EE 3143 Quiz #1
 
Please answer the following questions:
1. What features of integrated circuits have enabled them to revolutionize modern electronics?
Soln.: - Small Size, Low cost of manufacturing and low power.
2. What are the industries that depend most heavily on integrated circuits?
Soln.: - Computers, Communication, Microelectronics.
3. How is solid crystal silicon formed from molten silicon?
Soln.: - A seed crystal is dipped into a vat containing molten silicon and then slowly pulled out while rotating the seed and the vat containing the molten silicon in opposite directions. The molten silicon replicates the structure of the seed and cools as it is extracted from the vat.
4. What makes a material a semiconductor? What are dopants?
Soln.: - A semiconductor is a material that has electrical properties (conductivity) between a conductor and an insulator. It behaves as a insulator at low temperature and as a conductor at high temperature. Dopants are the impurities added to semiconductors to better control their electrical properties.
5. Integrated circuits contain a maze of interconnected, microscopic switches called transistors. What is the function of these transistors?
Soln.: - The transistors either act like switches by conducting or blocking signals or they act like amplifiers by amplifying the input signals.
6. Explain how a pair of complementary metal oxide semiconductor transistors work. What are n-channel transistors and p-channel transistors? What are source and drain regions? What role does the gate electrode play? (4)
Soln.: - In case of a CMOS, we have both a NMOS and a PMOS fabricated on a single wafer. When a low voltage is applied to the gate the PMOS is turned on where as the NMOS is turned off. Application of a high voltage reversed this. An n-channel transistor or a NMOS is a transistor with two heavily doped, electron rich n-type regions separated by an electron poor p-type substrate. PMOS has two heavily doped, electron poor p-type regions separated by an electron rich n-type substrate. The heavily doped regions on either side of the substrate act as the source and drains while the lightly-doped region acts as the gate electrode. Voltage applied to the gate electrode can flip the switch on or off.
7. Describe the process by which integrated circuits are designed. How do the designs make the transition from computer to chip?
Soln.: - IC design is performed using CAD tools. IC layout is drawn and checked for accuracy using various CAD tools. A mask designer uses the layout to create a mask design and this mask design is fed to another computer that controls instruments that are used to etch the design onto wafer.
8. Why must the environment in which integrated circuits are manufactured be as dust-free as possible?
Soln.: - Presence of even a small amount of dust in the manufacturing facility might change the dopant concentration in the IC being manufactured. This can lead to defective chips or chips with incorrect electrical properties.
9. At the beginning of the manufacturing process, silicon wafers are placed in a high temperature oxidation furnace to grow a layer of silicon dioxide. What common process is similar? Why is the layer of silicon dioxide valuable?
Soln.: - This process of oxidation is similar to the way the sunlight makes shiny paint of a car dull over time. This layer of oxidation acts as a shield to protect the silicon below during ion-implantation and other such process.
10. What purpose does photolithography serve in the integrated circuit manufacturing process? Explain this procedure.
Soln.: - Photolithography is a process used to mark the regions for transferring the design from the mask to the wafer. During photolithography a light sensitive material is uniformly applied to the wafer. The mask is properly aligned on the wafer and light is used to transfer the mask pattern onto the wafer. The resist reacts to the light and the exposed area (if positive photoresist or unexposed area if negative photoresist) is dissolved and pattern etched onto the wafer,
11. How are dopant ions added to wafers?
Soln.: - Dopant ions are accelerated and bombarded onto wafers so that they embed themselves in the wafer.
12. How is each new mask perfectly aligned to the pattern already on the wafer?
Soln.: - The machine looks for special patterns on the wafer to perfectly align each new mask to the pattern already on the wafer.
13. What is the advantage of dry-etching?
Soln.: - Dry-etching has the advantage of not leading to undercutting or over-etching.
14. What feature of the integrated circuit allows modern computers to be very fast? What role does insulating material between transistors play?
Soln.: - Small size and high density are the features of the IC that allow modern computers to be very fast. Having insulating material between transistors allows for higher density of transistors and the electrical contacts in a IC.
15. How are electrical circuits incorporated into the chip? How is the chip connected to wires from the outside world?
Soln.: - Wiring layers, created using photolithography and etching, are used to create the circuits. The circuit is then connected to the pad frame using gold wires and the pads are connected to the outside using aluminum pads on the edges of the chip. 

