithography.

UV Lithography: This is usually referred to as photolithography itself. UV light source is used.

Immersion UV Lithography: Optical immersion lithography utilizes liquids with refractive indices >1 (the index of air) below the last lens element to enhance numerical aperture and resolution, enabling sub-40-nm feature patterning. This shift from conventional dry optical lithography introduces numerous challenges requiring innovations in materials at all imaging stack levels.

X ray Lithography: It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photo resist, or simply "resist," on the substrate. A series of chemical treatments then engraves the produced pattern into the material underneath the photo resist.

Electron Beam Lithography: The primary advantage of electron beam lithography is that it is one of the ways to beat the diffraction limit of light and make features in the nanometre regime. This form of maskless lithography has found wide usage in photo mask-making used in photolithography, low-volume production of semiconductor components, and research & development.

Ion Beam Lithography:
Ion beam lithography, or ion projection lithography, is similar to Electron beam lithography, but uses much heavier charged particles, ions. Ion beam lithography has been found to be useful for transferring high-fidelity patterns on three-dimensional surfaces. Ion beam lithography offers higher resolution patterning than UV, X-ray, or electron beam lithography because these heavier particles have more momentum. This gives the ion beam a smaller wavelength than even an e-beam and therefore almost no diffraction. The momentum also reduces scattering in the target and in any residual gas. There is also a reduced potential radiation effect to sensitive underlying structures compared to x-ray and e-beam lithography.
Nanoimprint lithography is a method of fabricating nanometer scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting. Adhesion between the resist and the template is controlled to allow proper release.
Scanning probe lithography describe a set of lithographic methods, in which a microscopic or nanoscopic stylus is moved mechanically across a surface to form a pattern.
This type of method can be split in two different groups:

Constructive - In which the patterning is done by directly transferring chemical species to the surface (Dip Pen Nanolithography)

Destructive - In which the patterning is done by providing the substrate with energy (Either mechanical, or thermal, photonic, ionic, electronic, Xrays, and so on and so forth) to physically, chemically, electronically deform the substrate's surface.

Main parameters of lithography.

 Resolution
 Throughput (wafers per hour)

 Registration (alignment accuracy)

Three Basic UV Exposure Methods

Contact Printing

In contact printing, the resist-coated silicon wafer is brought into physical contact with the glass photomask. The wafer is held on a vacuum chuck, and the whole assembly rises until the wafer and mask contact each other. The photoresist is exposed with UV light while the wafer is in contact position with the mask. Because of the contact between the resist and mask, very high resolution is possible in contact printing (e.g. 1-micron features in 0.5 microns of positive resist). The problem with contact printing is that debris, trapped between the resist and the mask, can damage the mask and cause defects in the pattern.

Proximity Printing

The proximity exposure method is similar to contact printing except that a small gap, 10 to 25 microns wide, is maintained between the wafer and the mask during exposure. This gap minimizes (but may not eliminate) mask damage. Approximately 2- to 4-micron resolution is possible with proximity printing.

Projection Printing

Projection printing, avoids mask damage entirely. An image of the patterns on the mask is projected onto the resist-coated wafer, which is many centimeters away. In order to achieve high resolution, only a small portion of the mask is imaged. This small image field is scanned or stepped over the surface of the wafer. Projection printers that step the mask image over the wafer surface are called step-and-repeat systems. Step-and-repeat projection printers are capable of approximately 1-micron resolution. They print » 50 wafers/hour and cost $10M -20M.

The reticle and photomasks are the two things used to form the pattern on the substrate.

Feature Size is usually roughly half of the resolution. The minimum feature size is called Critical Dimension.

Registration: how accurately patterns on successive masks can be aligned (or overlaid) with respect to previously defined patterns.

Throughput: number of wafers that can be exposed/unit time for a given mask level.

Until now, most used portion of spectrum is the UV spectrum. This is due to the cheap and ready availability of UV sources.

UV spectrum is 10 nm to 400 nm.

Mercury Vapour Lamp Source: g line =436 nm

i line= 365nm

Deep UV (DUV) : Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light from excimer lasers with wavelengths of 248 and 193 nm (the dominant lithography technology today is thus also called "excimer laser lithography"), which allow minimum feature sizes down to 50 nm. Excimer laser lithography has thus played a critical role in the continued advance of the so-called Moore’s Law for the last 20 years.

