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An Exploration into the World of Microprocessors

Alex Pisarski, The Institute of Optics, The University of Rochester

 

Introduction:
Semiconductors and microprocessors are essential to modern technology; their absence would halt every aspect of modern life. Advances and improvements in the semiconductor industry have fueled the technological revolution of the 1990s. The decrease in the cost and size of transistors allowed for an every increasing amount of transistors to be put onto a silicon chip, thereby allowing for faster processors.
The ability to fit more transistors on a silicon wafer approximately follows Moore’s Law, which is named after Gordon Moore, co-founder of Intel Corporation. Moore’s Law states that the number of transistors on a silicon chip doubles every 18-24 months.

 

A sketch of Moore's original prediction from 1965 showing the increase in transistor capacitance per chip. Image courtesy of Intel Corp.

The semiconductor industry has roughly followed this law since its conception in 1965. In 2001 Intel published a paper in which is used electron microscopy techniques to verify and predict the future of Moore's Law. Intel estimates that Moore's Law will hold for another 10-15 years before physical limitations begin to slow progress.

The goal of this research was to study semiconductors under both light microscopy and Scanning Electron Microscopy (SEM) in order to verify Moore's Law. By studying two consecutive generations of semiconductors, it should be possible to note significant changes in transistor density and wire size.

 

Samples:

Sample 1- An Intel 486 DX processor produced in 1989. This processor represented the fourth generation of Intel processors and added significant improvements of the earlier 386 processor. It had a clock speed of 33MHz and contained 1,200,000 transistors.

Sample 2- An Intel Pentium processors (the original), produced in 1992. This processor represented the fifth generation of Intel processors and was the first to implement Superscalar architecture which allowed it to complete multiple instructions per clock cycle. The Pentium had a clock speed of 66MHz and contained 3,100,000 or double that of its predecessor, the 486 DX.

 

 

Image of the Intel 486 DX processor. Image courtesy of Intel Corporation.

Image of the Intel Pentium processor. Image courtesy of Intel Corporation.

Techniques:

Differential Interference Contrast Microscopy (DIC)- Uses beam-shearing interferometry to greatly enhance image contrast. This is used as part of the overall light microscopy system and is capable of producing brilliantly colored high contrast images.

Image Enhancement-Using Adobe Photoshop, various images were either colorized, sharpened, or enhanced in contrast and brightness in order to be more aesthetically pleasing.

Scanning Electron Microscope (SEM)- The majority of the collected images were taken using a scanning electron microscope. The SEM can provide much high magnifications than a traditional light microscope. Images were collected using the back scattered electron detector, the secondary electron detector or a combination of the two.

X-Ray Spectrometry- Using the EDAX system, which is an X-ray detector attached to the SEM, it is possible to determine the elemental composition of the sample as well as map the location of those elements in a sample.

 

Results:

Wire Size:

It wasn't possible to directly verify Moore's Law because the transistors were located on the bottom layer of the chip. However, measurements were made regarding the side of the wires used to connect the layers and transistors. Wire size should be a good indication of transistor size, since as transistors decrease in size, so to must the wires used to connect them. It was found that the size of wires on the 486 DX ranged from 3.0um to 3.5 um. The average size on the Pentium was 1um to 1.5um, or roughly half that of the 486 DX.

(a)

(b)

Images showing examples of line width measurements. Image (a) shows a series of measurements on the 486 DX processor. The average line width was 3.28um. Image (b) illustrates measurements taken on the Pentium processor with an average line width of 1.5um.

 

Elemental Composition:

By using energy dispersive X-ray spectrometry, it was determined that the wires etched onto the chip were composed primarily of aluminum. The base layers were found to be silicon with trace amounts of titanium and oxygen. These findings are consistent with data from Intel's website regarding the manufacturing of microprocessors.

Spectrograph of Pentium chip showing peaks at the Aluminum, Silicon, Titanium, and Oxygen lines.

 

Intel 486 DX

The 486 DX processor imaged very well under both the light microscope and SEM. The sample was sputter coated in order to reduce charging, which was a persistent problem on this sample.

Three DIC images showing the complex and beautiful circuitry patterns laid out on the chip.

 

 

Secondary electron image of wire leads. This image has been enhanced using Adobe Photoshop.

Mixed signal image, combining back scattered and secondary electron signals.

20,000x close up of the interconnect holes. These areas connect wires from different layers of the processor.

Interesting pyramid shaped wire structure. Please see the X-ray mapping part for a higher magnification and mapped image.

 

Pentium

The Pentium chip was more difficult to image because there was a layer of polyimide over the top. This protective coating, acted as an insulator and was thick enough so that the electron beam could not penetrate it. The polyimide layer was removed by exposing the beam to an oxygen plasma. This removed the layer without damaging the circuitry underneath. It did however, leave some surface contamination.

DIC image showing interconnect banks.
DIC image showing a large amount of wires.
Light microscopy image showing circuitry near the edge of the chip.

 

Image showing wires ending in interconnects. This was the first high quality image I obtained of the Pentium processor and it is one of my favorites!

Colorized and enhanced image showing wires located near the socket connections on the microprocessor.

Image showing the fine detail and precision in which these wires are produced. The wires conform to the shape of the layer below them.

Three wires spaced closely next to each other. It looks like a foot!

 

Pentium Edge

An attempt was made to image the edge of the silicon chip. This proved difficult for a number of reasons. First, it was difficult to break the chip in such a way as to leave an edge perfectly intact. Secondly, because of the large sample size, and the extreme tilt angle (~90 degrees) caution needed to be exercised so as to not run it into anything (i.e. detectors, or lenses). That being said, a few images were obtained showing the different layers of silicon.

Ghostly image of the edge of the Pentium chip. The close up images below, were taken near the corner edge.
The different layers are clearly visible. Microprocessors can have 20 or more layers of circuitry connecting the millions of transistors.
A close up of the layers. There was excessive surface contamination along the edges.

 

X-ray Mapping

(a)

(b)

(c)

(d)

X-Ray map showing three levels of gold leads progressing towards the chip. Image (a) shows the secondary electron image of the sample. Image (b) showing the location of aluminum within the sample. Image (c) shows the locating of gold. Image (d) shows the location of oxygen. Note that the wires running off the terminals are entirely composed of gold. The wires show up clearly in image (c) but appear black in images (b) and (d) since they contain neither aluminum or oxygen.

 

(a)

(b)

(c)

Image of wires on the 486 DX processor. Image (a) is secondary electron image of the region of interest. Image (b) maps the location of aluminum while image (c) maps the location of silicon. These images clearly indicate that the wires are indeed composed of aluminum.

 

Silicon Graffiti

The engineers who design microprocessors are notorious for inserting images, phrases, or even their own initials into the layout of the circuitry. The large amount area of the chip combined with the extremely small scale of the design make locating silicon graffiti extremely difficult. However, I was lucky enough too find examples on both the 486 DX and the Pentium processors.

486 DX
DIC image showing designers initials in lower right corner.
SEM image showing designers initials. This image was taken prior to the sample being coated and therefore suffered greatly from the effects of charging. I tried to locate this feature again, after the sample was coated, but I was unsuccessful. The letters measured 75.1um from the top of the "M" to the bottom of the "J."
Pentium

DIC image of Intel logo and chip model. The Pentium processor was developed in 1992 but not released to the public until March on 1993, hence the two dates on the chip.