Technical Field

This invention relates to facilities for growing

crystals for use in the semiconductor industry and, in

particular, to a crystal puller cell for housing a

Czochralski crystal puller and to a growing hall housing

multiple ones of such cells.

Background of the Invention

Integrated circuits are typically fabricated on

wafers of a single-crystal semiconductor material. The wafers are sliced from a single crystal ingot grown in a

Czochralski-type crystal growing machine, referred to as

a "crystal puller." Typically, many crystal pullers are

located within a single large room, called a "growing

hall." A control console located by each crystal puller

allows a crystal puller operator to control each crystal

puller in accordance with instrument readings and -

observations through an observation window of conditions within the crystal puller.

To grow an ingot, a seed crystal is dipped into a

crucible of molten semiconductor and then slowly

retracted as the seed and the crucible are rotated in

opposite directions. The semiconductor freezes onto the seed as it is retracted to produce a single crystal

ingot . Crystals are sometimes grown in the presence of a

strong magnetic field which, by controlling thermal

convection in the molten semiconductor, produces an ingot

having a more uniform amount of oxygen incorporated into

the ingot from the crucible walls.

The growing process begins with loading into a

crucible a "charge" of ultrapure polysilicon. A dopant

is often added to the charge to change the electrical

properties of the resultant crystal. A seed crystal

having the desired crystal orientation is then secured

within the crystal puller in a chuck attached to a cable

that is used to raise the seed, and a charged crucible is

loaded into the crystal puller. The section of the

crystal puller in which the seed crystal and the charge

are placed is called the "furnace tank." As the single--

crystal ingot is grown, it is received into a "pull

chamber" above the furnace tank. A "mechanical unit"

below the furnace tank includes motors and other

mechanical and electrical devices used in the crystal

growing process.

Before melting the charge, the air in the furnace

tank and pull chamber is evacuated and replaced with an

inert gas, such as argon. The crucible is rotated as the

charge within it is melted, typically using a resistant

heater. The seed crystal is dipped slightly into the

melted charge and slowly retracted to grow the ingot.

After the ingot is grown and cooled, the pull chamber is

opened and the ingot is removed.

Extremely small amounts of contamination in a

crystal can have severe adverse effects on the

characteristics of electronic circuits fabricated on the

crystal. It is critical, therefore, that airborne

contamination be minimized in the growing area. Although

clean room techniques for reducing airborne particulate

contamination are widely used in the semiconductor

industry, it is difficult and expensive to maintain a

high degree of cleanliness in a large growing hall . The

energy cost of providing the required volume of filtered

airflow is too great. Higher than optimum particulate

levels have, therefore, been tolerated in growing halls,

along with corresponding contamination of the grown

crystals .

Cross-contamination problems are much worse during

the installation of new equipment . Growing halls

typically include an overhead bridge crane that is used

to install new equipment and repair existing equipment.

Use of such an overhead crane produces particulate

contamination that settles down onto the active crystal

pullers .

To reduce contamination of the crystals during the

installation of new crystal pullers, one practice has

been to install several machines at one time in a

segregated area of the growing hall. Under these circumstances, the growing hall is not being continually

contaminated and disrupted by the installation of

individual machines.

Summary of the Invention

An object of the present invention is, therefore, to

provide a crystal-growing environment having a reduced

level of airborne particulate contamination.

Another object of this invention is to provide such

an environment at a reasonable cost and to achieve such

an environment at a minimum running cost .

A further object of this invention is to provide

such an environment capable of modular expansion.

Yet another object of this invention is to provide

such an environment that is capable of reducing exposure

of machine operators to magnetic fields.

Still another object of the present invention is to

improve the productivity of crystal puller operators.

The present invention is an economical, low

contamination environment for a crystal puller. The

environment includes a crystal-puller cell for housing a

crystal puller and a growing hall containing multiple

crystal puller cells.

The crystal puller cell comprises plural cell walls

that isolate the environment within the cell from the

environment in the growing hall outside the cell and a

base floor at a first level for mounting of a crystal

puller. Above the base floor is a second floor for

accessing the crystal puller. The second floor is

preferably a multi-level floor having a second, or

operator, level positioned at a height convenient for

loading the raw material and unloading the single crystal

and an intermediate, or maintenance, level positioned at

a height between the first and second levels and

convenient for maintaining the crystal puller. If a

crystal puller cell houses a crystal puller using a

magnetic process, the walls of the cell can be shielded-

to reduce exposure of workers and other equipment to the intense magnetic fields.