Alignment:

n  Mask for each layer must be aligned to previous layer patterns

n  For a minimum feature size ~ 1 mm => alignment tolerance should be +/- 0.2 mm

n  To align, wafer is held on vacuum chuck and moved around using an xyz stage

Overlay Budget: How much misalignment is allowed.

Positive and Negative Photoresists.

There are two types of photoresist: positive and negative. For positive resists, the resist is exposed with UV light wherever the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. In other words, "whatever shows, goes." The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer.

Negative resists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photoresists, therefore, contain the inverse (or photographic "negative") of the pattern to be transferred. The figure below shows the pattern differences generated from the use of positive and negative resist.

CLEAR FIELD/DARK FIELD

Masks where most of the Cr remains and the features of interest are defined by regions where the Cr is removed are referred to as positive tone or dark field masks.  Masks where most of the Cr is removed and the features are defined by regions where the Cr remains are referred to as negative tone or clear field masks.  This is illustrated in the following figure.

Eight Steps of Lithography:

Vapour Priming: Cleaning the sample to remove dirt dust and residual PR. If water is present on top, it causes poor photoresist adhesion. So we bake wafer at 200-250C (dehydration baking).

Another objective of Vapor Priming is to make the inorganic Silicon surface more adhesive to the organic PR being used.

1)Wafer surface is first cleaned to remove dirt dust and residual PR.

2)Pre bake at 200-250 C to evaporate off excess water. Complete moisture removal is quite impossible due to the strong –OH bond. So, after pre baking, the oxide will form bonds with moisture in the air.

3) Then we prime the wafer with HMDA –HexaMethylDiSilazane. HMDA is dispensed onto the wafer held by vacuum chuck. The excess liquid is then spun off.

The HMDS primer will bond with the –OH groups to seal out any moisture. The Si(CH3)3 molecules are compatible with the PR, creating adhesion between the two.

2. Spin Coating: The photoresist is then applied to the vapour primed wafer. The wafer is held on chuck and the nozzle applies ~5ml phototresist to the centre while chuck rotates at slow speeds of 500 rpm.

This speed is then ramped up to 3000-5000 rpm. Resist layer thickness depends on viscosity of resist and is inversely proportional to the squareroot of the spin speed, t ∝ 1/√ω2.

Edge Bead: The PR may sometimes form a cusp at the wafer edge or may even spil ot to the other side of the wafer. The edge beads can be removed at the end of spin coating at reduced spin speed. A jet of solvent suitable for the type of photoresist is directed to the top 2-5mm edge of the wafer to dissolve the front surface edge bead and another jet of solvent is directed to the backside of the wafer to remove the backside edge bead.

3. Soft Baking: Soft-baking is the step during which almost all of the solvents are removed from the photoresist coating. Soft-baking plays a very critical role in photo-imaging. The photoresist coatings become photosensitive, or imageable, only after softbaking. Improves Photoresist-to-Wafer Adhesion and Promotes Resist Uniformity on Wafer. Oversoft-baking will degrade the photosensitivity of resists by either reducing the developer solubility or actually destroying a portion of the sensitizer. Undersoft-baking will prevent light from reaching the sensitizer. Positive resists are incompletely exposed if considerable solvent remains in the coating. This undersoft-baked positive resists is then readily attacked by the developer in both exposed and unexposed areas, causing less etching resistance.

Typical Bake Temperatures are 90 to 100°C for about 30 Seconds, On a Hot Plate, Followed by Cooling Step on Cold Plate

4. Alignment and Expose: One of the most important steps in the photolithography process is mask alignment. A mask or "photomask" is a square glass plate with a patterned emulsion of metal film on one side. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern.

After prebaking, the photoresist is exposed to a pattern of intense light. The exposure to light causes a chemical change that allows some of the photoresist to be removed by a special solution, called "developer" by analogy with photographic developer. Positive photoresist, the most common type, becomes soluble in the developer when exposed; with negative photoresist, unexposed regions are soluble in the developer.