 EE 3143 Quiz #2
 
Please answer the following questions:
1. In SILICON RUN II we saw individual chips placed into packages followed by the assembly of packaged chips onto printed circuit boards and finally the assembly of complete systems from a set of printed circuit boards, power supplies, and other components. We also saw that testing occurred at several stages of this process. Why is testing done throughout the manufacturing process rather than simply after final system assembly?
Soln.: - Testing is done to find and remove defective pieces. Finding defects early in the processes leads to savings in cost. If testing is to be done only after final system assembly then there is a great possibility that even defective chips might have been packaged and assembled onto PCBs which would make the PCB defective and it will have to be discarded thus increasing the cost.
2. Some of the electrical testing was performed at higher-than-normal temperatures or at higher-than-normal voltages. Why would this testing be performed under such stressful conditions even before the chips, boards, or systems have left the manufacturing area?
Soln.: - Performing testing under such stressful conditions makes weak parts fail before further processing, thus leading to savings. In case parts fail after systems at the end user location the manufacturer might have to replace them, if under warranty, which would cost more money.
3. Virtually all of the automated pieces of manufacturing equipment shown in SILICON RUN II are controlled by computers. For example, the die attach machine uses a computer-controlled camera to "look at" the individual dice that are mounted on the sticky tape and then uses a computer-controlled arm to pick up the selected die and mount it on the adhesive of the lead frame. Describe a number of the decisions and actions that the computer must control to successfully complete this process.
Soln.: - The decisions and actions that the computer must control to successfully complete this process are: 1) Discard die that have been marked defective (i.e., must not pick up die that have ink mark) by the presence of ink dot; 2) Move the robotic arm to pick up the good die; 3) Move the robotic arm to properly place the die on the adhesive of the lead frame; 4) Move the dice mounted on the sticky tape to bring the next die under the camera lens.
4. Printed circuit boards were assembled using both surface mount and through-hole processes. Describe the differences between these two mounting approaches.
Soln.: - In case of surface mount process the leads of the chip are connected only to electrical connections (wires) on the top of the board (i.e., the side on which the chip is mounted). In case of through-hole process the leads pass through the board and can have electrical contacts over several layers.
5. The four basic elements of a computer are input devices, output devices, the memory, and the central processing unit. Which of these units is generally thought of a being the "brain" of the computer? Which of these units is sued to store the data and program instructions?
Soln.: - The central processing unit is thought to be the brain of the computer. The memory is used to store the data and program instructions.
6. List several examples of "input devices" that send signals to the computers that control the automated manufacturing equipment shown in SILICON RUN II. What are examples of "output devices" on these same pieces of equipment?
Soln.: - The input devices are 1) Sensors, 2) Camera, 3) Keypads & 4) Mouse. The output devices are 1) Monitor, 2) Robotic Arm & 3) Timer Alarms.
7. Application software programs control the computer so that it performs tasks that are of interest or used to the person using the computer. List a variety of tasks performed by application programs.
Soln.: -Examples of tasks performed by application programs are: 1) Word Processing, 2) Accounting, 3) Picture, Video and Sound Editing, 4) Numerical Calculations and Simulations etc.
8. With their large number of solder bonding pads and high density signal carrier substrates, multipchip units (MCUs) allow chips to be packaged closer to one another when compared to conventional printed circuits boards. Why is that an advantage in high speed computer systems?
Soln.: -MCUs decrease the distance travelled by a signal on the board. Also they decrease the resistance that a signal has to overcome to travel from chip to chip. Due to these factors the chips can work at higher frequencies as the signals flowing between them do not face any bottlenecks.
9. We saw two processes used for electrically connecting signals on the chip to the interconnection wires closest to the chip: wire bonding and tape automated bonding (TAB). What are some advantages of tape automated bonding compared to conventional wire bonding?
Soln.: - Some of the advantages of tape-automated bonding over wire bonding technology are: 1) smaller bonding pad, 2) smaller on-chip bonding pitch, 3) a decrease in the quantity of gold used for bonding, 4) the reduction of variations in bond geometry, 5) increase in production rate because of area or `gang' bonding, 6) a stronger and more uniform inner lead bonding strength. Also TAB has lower labor costs, higher I/O counts and lighter weight, greater densities are achievable and the chip can be attached in a face-up or face-down configuration.
10. Electrical signals can travel no faster than the speed of light and often travel slower than this due to resistance, capacitance, and inductance encountered in real systems. The speed of light is 3 x 10^8 meters/second (i.e., 300,000,000 meters/second). What is the maximum distance that an electrical signal can travel in 100 picoseconds (1 picosecond = 1 x 10^-12 seconds)?
Soln.: - The maximum distance that an electrical signal can travel in 100 picoseconds is 3cm.