ECE Theory and Fabrication of Integrated Circuits

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Theory and Fabrication of Integrated Circuits
       Where students create integrated circuits from scratch…

Electrical and Computer Engineering Department
University of Illinois Urbana-Champaign
The ECE444 laboratory teaches core concepts for the
production of integrated circuits:

          •   Thermal Oxidation
          •   Photolithography
          •   Etching
          •   Dopant Diffusion
          •   Metal Evaporation
          •   Electrical Testing
Thermal Oxidation
Silicon is the dominant semiconductor used in integrated circuit processing,
    in large part due to its ability to form a robust (tough) native oxide.

This oxide is used for multiple purposes in the fabrication of ICs:

    – Diffusion barrier for selectively doping (adding impurities to) silicon
    – Dielectric (insulator) for MOS devices
    – Passivation and protection of the silicon surface

Of particular importance in semiconductor processing is cleanliness. For
   oxidation, cleanliness must be targeted to the molecular level. A
   specialized process called the RCA Clean is implemented before
   oxidation to remove

    – organic contaminants (oils)
    – trace metals
    – alkali ions (sodium).
Thermal Oxidation
After cleaning with the RCA clean, silicon wafers
   are placed into a high temperature furnace
   (900ºC < T < 1200ºC) in the presence of
   oxygen or water where the following reaction

               Si + O2 → SiO2

          Si + 2H2O → SiO2 +H2
By controlling temperature and oxidation time
   precisely, oxide thickness can be predicted and
   controlled accurately.

                                                     ECE444 students performing an
                                                     oxidation process in a high
                                                     temperature furnace.
Photolithography and Etching
After oxidation, the silicon wafer is completely covered with silicon dioxide. This
   oxide will prevent dopants from reaching the underlying silicon wafer.

In order to create integrated circuits, the silicon wafer must be doped with
    impurities (boron and phosphorus are the most common) selectively – this is
    accomplished by removing the oxide in specific areas so the dopants are
    allowed to diffuse (movement due to high temperature) into the exposed

Selective removal of the oxide is performed using
    – Ultraviolet light (UV) sensitive photoresist (PR) to coat the wafer
    – An alignment/illumination system (mask aligner or stepper) to expose
        the PR
    – A mask or reticle with desired circuit pattern that only allows the UV light
        to be transmitted in the shape of the circuit pattern
    – Acid to etch the oxide through openings created in the PR mask by
Photolithography and Etching
Photolithography is very much like taking a picture:

    – PR coated wafer = film
    – Mask aligner or stepper = camera
    – Mask or reticle = subject of the picture

But photolithography is binary – either the film is exposed or not exposed; there
   are no shades of gray.
Mask or Reticle:

The mask is an opaque plate of fused silica
   (high purity glass) with an optically
   opaque film applied to one surface. A                   CAD full adder layout
   detailed layout of the circuit is created
   using computer aided design (CAD)
   software, and this pattern is etched into
   the opaque film. The etched film on the
   mask creates the hard copy of the circuit

The mask/reticle is then used to transmit
   UV light in the pattern of the circuit.
                                               Isometric detail view of the ECE444
                                                                         CAD layout
PR Application:

Photoresist (an organic polymer sensitive to
   UV light and resistant to attack by acids)
   is applied to the oxidized wafer using a
   photoresist spinner. This process uses
   centrifugal force from high speed rotation
   of the wafer.

The PR is applied as a small puddle in the
   center of the wafer. When the wafer
   spins, the PR spreads out over the wafer      ECE444 student dispensing photoresist
                                                 onto an oxidized silicon wafer. Note the
   due to centrifugal force. After spinning, a   yellow cast to the picture – short
   uniform layer of PR remains on the            wavelength light (green, blue, violet, and
                                                 ultraviolet) exposes PR, so it has been
   surface.                                      filtered out of the room light, leaving only
                                                 red, orange, and yellow to see with!
Alignment and Exposure:
The PR coated wafer is placed into a
   system (mask aligner or ‘stepper’)
   which allows the mask to be aligned
   to the wafer. After alignment, the
   system opens a shutter to allow UV
   light to illuminate the PR through the
   mask for a controlled period of time.

The PR which is exposed to UV light
   undergoes a photochemical reaction
                                            ECE444 student loading PR coated wafer into an
   to make the PR more acidic (indene       Ultratech 1000WF Step and Repeat Projection
   carboxylic acid is produced).            Alignment system (also known as a ‘stepper’).
After the wafer is exposed to UV light through the mask, the acidic regions of
   PR are removed by dipping the wafer into an alkaline (base) developing
   solution. The acidic PR reacts chemically with the basic developer to
   form water soluble salts that dissolve in the developer.

At this point the mask image can be seen in the PR (remember that the PR
    was illuminated with UV light through the mask, so only light in the shape
    of the circuit reaches the PR – the rest of the PR did not change!).