The areas that are exposed to the UV light will undergo a chemical reaction. The water in this reaction is obtained from humidity in the air. If the air is not humid enough, the remaining carbon bond will bond with the resin, creating an insoluble material.

5. Post Expose Bake:

In this the wafer is heated to Typical Temperatures 100 to 110°C on a hot plate immediately after Exposure . In chemically amplified resists, the PEB catalytically performs and completes the photo reaction initiated during exposure. A PEB performed near the softening point of the photo resist reduces mechanical stress formed during softbake and exposure of especially thick resist films due to the expanding nitrogen and therefore improves resist adhesion and reduces underetching in subsequent wet chemical etching. However, a certain delay between exposure and PEB is required to outgas N₂. Otherwise, during PEB the N₂ in the resist will expand and increase mechanical stress in the film!

The PEB promotes the thermally activated diffusion of carboxylic acid formed during exposure from the photo active compound. This diffusion step smoothens the spatial periodic pattern of carboxylic acid having their origin in standing light waves during monochromatic exposure especially in case of highly reflective substrates. These patterns otherwise would transfer to the resist profile

6. Develop: Soluble areas of photoresist are dissolved by developer chemical. Commonly used chemical for positive resists is Tetramethylammonium hydroxide. Tetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt with the molecular formula [(CH₃)₄N]⁺[OH]^-, and is highly effective in stripping photoresist.

7.Hard-Baking

Hard-baking is the final step in the photolithographic process. This step is necessary in order to harden the photoresist and improve adhesion of the photoresist to the wafer surface. The hardbake sometimes performed after development intends to increase the thermal, chemical, and physical stability of developed resist structures for subsequent processes (e.g. electroplating, wet-chemical and dry-chemical etching). Evaporates Remaining Solvent and Improve Resist-to-Wafer Adhesion .

8. Develop Inspect: Inspect the wafers for defects, and redo if needed.

By Sharath with No comments

onX: way to go Microsoft.

Microsoft has stunned everyone by launching this wonderful platform on{X}, that lets you write custom code and automate your Android phone... Wait did I just hear Microsoft + Android? Seriously?

Looks like it is happening and whatever happened, seems very good and promising so far!

The site's documentation page teaches you everything you probably want to know before you can start writing your own code.

A number of common recipes are also available for download on the website, for use right away.. The codeplex community for onX is just maturing and there is no doubt it will soon become the next big thing among geeks.

You need to install onX(~7MB) on your phone before you can start using it and writing custom codes online.Oh yeah, you write code online and push it to your phone (which is very simple in fact) so we can call this remote programming...

Shira Weinberg, the on{X} team’s Program Manager, explained that the less strict security model of the Android platform is well suited for deploying early stage technology previews. This means that on{X} should be expected to be implemented on Windows 8 and Windows Phone, which require a more mature implementation of the technology.

All users need to do is download the Android app and then configure rules on the on{X} site. Rules are called ‘recipes’ and there are 11 templates ready to go. These include ones such as ‘Launch the music app when I am walking’ and ‘Remind me to visit the gym if I haven’t been there in 3 days’. Check out the video at the bottom of the post for a demo.

Each recipe can be easily edited by novices before being pushed to the user’s device. The rules and all the data they hold are private, and are available only to the user.

By Sharath with No comments

Must Read:Higher Ed Roadmap!

So, the grand finale of your B.Tech life is getting closer every day, and everybody is busy saying goodbyes and trying to have one last shot of college life before it all turns into sweet memories of some of the best days of our life...

Here are some ten-things you can squeeze in-between while the apocalypse nears... if you plan to do Masters or some other higher education. Even if you don't just do these 'coz you never know when these will come in handy!

Get all your papers and their certificates.

Because in the end, certificates are all that matters boss, clear as much of "supplys" you can, correct all those mark-sheets if the university somehow messed up any figures, don't wait for college to get over, do it now and get ready for that final piece of paper that turns you into..... just another engineer among the millions in India :P

Get as many Letter of Recommendations as you can.

