Linear Integrated Circuit Questions and Answers – Basic Planar Process – 2

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This set of Linear Integrated Circuit Interview Questions and Answers for Experienced people focuses on “Basic Planar Process – 2”.

1. The process involved in photolithography is
a) Making of a photographic mask only
b) Photo etching
c) Both photo etching and making of photographic mask
d) None of the mentioned

Explanation: Photolithographic involves both processes in sequence. First photographic mask is used for artwork and reduction. Then Photo etching for removal of SiO2 from designed region.

2. How will be the initial artwork done for a normal IC?
a) Smaller than the final dimension of chip
b) Same as that of final dimension of chip
c) Larger than the final dimension of chip
d) None of the mentioned

Explanation: The initial artwork of an IC is done at a scale several hundred times longer than the final dimensions. This is because for a tiny chip, larger the artwork, more accurate is the final mask.

3. Find the area of artwork done for a monolithic chip of area 30mil × 30mil.
a) 16 cm × 16 cm
b) 60 cm × 60 cm
c) 12 cm × 12 cm
d) 36 cm × 36 cm

Explanation: Drawings are magnified by a factor 500.
=> 1mil = 25µm
Therefore, 500mil = 1.2cm.
In an area of 30mil × 30mil, the area for artwork required = 30mil × 1.2cm = 36cm × 36cm.
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4. Mylar coated with a sheet of red photographic Mylar is used for artwork (layout) because,
a) It is used to get a colourful layout
b) It can be easily peeled off from layout
c) It is recommended colour for layouts
d) It is used for highlighting layout

Explanation: For photographic purpose, artwork should not contain any line drawing but must be of alternate clear and opaque region. The red layer can be easily peeled off thus exposing clear areas with a knife edge from regions where impurities have to be diffused.

5. Find the coating material used for photo etching process along with its thickness range.
a) Kodak photoresist (5000-10000Å)
b) Kodak photoresist (1000-5000Å)
c) Kodak photo etchant (1000-5000 Å)
d) Kodak photo etchant (500-1000 Å)

Explanation: The coating material is Kodak photoresist. It is a photosensitive emulsion film coated on wafer to remove SiO2 from desired region.

6. Which type of etching process is preferred to make the photoresist immune to etchants?
a) None of the mentioned
b) Wet etching
c) Plasma etching
d) Chemical etching

Explanation: Plasma etching is also called as dry etching. The major advantage of dry etching process is that, it is possible to achieve smaller line openings (<1µm) compared to other process.

7. Which of the following statement is not true?
a) X-ray and Electron beam lithography technique, produce device dimensions down to submicron range.
b) Ultraviolet lithography has limitation due to diffraction effects of wavelength.
c) The cost of X-ray or Electron beam is less compared to Ultraviolet photolithography.
d) The exposure time is less in Ultraviolet compared to X-ray or Electron beam lithography.

Explanation: The cost of X-ray or Electron beam is very high and thus, it is an expensive process. Therefore, it is used only when very small device dimension (<1 µm) are needed.

8. For photographic purpose usually coordinatograph is preferred for artwork because,
a) It is a precision drafting machine
b) Cutting head can be positioned accurately
c) It can be moved along two perpendicular axes
d) All of the mentioned

Explanation: The coordinatograph is a drafting machine that outlines the pattern cutting through the red Mylar without damaging the clear layer underneath.

9. Which of the following is added as an impurity to p-type material in diffusion process?
a) Phosphorous pentaoxide (P2O5)
b) Phosphorous oxychloride (POcl3)
c) Boron oxide (B2O3)
d) None of the mentioned

Explanation: Boron is a p-type material, whereas Phosphorous is an n-type material.

10. In the fabrication of monolithic ICs, Boron chloride is added as an impurity in the diffusion process. Find the diffusion time, if the furnace is heated up to 1200oc.
a) 1 hour
b) 2 hours
c) 45 minutes
d) 30 minutes

Explanation: In diffusion process, the depth of diffusion of impurities depends upon the time taken for diffusion, which normally extends for more than 2 hours.

11. Which component is not used as an impurity in diffusion process?
a) Phosphorous
b) Boron chloride
c) Phosphorous pentaoxide
d) Boron oxide

Explanation: Elemental form of Phosphorous is not added directly as an impurity in diffusion process.

12. In ion implantation method, penetrating the ions into the silicon wafer depends upon
a) Accelerating voltage
b) Accelerating speed
c) Accelerating current
d) All of the mentioned

Explanation:The depth of penetration of any particular type of ion increases with increasing accelerating voltage.

13. What is the advantage of using Ion implantation process?
b) Performed at high temperature
c) Beam current controlled from outside
d) Performed at low temperature

Explanation: In diffusion process, temperature has to be controlled over a large area inside the oven, whereas in ion implantation technique, accelerating potential and the beam current are electrically controlled from outside.

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