Note: the image from the mask has only been transferred to the PR. The
   PR will be used as a mask for etching the underlying oxide in an acid
The previous steps produced a pattern in the PR
   layer coating the oxidized wafer. This patterned
   PR will now be used for selectively etching the
   oxide areas that are exposed.

The patterned, PR coated wafer is placed into a
   hydrofluoric (HF) acid bath to remove the
   exposed oxide. HF will react chemically with
   the oxide to form water soluble products that
   dissolve in the water used to dilute the acid.

When the oxide is etched away, the silicon beneath
                                                      Oxidized and etched 100mm diameter
  the oxide can be seen. Fortunately, HF does         wafer fabricated in the ECE444
  not react with silicon (this is ideal – the HF is   laboratory. Mask level 1 used for
  selective with regards to the two materials         photolithography. Purple areas are
                                                      silicon dioxide, silver areas are exposed
  present on the wafer).                              silicon.

The PR is then removed, leaving a permanent
   pattern etched into the oxide.
Dopant Diffusion – physics
Silicon is a column IV element – this means there are four electrons in the
    outermost shell of the atom. It is these electrons that are used when
    bonding to other atoms. In a wafer, each silicon atom bonds to four other
    silicon atoms (each Si-Si bond shares one electron). So in an intrinsic
    (pure) silicon wafer, all the electrons in the outer shell are part of a bond –
    they are ‘stuck’ between the bonded silicon atoms.

In order for current to flow in a material there must be ‘loose’ electrons. But all
    the electrons in silicon are working at holding the atoms together, which
    means it is not a good conductor of current.

So what can be done to allow the silicon to conduct current more easily? Free
   ‘carriers’ of current must be added. The goal is to find an element about the
   same size as a silicon atom so that it fits together well with the silicon, but
   with more electrons in its outer shell.
Dopant Diffusion – physics
In the periodic table, the closer elements are to each other, the more similar
    they are. So the best candidates would come from column V (which have
    five outer shell electrons). The element closest to silicon in column V is

If phosphorus is inserted into the silicon wafer in a certain way, it will take the
    place of a silicon atom and bond with its four neighbor silicon atoms. After
    bonding, phosphorus has an electron left over that is not bonded to a silicon
    atom. It turns out this extra electron is not strongly held by the phosphorus
    atom any more, so it can be removed easily. This electron then becomes a
    ‘carrier’ for current – it is free to move around the wafer. So the conductivity
    of the silicon wafer increases. This type of silicon ‘doped’ with phosphorus
    is called an n-type semiconductor.
Dopant Diffusion – physics
Extending this idea of inserting an element with a different number of valence
     electrons, a column III element (such as boron) could be added to the
     silicon wafer. In this case, the boron will try to bond with four silicon
     atoms, but it only has three electrons to bond with. This means there is
     an incomplete bond with one of the silicon atoms – a ‘hole’ where an
     electron would normally be. This ‘hole’ behaves much like an electron
     and can move around the wafer, but with an opposite charge (+). So a
     different type of current carrier is present in the wafer that increases the
     wafer’s conductivity. This type of silicon ‘doped’ with boron is called a p-
     type semiconductor.

By adding impurities to silicon, the conductivity increases. This conductivity
     can be adjusted by the amount of impurity added.
Dopant Diffusion – physics
Now for the interesting part - when n-type silicon comes into contact with p-type
  silicon. A built-in potential (voltage) develops that must be overcome before
  current can flow from the n-type to p-type regions.

Think of carriers as being able to only move across a flat surface or down a
   slope. The built in potential is a hill that the carrier can not go up. So in
   order for the carrier to keep moving, the low part must be pushed up to be
   level or higher than the top of the hill. In the case of an n-type / p-type
   junction, the energy to push up the low side comes in the form of a voltage
   applied to the wafer. The voltage is used to ‘push up’ the ‘ground’ on the low
   side of the hill before current flows from n-type to p-type regions.

But if the voltage is reversed, the energy is used to push the low side lower
   while keeping the high side at the same height! That means the carrier
   probably won’t ever make it up the higher hill, so it is stuck (no current
Dopant Diffusion – physics
So when n-type silicon is brought into contact with p-type silicon (a pn junction),
   current can flow in only one direction. This is the fundamental
   semiconductor device – a pn junction diode – a one way switch for current.

The devices used in integrated circuits are specialized combinations of pn
   junctions. The junctions are formed by the addition of impurity atoms from
   columns III and V of the periodic table into the silicon wafer through
Dopant Diffusion - Predep
The goal of the dopant predeposition diffusion is to
    move dopant atoms from a source to the wafer,
    and then allow the dopants to diffuse into the

The source of dopant can be in several forms – solid
    (boron nitride and phosphorus oxide ceramic
    discs), liquid (boron tribromide and POCl3), or gas
    (diborane or phosphine).