Get as many as you can... In all forms and shapes and sizes. Because you never know what fits whom. I'm sure not many of your professors are gonna sign off whatever acts of academic bravado you have put in your LOR as authentic... Hell I don't think all of them are gonna give you an LOR in the first place even :D So, get those LORs, the good ones, and get the contact numbers and mail ids of those benevolent professors who agreed to vouch for you despite.... you know! Also prepare a 'general' LOR so that you can produce it for more than one course.

Get all the official documents from your college.

By this I mean conduct certificate, migration etc. Also don't forget to get back your tenth and Plus two certificates if you were forced to give them at the time of joining.

Get the most important document after your degree certificate: the transcript.

Most foreign universities don't accept the mark sheets for individual semesters from your university. Instead they ask for the transcript of marks. The transcript is a consolidated mark-sheet that tells us what all subjects you took and how did you score through the whole eight semesters at one look. Transcripts are usually single page documents on a university letterhead.

As far as I know, M.G university and most other Indian universities requires you to prepare the transcript on your own in the prescribed format, put it on a C.D and produce it with all the supporting mark list copies after applying for transcript at the university office.

Don't get scared at the sound of this, it is most probable that the DTP centre in front of your university will do this for you for a small fee. Apply for this along with your application to issue degree certificate.

By this time, you will be at home.... either wondering what to do now without the faintest of clues, or rabidly facebooking away alll your time. Any way there is gonna be one day when you finally get borred with both and decide to do something about it. :)

And that is the day we start part two....

Get yourself a passport, PAN Card and all those umpteen cards.

And be careful not to mess up the procedures of any of these, like the passport especially. Applying for passport and then 'accidentally' missing the police verification or anything are the last thing you want to do.

Get all the government documentations ready.

Cross check all your old but still important documents like birth certificate, etc and see your name spells the same everywhere. If possible get documents from your village office proving residency etc. Never know these will come in handy.

Prepare yourself for GRE/TOEFEL/IELTS or whatever.

First decide which of these is necessary for you depending on which part of the globe you are targeting. Keep in mind that for applying for the above exams, you need to produce a copy of your passport.

Open a new e-mail account and dedicate it for course hunting.

Use this inbox for official communications with universities only; which means no stupid ids also. It should preferably be your name with normal characters and preferably on gmail . And for gods sake avoid cluttering the inbox with all sorts of spam by registering for all sorts of free Facebook t-shirts and other sites... you know :D

Upload all scanned copies of these certificates and documents to the Google docs account associated with your new id so that these are always accessible from anywhere.

Prepare the killer Statement Of Purpose and C.V.

You get lots of templates on-line but its best you write one for your own, and try to be as convincing as possible about the fact that 'the sole intention of you joining the course is your un-quenchable thirst for knowledge, and that you have no plans to stay back in that country for job since you will be very busy contributing to and serving mother India after the course'!!.......... :D 

This depends, since some countries may actually want you to stay sometime after you complete the course. So just get a good SOP. Keep in mind that big time lying will screw you up big time in the future so keep it real.

Prepare a small general purpose resume' and C.V that is quite detailed and that which can be modified for specific courses. Keep these also in your Google docs account.

Now you are almost ready. Get out there, and start firing your applications far and wide.

Oh, and by the way don't 'acccidentally' mix up applications and screw yourself..... Believe me, Shit Happens :D

ALL THE BEST FOLKS!

By Sharath with 1 comment

My Pick: Windows phone; Cool Tool in the wrong hands!

Windows phone had been the hype-du-jour for the past few months. While admittting that it succeeded in creating its own fan-base, we should also point out that being proprietary has kept the potentials of this platform leashed to a the market interests of Microsoft.

Read this article about how the platform could have been made more attractive, now that Windows 8 is gonna come, this might be what you can expect.
Presenting… a vision of what Windows Phone 8 might look like: Read ON>>>

By Sharath with No comments

My Days at Infosys.

I joined Infosys Technologies Ltd. on November 28th 2011 as Systems Engineer. Like any other fresher, who was just out of his college days, I didn't have a clue about what Infosys was all about, or what corporate atmosphere is. All I had was the impression that all ITES companies like Infosys belonged to a class off companies that made their living out of the shitty brainless and monotonous jobs outsourced by Europe or North america. In fact I looked upon Infosys (and other IT companies) with much contempt and disgust much akin to that shown to his humble servant (who makes a life out of washing under wears from all over the world )by his aristocratic master (who happened to be a fresh Electronics & Communication Engineer). Well, the servant might be damn richer than the master here, but who cares! He made all of it washing under wears!