The crystal puller cell also includes an adjustable

air contamination control system for maintaining airborne

particulate within the cell below specified levels. The

particulate level specified at any particular time is

appropriate for the operation being performed within the

crystal-growing cell. For example, the specified

particulate level would be lowest, i.e., the environment within the cell would be cleanest, when the semiconductor

material is exposed to the environment, i.e., when the

raw material is being loaded and the single crystal is

being unloaded. A higher particulate level can be

tolerated when the crystal puller is operating and the

furnace tank and pull chamber are closed. An even higher

particulate level could be tolerated when the machine is

idle. By maintaining only the level of cleanliness

required, the costs associated with maintaining suitable

particulate levels are greatly reduced.

The area within the growing hall is divided into

three types of areas: puller cells, clean aisles, and

maintenance aisles. Each area has an environment

characterized by an airborne particulate level, which

corresponds to a level of cleanliness. A relatively high

degree of cleanliness is maintained in the clean aisles,

which are used to bring raw material to and finished

products from the puller cell. A lower degree of

cleanliness is maintained in the maintenance aisles,

which are used by maintenance workers to maintain the

crystal pullers.

Each crystal puller cell has two doors: a first

door, at the operator level, that opens onto a clean

aisle and a second door, at the maintenance level, that

opens onto a maintenance aisle. Cross contamination is

prevented by logic circuitry that electronically controls

the door locks to prevent both doors from being opened

simultaneously and to prevent either door from opening

when the environment within the puller cell is not

compatible with that outside the door.

By maintaining separate environments in areas in

which the semiconductor material is exposed to the

environment, a low particulate level can be maintained in

appropriate areas at a lower overall cost. Providing a

separate environment for each crystal puller not only

isolates each puller from contamination in the growing

hall, it allows a reduction in airflow when appropriate

in each cell, thereby further reducing costs. Moreover,

additional crystal puller cells and crystal pullers can

be installed with minimal contamination of existing

machines.

In accordance with another aspect of the invention,

a remote control console capable of controlling multiple

crystal pullers is located in a clean aisle away from the

doors of the puller cells. By observing instruments and

video images transmitted to the remote control panel from

a television camera positioned to observe conditions

within the crystal puller, an operator can operate the

multiple crystal pullers from one remote location,

thereby reducing contamination in the immediate vicinity

of the pullers, reducing operator exposure to magnetic

fields, and improving productivity by allowing a single

operator to operate more machines.

Additional objects and advantages of the present

invention will be apparent from the following detailed

description of a preferred embodiment thereof, which

proceeds with reference to the accompanying drawings.

Brief Description of the Drawings

Fig. 1 is a plan view of a growing hall ^• designed in accordance with the present invention.

Fig. 2 is a cross sectional view of a crystal puller

cell taken along line 2--2 of Fig. 1.

Fig. 3 shows a prefabricated ceiling unit used in

the crystal puller cell of Fig. 2.

Fig. 4 is a block diagram of a control unit used to control the puller cell of Fig. 2.

Fig. 5 is a schematic view of a remotely controlled

bay including several of the crystal puller cells of Fig.

2.

Detailed Description of Preferred Embodiments

Fig. 1 shows the layout of a crystal-growing hall 10

of the present invention. Hall 10 is contained within a

shell 12 formed from multiple walls 14 and a high ceiling

16 (Fig. 2) that provides an unobstructed, column-free

expanse for containing multiple crystal puller cells 18

of the present invention. Fig. 2 shows a cross sectional

view of a preferred crystal puller cell 18 of the present

invention within crystal-growing hall 10. The

environment within crystal puller cell 18 is isolated

from the environment within hall 10 by prefabricated

walls 24 and a prefabricated ceiling unit 26. Crystal

puller cell 18 houses a multi-story crystal puller 28,

which rests upon a prefabricated concrete pad 30, which,

in turn, rests upon a base floor 32.

The use of prefabricated components to construct

puller cell 18 eliminates the need for heavy construction

in growing hall 10, thereby reducing sources of

contamination. The use of prefabricated components also

reduces the time required to construct puller cell 18.

