GROWTH OF SEMICONDUCTOR MATERIALS
The success in fabricating very large scale integrated ( VL SI) circuits is a result, to a
large extent, of the development of and improvement in the formation or growth of
pure single-crystal semiconductor materials. Semiconductors are some of the purest
materials. Silicon, for example, has concentrations of most impurities of less than
1 part in 10 billion. The high purity requirement means that extreme care is necessary
in the growth and the treatment of the material at each step of the fabrication process.
The mechanics and kinetics of crystal growth are extremely complex and will be described
in only very general term in this text. However, a general knowledge of the
growth techniques and terminology is valuable.
Growth from a Melt
A common technique for growing single-crystal materials is called the Czochralski
method. In this technique, a small piece of single-crystal material, known as a seed,
is brought into contact with the surface of the same material in liquid phase, and then
slowly pulled from the melt. As the seed is slowly pulled, solidification occurs along
the plane between the solid-liquid interface. Usually the crystal is also rotated slowly
as it is being pulled, to provide a slight stirring action to the melt, resulting in a more
uniform temperature. Controlled amounts of specific impurity atoms, such as boron
or phosphorus, may be added to the melt so that the grown semiconductor crystal is
intentionally doped with the impurity atom. Figure 1.20 shows a schematic of the
Czochralski growth process and a silicon ingot or boule grown by this process.
Some impurities may be present in the ingot that are undesirable. Zone refining
is a common technique for purifying material. A high-temperature coil. or r-f induction
coil, is slowly passed along the length of the boule. The temperature induced by
the coil is high enough so that a thin layer of liquid is formed. At the solid-liquid interface,
there is a distribution of impurities between the two phases. The parameter
that describes this distribution is called the segregation coefficient: the ratio of the
concentration of impurities in the solid to the concentration in the liquid. If the segregation
coefficient is 0.1, for example, the concentration of impurities in the liquid
is a factor of 10 greater than that in the solid. As the liquid zone moves through the
material. the impurities are driven along with the liquid. After several passes of the
r-f coil, most impurities are at the end of the bar, which can then be cut off. The moving
molten zone, or the zone-refining technique, can result in considerable purification.
After the semiconductor is grown, the boule is mechanically trimmed to the
proper diameter and a Rat is ground over the entire length of the boule to denote the
crystal orientation. The Rat is perpendicular to the [ 1101 direction or indicates the (I 10)
plane. (See Figure 1.20b.) This then allows the individual chips to be fabricated along
given crystal planes so that the chips can he sawed apan more easily. The boule is then
sliced into wafers. The wafer must he thick enough to mechanically support itself. A
mechanical two-sided lapping operation produces a Rat wafer of uniform thickness.
Since the lapping procedure can leave a surface damaged and contaminated by the mechanical
operation, the surface must be removed by chemical etching. The final step is
polishing. This provides a smooth surface on which devices may be fabricated or further
growth processes may be carried out. This final semiconductor wafer is called the
substrate material.
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large extent, of the development of and improvement in the formation or growth of
pure single-crystal semiconductor materials. Semiconductors are some of the purest
materials. Silicon, for example, has concentrations of most impurities of less than
1 part in 10 billion. The high purity requirement means that extreme care is necessary
in the growth and the treatment of the material at each step of the fabrication process.
The mechanics and kinetics of crystal growth are extremely complex and will be described
in only very general term in this text. However, a general knowledge of the
growth techniques and terminology is valuable.
Growth from a Melt
A common technique for growing single-crystal materials is called the Czochralski
method. In this technique, a small piece of single-crystal material, known as a seed,
is brought into contact with the surface of the same material in liquid phase, and then
slowly pulled from the melt. As the seed is slowly pulled, solidification occurs along
the plane between the solid-liquid interface. Usually the crystal is also rotated slowly
as it is being pulled, to provide a slight stirring action to the melt, resulting in a more
uniform temperature. Controlled amounts of specific impurity atoms, such as boron
or phosphorus, may be added to the melt so that the grown semiconductor crystal is
intentionally doped with the impurity atom. Figure 1.20 shows a schematic of the
Czochralski growth process and a silicon ingot or boule grown by this process.
Some impurities may be present in the ingot that are undesirable. Zone refining
is a common technique for purifying material. A high-temperature coil. or r-f induction
coil, is slowly passed along the length of the boule. The temperature induced by
the coil is high enough so that a thin layer of liquid is formed. At the solid-liquid interface,
there is a distribution of impurities between the two phases. The parameter
that describes this distribution is called the segregation coefficient: the ratio of the
concentration of impurities in the solid to the concentration in the liquid. If the segregation
coefficient is 0.1, for example, the concentration of impurities in the liquid
is a factor of 10 greater than that in the solid. As the liquid zone moves through the
material. the impurities are driven along with the liquid. After several passes of the
r-f coil, most impurities are at the end of the bar, which can then be cut off. The moving
molten zone, or the zone-refining technique, can result in considerable purification.
After the semiconductor is grown, the boule is mechanically trimmed to the
proper diameter and a Rat is ground over the entire length of the boule to denote the
crystal orientation. The Rat is perpendicular to the [ 1101 direction or indicates the (I 10)
plane. (See Figure 1.20b.) This then allows the individual chips to be fabricated along
given crystal planes so that the chips can he sawed apan more easily. The boule is then
sliced into wafers. The wafer must he thick enough to mechanically support itself. A
mechanical two-sided lapping operation produces a Rat wafer of uniform thickness.
Since the lapping procedure can leave a surface damaged and contaminated by the mechanical
operation, the surface must be removed by chemical etching. The final step is
polishing. This provides a smooth surface on which devices may be fabricated or further
growth processes may be carried out. This final semiconductor wafer is called the
substrate material.
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