During my never ending wait for Nov 28th which lasted almost 5 months, I was busy renovating this blog, trying to learn some media-graphics tools like After Effects and Maya, scouring the web for what to do after B.Tech, sometimes applying for some Masters programmes with no expectation of studying at one ever, Facebooking, Sleeping and more Facebooking. Finally it seemed like I was bored with whatever I could think of to spend a day. It was so much that soon I was asking myself questions like "What is the meaning of this life...." and the sorts which probably should be happening after a few more decades.

Infosys Mysore was a turning point... no, this is not the turning point for the sake of my probably stupid article sounding good; it really did change some things in my life forever.

Infosys Mysore allowed me to have a glimpse of what professionalism is. Pretty sad that even after studying a 'professional course' in a 'professional college' I had to come here to actually feel it.


Infosys pampered me with lots of luxury care and friends all around. It is probably the best place in India to make friends, with people coming in from all states for their training.

And finally, the most important thing Infosys taught me was that I am capable of more.


 More than what challenge the I.T sector would probably offer me... maybe not in terms of money, but still I felt that in the end I will be as bored as I was when I sat at home waiting for November 28th. During the initial months of training, I religiously stuck to GECs, reviewing the study materials and routines. But soon I found the whole thing to be too  easy; I was even put in a fast-track batch that required good performance in the Generic Training sessions.

While I still admire the way some young entrepreneurs from India started a small company with almost 100$ and the way it grew into a billion dollar global enterprise, and the way Infosys 'cared' for its employees, there was always this lingering feeling in my mind that kept reminding me that none of this success belongs to me.... At Mysore, I was still doubtful about this queer feeling.
But when I came to Trivandrum (TRINFY) these doubts became proven facts. Infy never cared (and it could not; practically thinking) about the interests of the 'employee'... Many of the straight '5 point CGPA' trainees were assigned to the dreaded testing stream, and people who never wanted to work were put into 'hot technologies'... Pretty soon, the picture became clear before me

Infosys and the many other ITES companies are basically organisations giving engineers for hire... it doesn't matter whether this engineer likes to work in this field or that, or even whether he knows anything at all about the field is of little concern. This fact gains more significance when you add to it the fact that most of the work or the so called 'Projects' done at these companies are basically clerical jobs or requiring similar brain work. You don't do anything, no, you don't even think! As the funny chain-mail says, the whole IT employee community owes their livelihood to who ever invented Ctrl+C and Ctrl+V.

I don't want to think further about what will happen to you sitting in front of a computer, going through code all day long, but it sure will strip you off whatever you gained in Mysore. (You'd gain some weight around the tummy for sure :D )

So when I got an offer from a University which I thought probably wouldn't even take a look at my resume, I just couldn't let it pass. Life is always filled with twists and turns... and for me most of it has been unexpected ones. But without them, I'd become as bored as I was, when I waited for that November 28th. :)
So, its time to bid adieu to Infosys although these memories of Mysore will always be close to my heart.....

By Sharath with No comments

FLIPBOARD FOR ANDROID LEAKED!

Samsung had announced earlier that its latest gadget Galaxy SIII will have the Android version of the popular iPhone app flipboard.

Flipboard is a pretty app that collects the content of social media and other websites and presents it in magazine format and allows users to "flip" through their social-networking feeds and feeds from websites that have partnered with the company.

Originally designed specifically for the iPad, in December 2010, the application was updated to add support for the iPhone and iPod Touch. On May 5th 2012, it was announced that Flipboard would be released for select Android phones, beginning with the Samsung Galaxy S3.

It is produced by Flipboard, Inc., a United States-based software company founded in 2010 by Mike McCue and Evan Doll and headquartered in Palo Alto, California.

Unfortunately, the app was announced as Galaxy S III exclusive at launch, which meant the only way for you to get it is buy a Galaxy S III. Thankfully, though, one XDA user managed to extract it from the device and made it available to everyone right now. 

For download links, CliCk Here.

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