If a magnetic process is used in crystal puller

machine 28, prefabricated walls 24 can include a magnetic

shielding material, such as low carbon, mild steel, or

other appropriate metal alloy to prevent exposure of

machine operators and other machines to intense magnetic

fields. Moreover, physically separating an exemplary

puller cell 18a from the other puller cells 18 can

further reduce the propagation of magnetic fields from

puller cell 18a through the metal walls to the other

puller cells 18.

Crystal puller 28 comprises a pull chamber 42a into

which the grown single crystal is received, a furnace

tank 42b in which a semiconductor material is melted and

grown into a single crystal, and a mechanical unit 44,

positioned below furnace tank 42b, that contains

mechanical and electrical components that operate crystal

puller machine 28.

Puller cell 18 also includes a discontinuous multi¬

level floor 46 comprising an operator floor 48 positioned

at an operator level convenient to pull chamber 42a and a

maintenance floor 52 at a maintenance level convenient to

furnace tank 42b. Operator floor 48 is typically level

with the bottom of pull chamber 42a so that an operator

or a material handling device (not shown) can easily

access the pull chamber 42a. Such an operator floor 48

is particularly useful with crystal pullers 28 that use a

door-type pull chamber 42a. It will be understood that

ingots grown in crystal puller 28 can be quite large,

some being longer than two meters and weighing over one

hundred kilograms. Access to crystal puller 28 is

provided through two doors in puller cell 18, a

maintenance door 62 that opens onto the maintenance level

and an operator door 64 that opens onto the operator

level .

Fig. 3 shows that prefabricated ceiling unit 26

includes a filter housing 78 for housing high-efficiency

particulate air ("HEPA") filters 80, lighting fixtures

82, and fire-prevention sprinklers 84 all mounted on a

common ceiling grid 86. Grid 86 is mounted at the top of

walls 24, above crystal pulling machine 28 and below a

hall ceiling 16, and attached to a plenum 88 to provide

air flow. Electric power is supplied to grid 86 to

provide lighting within puller cell 18, and grid 86 is

attached to a water supply for use by fire-prevention

sprinklers 8 . Because ceiling units 26 are

prefabricated, the amount of heavy construction required

in growing hall 10 is reduced. By mounting ceiling units

26 on walls 24 of an appropriate height, puller cell 18

can accommodate crystal puller machines 28 of various

heights. Free ceiling 16 permits the use of walls 24 of

various heights, thereby providing flexibility in

constructing within a single growing hall 10 puller cells

18 that accommodate machines of different sizes,

including larger machines that may be required in the

future.

A bridge crane 94 is typically installed below

building trusses 92 and is used in the installation and

repair of crystal pullers 28 and in the construction of

puller cells 18. Bridge cranes are inevitably a source

of particulate contamination but, because bridge crane 94

is above ceiling unit 26, particles shed by bridge crane

94 do not contaminate the environment within crystal

puller cell 18.

Filtered air is supplied to HEPA filters 80 in

ceiling unit 26 by plenum 88. The airflow within each

puller cell 18 is individually controlled by airflow

control circuitry 98 within a control unit 100 (Fig. 4) ,

airflow control circuitry 98 controlling an air supply

system comprising variable airflow dampers 104 and a

variable frequency motor 106 in an air handler unit 102

to produce an appropriate airflow at any particular time.

Airflow control circuitry 98, an air handler unit ("AHU")

102, variable air flow dampers 104, an airborne particle

counter 131, and HEPA filters 80 together comprise an

adjustable air contamination control system 108.

In one embodiment, a low airflow mode is used when

puller cell 18 is idle or is being cleaned. A moderate

airflow rate is used when crystal puller 28 is operating

with the pull chamber 42a and furnace tank 42b closed. A

high airflow rate is used when maximum cleanliness is

required, i.e., when raw material is being loaded or a

single crystal is being unloaded. A purge airflow rate,

which is greater than the high airflow rate, is used to

purge the interior of puller cell 18 after it has been

contaminated. Each airflow operation made at each puller

cell 18 is controlled by a status signal from a

controller 29 of crystal puller 28 and by an airborne

particle counter 131 or other device which indicates the particle level in the puller cell.

