So, your like, a lot of our other lectures, I want to start with some news.

This was over the summer, this article came up On the front page.

Not the front page of Wall Street Journal, but the front page of the

technology section and it says that you

know flash memory have suddenly become hot.

Especially for for application into the storage space, cloud

computing and it said you know flash memory celebrating its 25th year.

since its invention this year.

So we'll talk later.

I'll try to you know, give you some perspective

why this is the case, why this is happening.

And, this is

another thing which was later part of last year actually.

More than a year ago, and it says this the

latest flash memory chip you can buy, the most advanced one.

It says it's a 64 Gigabit die, or if you

convert it into byte, it's a 8 Gigabit. Eight Gigabyte times the

different between beta byte is beta usually a small b byte of the capital b.

So keep those in mind when you read news articles like that.

It also says that It's a two bit per cell, so x2

and you'll see this done again, x2, x3, x4, so we'll talk about what that means.

And another thing is that it says 90 nanometer but we'll

see later that it's not exactly 90 nanometer, bi 90 nanometer.

It's actually one of these two.

Either the bit line or the word line.

One of them is more closer to each other rather than the other.

So,

[COUGH],

earlier in the first lecture, I asked you

to name semiconductor companies, in the Bay area.

None of you, actually, I was very disappointed

that none of you named any of these companies.

There'll all flash memory companies, mostly located, in the Bay area.

In fact, this is a very hard segment of the market now.

Last year it says four, $400 million of venture capital were invested in

these companies. I see the number, correct number is 389

million, but That doesn't sound as good but so, the thing we were answer is what

do they do and why, why is there so much venture capital action in this space?

So you, these are common application of flash memory

you must have seen USB back in the 2000 2004.

IPod Nano. IPhone, MacBook era,

more recently iPad and tablets. And what has resulted,

because of that, is that this market, which has NAND flash.

So there's two kinds of flash, the NOR, which is

in white and the NAND flash which is in blue.

So that has really exploded in this post-PC era.

In 2010 it was close to a $20 million market.

And the question now is that is it going to saturate or is it going to.

That going to actually saturate or its going to continue growing.

So think about it.

You know, you already have all this tablets,

and smart phones, but they, so they put

out different models, right?

It's 8 gig, 16 gig, 32 gig most of the people actually

seventy, more than 70% of the people buy the lowest capacity one.

And their preferring to store their data over the cloud.

Apple has a solution right?

Dropbox. Box google drive.

And all these companies are not they're still hard disk based.

The company which stored in the clouds.

The questions is it going to saturate or is it going to continue growing.

And the good answer is a lot of cloud storages are soon moving to flash memory.

So, you might have I'm sure all of you use You know, some

of you might use Chrome, a lot of you might have used Google search.

This product right, where you start typing and it gives you the

[INAUDIBLE].

And it's all enabled by storing the whole internet on a NAND flash memory.

Google in fact is one of the largest buyers of

enterprise. Kind of, flash storage disk.

And there's a huge advantage you get in terms of,

power if you move from hard disk drive to flash.

And that's actually a more of a driving factor then, then the,

then the speed because.

As we'll see later, storing or writing into flash is much more energy

efficient as compared to a writing on a, on a flash disc drive.

So with that introduction, what I will do first, is to cover some basics.

Of flash memory, so we will cover four

things the different, two different architecture, NAND and NOR.

I will

cover how do you program a flash memory cell, how do you erase it.

Then how long you can store it which is also called retention.

How many times you can read and write to it, which is called endurance, or

[INAUDIBLE]

cycling.

And then the last thing is this multi-level flash, that

is, having one cell but storing multiple bits, into it.

So, there essentially two different architectures that you can help a flash.

One is a NAND and the other is NOR.

The differences is that in the NAR architecture, you have your

bit line, which is the, this black contact or her connecting to each of your cells.

So you can, if you look at the. Take diagram here, you can essentially,

if you look at the circuit diagram here

you can individual address each of the cells.

So you can select a particular cell, selecting this bit line and you

can select the board line and you can address each of the other cell.

Compared to a NAND, essentially you get rid of all these black contacts, so you

have All these cells connected in series. And the only way you

can address one cell is that if you address all of the rest of the others.

So if want to you read this cell you first have to turn on all these other cells.

What the advantages are as you can see this

is much more compact as compared to this one.

So this has a feature size of close to 4F squared, between four and 5F squared.

This one has a feature size of 10F squared even though

these two cells share one bit line contact, so you drop one for two.

But since the design rules for dropping a contact are

more relaxed, since this is dropping from a different layer.