In order for the dopants to move into the silicon, they
     must be given energy, usually in the form of heat.
     In order for the diffusion to occur in a reasonable
     time, the temperature must be very high (900ºC

At this temperature the dopant (in the form of an oxide)
     reacts with the exposed silicon surface to form a
     highly doped glass. It is from this glass that the
     dopants can then diffuse into the wafer.              ECE444 Diffusion furnace

The first dopant diffusion in the ECE444 process is a
    boron diffusion (the wafer is originally doped at a
    low level with phosphorus). This diffusion forms
    the first pn junctions selectively on the wafer
    through the openings in the oxide defined by
Dopant Diffusion - Drive
After the predeposition diffusion the dopants are situated
    close to the surface of the wafer. However, they must
    diffuse even farther to lower the overall concentration
    in order for some of the devices to work properly.

The first diffusion (predeposition) introduces dopants into
    the wafer.

The second diffusion (drive) redistributes the dopants and
    allow the dopants to diffuse into the wafer more
    deeply (up to ~3 micrometers)

In addition, oxygen and water vapor are introduced during
    the drive diffusion to grow a new oxide over the areas
    which were exposed to bare silicon during the
    photolithography process. This new oxide can be
    patterned again so that other selective diffusion         100mm diameter wafer fabricated in
    processes can be performed to create other types of       the ECE444 laboratory following
    devices.                                                  boron predeposition, boron drive,
                                                              and re-oxidation.
Repeat the process…
At this point, the process of oxidation-photolithography-etching-diffusion can be repeated to produce
the various types of electronic devices required for a circuit.

Some modern processes may require more than 20 iterations of this sequence!




                           Diffusion (Ion           The following slides show the rest of the processes
                           Implantation)                                  performed in the ECE444 lab.
ECE444 Process
    Phosphorus Diffusion
    • Photolithography Mask 2
    • Etch
    • Phosphorus Predeposition

                       Gate Oxide Formation
                       • Photolithography Mask 3
                       • Etch
                       • Gate oxidation
ECE444 Process
  Electrical contact vias (holes) to silicon
  • Photolithography Mask 4
  • Etch

                               Metal definition
                               • Photolithography Mask 5
After all diffusion and oxidation steps are
   completed, metal is deposited onto
   the surface of the wafer. This metal is
   used to ‘wire’ the devices fabricated in
   the silicon together.

The wafers are put into a large chamber
   and the air is pumped out of the
   system. A piece of aluminum located
   on a tungsten ‘boat’ in the system is
   heated to high temperature, causing
   the aluminum to melt and evaporate.
   The evaporated aluminum will solidify
   into a thin film when it touches the
   silicon wafer.
                                              Thermal evaporation vacuum system used
                                               in the ECE444 laboratory. This tool was
                                                 designed and built for an independent
                                                            study project.
After metallization, the wafer is completely covered by the aluminum.

It must be patterned and etched to form the actual ‘wires’ connecting
    individual devices into a circuit.

                           Wafer after aluminum evaporation
Completed wafer
Steps to create ECE444 wafer:

1.    Initial oxidation
2.    Photolithography Mask 1
3.    Oxide etch
4.    Boron predep
5.    Boron drive and re-oxidation
6.    Photolithography Mask 2
7.    Oxide etch
8.    Phosphorus predeposition
9.    Photolithography Mask 3
10.   Gate oxidation
11.   Photolithography Mask 4
12.   Etch
13.   Photolithography Mask 5
14.   Metal evaporation
15.   Metal definition

                                     Time to see if it works…
After completion of the wafer, it must be
   tested to verify operation.
The devices fabricated are extremely
   small (some dimensions are as small
   a 1micrometer!), so specialized
   probes are used to make electrical
                                                              ECE444 Probe station

Once contact is made, several different
  instruments are used to measure
  electrical performance.

                                            Example of electrical data from a device
                                               fabricated in the ECE444 laboratory
ECE444 - notes
The ECE444 laboratory was started in the mid-1960s with the goal of teaching
   future engineers the processes required to fabricate integrated circuits.
   Over time the lab has gained recognition as one of the premier courses in
   this demanding field.

This recognition is due largely to the reinforcement of theory learned in lecture
   with practical hands-on experience in the lab (each student processes their
   own wafer from beginning to end).

Industry provides strong support through past and continuing donations of
   actual industrial processing equipment.

                                                        Wafer Photo credits: Mark Hart
ECE444 - equipment

Diffusion/oxidation furnace        Prometrix FT650

Ultratech stepper             Photoresist spin station    Thermal evaporator
ECE444 - equipment

Probe station        Optical microscopes         Electrical test station

Four point probe   Photoresist process station     Plasma asher
For more information…

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