Referring to Fig. 1, the area within growing hall 10

is divided into puller cells 18, clean aisles 114, and

maintenance aisles 116. Puller cell maintenance door 62

opens onto maintenance aisle 116, and operator door 64

opens onto clean aisle 114. A relatively high degree of

cleanliness is maintained in clean aisles 114, which are

used to bring raw materials and finished products to and

from puller cell 18. The raw material typically

comprises ultrapure polycrystalline silicon, precharged

in a quartz crucible; the finished product is typically a

single crystal silicon ingot. A lower degree of

cleanliness is maintained in maintenance aisle 116, which

is used by maintenance workers to maintain crystal puller

28. To maintain their cleanliness, clean aisles 114 have

clean aisle ceilings 118 (Fig. 2) at a suitable height

120 with particulate filters 121 that continuously

provide filtered air to clean aisles 114. Ceiling height

120 is typically much lower than that of a prior art

growing hall, so less airflow is needed to maintain the

required level of cleanliness .

Control unit 100 includes logic circuitry 122 that

controls locks 124 and 126 on respective doors 62 and 64

to eliminate cross contamination among puller cell 18,

clean aisle 114, and maintenance aisle 116. Logic

circuitry 122 prohibits opening either door when

conditions within puller cell 18 do not match those

outside that door. For example, if puller cell 18 is

operating in a high flow mode to obtain a high degree of

cleanliness or if puller cell 18 is operating in a

moderate airflow mode to obtain a medium degree of

cleanliness, logic circuitry 122 will prevent first door 62 from opening onto maintenance aisle 116. Similarly,

if puller cell 18 is operating in a low flow mode and

first door 62 is opened, logic circuitry 122 will prevent

second door 64 from opening onto clean aisle 114. Logic

circuitry 122 also prevents first and second doors 62 and

64 from being opened simultaneously. Furthermore,

airborne particle counters 131 monitor particle level in

puller cells 18. Logic circuitry 122 has a data

interface with airborne particle counter 131 and prevents

second door 64 from opening onto clean aisle 114 unless

the particle level in puller cell 18 is compatible with

that of clean aisle 114. Also, logic circuitry 122

prohibits opening either door while a magnet device in

the puller cell is energized. This feature can keep

operators away from areas in which a strong magnetic field is present.

Fig. 5 shows schematically a remote control console

136 for controlling several puller cells 18 within a bay

138. Remote control 136 is located in clean aisle 114

and electrically connected by conductors 139 to

individual puller controller 29 through control consoles

140 positioned adjacent to each crystal puller cell 18.

A television camera 142 located within each puller cell

18 transmits images of the conditions within each crystal

puller 28 to an operator, who can observe the images on a

video display by remote console control 136.

By allowing the operator to monitor multiple crystal

pullers 28 from one location, the operator's productivity

is greatly increased. By eliminating the requirement for

operators to move among the crystal pullers 28 to monitor

the pulling operations, much of the particulate

contamination caused by the operators is eliminated. The

operator is also removed from magnetic fields used in any

of the puller cells 18. Because remote control console

136 is located in clean aisle 114, the operator can

quickly proceed to an individual puller cell 18, when

remote control console 136 indicates such a visit is

required.

By individually controlling the environment

surrounding each crystal puller machine 28, the present

invention allows the areas in growing hall 10 to maintain

appropriate cleanliness while reducing the overall

facilities cost. Because smaller areas are kept clean,

the cost of providing the necessary flow of filtered and

conditioned air to maintain the cleanliness level is much

less expensive, and those smaller areas can be maintained

in a cleaner condition. Adjusting the airflow to a level

appropriate to the activity with puller cell 18 further

reduces costs.

Additional crystal puller machines 28 can be added

while previously installed machines continue operating

without contamination, thereby allowing flexibility in

scheduling installation of new machines. It is more

effective to invest capital in crystal pullers and puller

cells only as required to meet production schedules.

It will be obvious that many changes may be made to

the above-described details of the invention without

departing from the underlying principles thereof. For

example, a puller cell 18 could also accommodate two or

more crystal pullers 28. Such an arrangement would

increase operating costs, but reduce construction costs.

Puller cell 18 could be divided by an internal wall into

a maintenance area, defined by maintenance floor 52, and

an operator area, defined by operator floor 48, with the

environments of the maintenance and operator areas being

separately controllable. The scope of the present

invention should, therefore, be determined only by the

following claims.