DIY Scanning Electron Microscope - Electron Gun Detail--- make money online

hey guys it is Saturday morning I've got

my cup of coffee and I want to talk

particle physics so let's head over to

the bench and I'll show you what's going

on with the electron gun in my scanning

electron microscope project so here is a

schematic of the electron gun in the

electron microscope and I will run

through the parts of it and how it's

built first and then talk about

operation so at the top this curved

piece represents the filament also

called the cathode in this case they're

the same then there is a metal plate

with a hole and this is known as the

when old cylinder it could also be

thought of as a grid and then below that

there is the anode so let me put these

pieces out here the actual pieces this

is the filament the when'll cup or when

alt cylinder in my case it's just a flat

plate with a hole in it very small hole

and the anode itself is really just a

plate with a hole - it could be cylinder

cylinder shaped like this so in the

actual electron microscope these items

are spaced out about like this so the

filament is very close to the when'll

and we'll talk about that and then

there's a fair bit of space between the

when'll and the anode so I'll put these

out and talk about what kind of voltages

are on these so the the filament is

heated up by a DC power supply which is

I'm representing with a battery with a

arrow through it and there's nothing

special about heating up a filament with

DC power it's just convenient if there

was some other magic way to make the

filament hot that would work too and

then there's a couple of balancing

resistors just so that current flowing

into this little circuit here is

balanced to both legs of the filament

and another variable resistor a current

meter the actual high voltage supply and

the high voltage supply connects to the

anode so one one can

I wanted to talk about first is how to

measure voltages in a system like this

you notice I made a ground symbol here

but I could have put the ground symbol

anywhere so when you talk about five

kilovolts you're really saying that the

thing you're talking about is five kilo

volts in potential above a known ground

point and that ground could actually be

the earth itself represented by a symbol

like this or it could be something else

so when I first drew this diagram out I

wrote down +5 kV for the anode and zero

volts for the when 'old but actually it

would be probably better to specify it

as zero volts for the anode since we're

talking about that as earth ground and

negative five kilovolts for the Reynolds

I guess that's actually a very good

concept to be familiar with is that

voltages are really just a potential

difference between two different

conductors so it doesn't make sense to

say well it's at 12 volts or it's at

five thousand volts you really have to

say it's five thousand volts with

respect to something else so in this

case we're going to call the anode

ground zero and everything will be

measured to that so these are all just

metal conductors we're not going to talk

about insulators yet or how this thing

is built and I'll try to you know give

some dimensions here actually first so

the hole in the in the anode is about I

think I settled on maybe one and a half

millimeters and the hole in the reynolds

cylinder is about 500 microns or half a

millimeter I think I might have gone up

to 750 the spacing between the the

filament tip and the one alts cylinder

bottom the actual distance between the

very tip of the filament and the actual

bottom of the lentil plate is quite low

in fact it's it's so low that the the

film it actually protrudes into that

hole a little bit it's it's on the order

of the hold

so if the hole is 500 microns ideally

that the filament would only be 500

microns above the bottom of this plate

here which is I mean these are small

distances so that's that's a tricky

machining job

the filament itself I think is about a

hundred microns in diameter I'll get

some close-up shots of these okay so I

hope that gets the basic idea down and

let's talk about the operation now so in

it when you read a description of how

this works on the web almost all of them

say something like well the electrons

are liberated from the filament and

they're attracted to the anode and

that's true but only in a very limited

sense and it kind of bothers me because

if you think if you take it literally

the electrons are boiled off of the

filament just think of these little

specks coming off the filament and

they're drawn towards the anode so

they're shooting down here but as soon

as they go through the hole in the anode

if they were attracted to the anode in

the general case they would be pulled

back up towards the anode again that

doesn't make any sense that's not how it

works so actually what's happening here

is the electrons are liberated from the

cathode and accelerated by the field the

electric field that's between the anode

and the when 'old so if we're calling

this anode zero volts and the reynold is

negative 5 kilo volts there's a very

strong electric field between these

plates and the field looks kind of like

this so this is a close-up of the the

action here and again I sorry about the

voltage conventions I said 5 kV at the

bottom and zero at the top and I hope

that's okay the field is very flat so if

you're an electron and you're sitting

right here you're experiencing a very

high acceleration in the

downward direction like this but as soon

as you go through the hole in the anode

there's no more field so for example if

if this is the anode inside this metal

cup there's no electric field because

it's all the same potential but above

the above this Cup if this plate is set

up like this and this plate is at

negative 5 kV and this is at zero

there's a very high field and that's

what actually causes the electrons to

move so it's important not to think in

terms of electrons are attracted to

anodes it's really just an acceleration

due to the field so maybe we can talk

about this control circuit here so we

know that if there's a if there's a big

electric field between here the

electrons will be accelerated in a

linear fashion so what's the deal with