You essentially get twice the area as compared to this one.

So you get A feature size of 10 f square so the

[INAUDIBLE]

is described as nine.

You get five squared or you get 10 squared.

Similarly if you want to read a cell here, you can

very quickly read it in a matter of tens of nanoseconds.

Data memory if you know it is has a similar read latency.

Nine flash if you read once you need to turn all

of them and then only you can access so it has a

read time of a few microseconds. So its as you can see it is much

slower to read but The best analogy

I've seen is that, this is like living in your own house.

You can get out on the road as you want.

This is like living in an apartment building,

multi-story apartment building, whenever you get out you

have to walk through all of the other floors.

You have to disturb all of the people

around you, but that's a good analogy to understand.

>> Question. >> Yes.

>> When you talk about the cell size >> Yes.

>> What's the definition of one f

>> So, F is the minimum feature size you can print, so suppose you can print

feature size of f, you have to separate them by the feature, by a feature size

of F2.

So you get like two F, and similarly two F in this direction for four F squared.

[COUGH]

So, the reason why it's called NOR and NAND is because it, the circuit

diagram, at least, has some resemblance to how NAND and NOR gates look.

If you look at.

From our NOR gates, from your basic

electronic class, it has essentially, if you need

to perform any operation on these, signal has to pass through both A and B.

As compared to NOR,

each of them has their individual contact to the ground.

So that's why it's called NAND or NOR, even though it's a memory

device, because it looks similar to this stick, similar to this circuit diagram.

So, as I described, you know, that

Since you, don't have this base line contact between two adjacent

scales, you can pack the total Say, so, in this case

if you have eight cells and if I draw them in a NAND fashion.

I'm convening this much length, 59 micron.

But if I disconnect them in a NAND

fashion, I'm convening half the length or five microns.

So. Even though there are

disadvantages of living in an apartment

building you can get significant cost advantages

and this is a more realistic Picture of actual cross-section of cells connected.

And here you can see there's in a NOR flash you need to drop this contact

between two, between each of these two cells you have to drop these Contacts,

as compared to a NAND it looks very

clean the NOR contacts.

The only contact you have is towards the beginning and toward the end of a string.

So you get half the area, as compared to 10F square,

you get 5F square, and you get half the price as well.

Since Half the area and it's you can pack twice the

cells so it's half the price. And that's why this market has NAND,

has grown much more faster at as

compared to NOR. Sometimes at the let's

try to displace both NOR. And the NAND because it's essentially

so cheap because you can fabricate it so compactly.

So

[INAUDIBLE]

come to how, how this works. So to understand how it works.

We can borrow this equation from our from 216 or similar

device class, where you might have studied the threshold voltage.

And it depends upon these normal terms, but it also has these

terms so what if there's some charge trapped in your mass capacitor?

So if there's some

charge trapped in your mass capacitor at the distant of DT from your top gate.

Your special threshold will shifts by this amount.

That is Q is the amount of charge trapped into

your distant divided by the direct constant of your insulator.

And you can re-arrange this down, that is you can divide

this epsilon by DT, and that will give you that capacitance term.

So what happens is

that if you don't have any charge, you have threshold, if

you have some charge trapped in there, you get a different threshold voltage.

So, the flash memory is essentially, is nothing but, it's a

more convenient way of Trapping charge in your gate stack.

So, this is

how it looks like.

So you have a source hindrance and alert to

what you have a nanometer transistor, but you have

this extra tank, all of the floating it and

it's floating because there's no terminal available to it.

You can see that three terminals sticking here,

[UNKNOWN]

drain and again.

So there's no terminal connecting the floating gate.

And it has, it's separated from this channel by this oxide which is called as

tunnel oxide and it's called tunnel oxide because

you can, most of your tunneling of electron

[UNKNOWN].

It occurs from the channel into the floating gate through this oxide.

So that's why it's called tunnel oxide.

This one is called IPD or inter poly dielectric.

this floating gate and the control gate are made out of poly silicon.

So, it was separating to poly materials.

That's why it's called a interpoly dielectrics.

So, again, what will happen if you don't

have any charge, you get one threshold voltage and

if you store charge, you get another threshold voltage.

So, that's how this otherwise works.

And so depending on upon whether, if you have charge no charge stored

over here, your electrons can easily flow from your source to your drain.

As compared to if you have charge

present in your control gate.

It prevents that electrons from flowing, because these

electrons will try to repel electrons in the channel.

And you have to apply a extra gate voltage to allow that flow of the current.

So what you do is essentially you, how you read the sale

is you apply a certain voltage which is in between these two states.