all this the the win all combination of

the one alt and the voltage on the

cathode controls how many electrons are

produced by this gun and the setting is

actually this this variable resistor

here known as a bias resistor

so our high voltage supply is connected

between the anode and the when old and

that's always some fixed value like

let's say five kilovolts but what's the

voltage on the cathode in this case this

bias resistor sets up something known as

automatic biasing so as this gun is

running and electrons are coming out of

the filament it's potential is actually

going negative

excuse me positive okay sorry about that

guys I corrected an error here this is

actually supposed to be positive the

filament ends up becoming more positive

and potential relative to the rest of

the system and the reason for that is as

it's boiling off electrons think of

getting rid of a negative charge it's

actually going to end up more positive

and this variable resistor controls how

positive it's going to get so if this if

this resistor is a high value very high

value

like let's say ten mega ohms any current

flowing through here will cause a large

potential difference so as this filament

is boiling off electrons at us at some

current level let's just say 100

microamps the 100 microamps flowing

through this variable resistor will

determine what voltage the cathode ends

up being so some round numbers for you I

think the cathode is normally about 400

volts higher than the when'll I don't

actually know because in my system it's

it's not measured it's it's I'm just

using this resistor but you do want to

know the emission current so I have that

micro ammeter which shows how much

current is actually going into the

cathode and that tells you how much how

many electrons are being emitted from

there okay so hopefully that is starting

to make sense basically what we've got

here is three different voltages on the

filament when all an anode and I said

that by adjusting this variable resistor

the voltage on the filament will change

all by itself because it's emitting

electrons and it's it will buy us itself

basically so how does this actually work

so if we look at the close-up here this

is the tip of the filament the 1-all and

the anode and I said that if an electron

is in this field it's going to

experience a very high acceleration and

that's what actually causes the gun to

work so how do they actually make it

from the tip into this field into the

accelerating field and that that's

controlled by the potential difference

between the filament and the 1-all which

is adjusted by this resistor so let's

say the resistor is a huge value in fact

let's just say it's open infinite

resistance what's going to happen the

filament is going to get more and more

positive as it boils off electrons and

gets rid of its negative charge it has

no way of replenishing the charge

because it's not connected to the rest

of the circuit this is infinite so

what's going to happen is the filament

is going to get very very positive even

more positive in the anode maybe well I

won't get that high but it's going to

shoot up pretty high and electrons that

the surface boiled off are going to

experience a field that actually causes

them to go back to the filament so if

this filament is let's just say 800

volts and when alt is 0 any electrons

that leave this filament are not going

to be accelerated down the column

they're actually going to be pushed back

into the filament because you know the

filament is more positive than the one

all so let's say we I did have a value

here let's say it's zero actually let's

say we shorted this out that means that

the filament is always going to be the

same potential as the one all because

this is a short now what's going to

happen now these field lines would be

very flat in fact they would not be

drawn the way that they look now because

this is all the same potential so you

just have straight field lines running

across here these are actually

equipotential lines that just show you

know the shape of the field so any

electrons that are boiled off in that

case all of them get accelerated pretty

much or any of them that make it down

into this area so if we look at the two

extremes of the situation we either have

no electron emission or tons of electron

emission we can guess that if we set

this value to be something appropriate

we can actually control the electron

emission by regulating the voltage

between the filament and then when all

so at some happy medium and in this case

with these sizes and voltages it comes

out to be on the order of a couple mega

ohms so if this is 2 mega ohms the

filament voltage might be something like

400 volts higher than the 1-all and that

causes the field to look like this where

there's sort of like a dip in there and

electrons that are boiled off in a

region where the dip touches the

filament actually make it down into the

field and are accelerated and electrons

that are boiled off over here on the

side they don't get sucked in they

actually get pushed back into the

filament or they just stay in a cloud

around here so it's a self-regulating

adjustable self-regulating electron gun

okay so here we are at the actual

microscope I'm going to point out some

of the parts that we just talked about

so these two conductors are the the

filament you know lines and you can see

just barely the pins of the actual

filament sticking up so the filament is

aiming downwards in this cup area here

and this is the one alt cylinder

connection and all of the metal in this

entire microscope is the anode so we're

calling this zero volts ground the one

alt is you know negative five kV or

whatever the acceleration voltage is and

the voltage on both of these is very

close to about negative 4500 volts or

whatever the bias voltage is on top of

the one alt voltage and the voltage

between these is about one volt at two

amps or two volts at one amp or

something like that just to heat up the

filament so I hope this was interesting

to you and made some sense I certainly

had some difficulty explaining it so let

me know if you have any questions and I

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