Method and system to monitor and control electro-static...

Details Method and system to monitor and control electro-static... Method and system to monitor and control electro-static...

2002-01-16

2004-05-25

Toatley, Jr., Gregory J (Department: 2836)

Electricity: electrical systems and devices

Electric charge generating or conducting means

C361S230000

Reexamination Certificate

active

06741445

ABSTRACT:

FIELD OF INVENTION
The present invention relates generally to development processes for semiconductor fabrication and in particular to systems and methods to monitor and control electro-static discharge during semiconductor fabrication.
BACKGROUND OF THE INVENTION
Semiconductor devices are becoming smaller and faster. This decrease in size requires that the dimensions and critical dimensions (CD) of semiconductor devices must also decrease. However, as the dimensions and size semiconductor devices become smaller, static charges become more of a problem.
Static charge is generally produced when two materials initially in contact are separated. One surface becomes positively charged as it loses electrons. The other becomes negatively charged as it gains electrons. Static charges can build up on wafers, storage boxes, work surfaces, fabrication equipment and the like. Static charges can reach voltages as high as 50,000 V.
Static charge can create device operational problems. It is possible that an electrostatic discharge (ESD) of up to 10 A can occur on a semiconductor device. Such levels of ESD can physically destroy or damage semiconductor devices and integrated circuits. Additionally, photomasks and reticles are sensitive to ESD. A single ESD can vaporize and destroy portions of a photomask or mask.
Static charge also creates a problem for measuring and inspecting semiconductor devices. As these CDs get closer to the resolution limits of optical lithography and microscopy measurement techniques, great care must be taken to eliminate all possible sources of measurement error in order to obtain accurate and reproducible CDs. A common measurement technique is scanning electron microscopy (SEM), which utilized highly focused energetic beams of electrons impinging on the sample and measures the yield of secondary emitted electrons. SEM is widely used for measurement due to its high resolution, about 10-30 Angstroms, and relative ease of use.
FIG. 1
illustrates a SEM system, showing the electron source and the acceleration, focusing, and detection electronics. The system includes an incident electron beam
2
impinging on a sample
4
. Secondary electrons are collected and detected at detector
6
. The system also includes an electron beam source
8
, accelerating and focusing electrodes
10
, lens apparatus
12
, scan control
14
and monitor
16
.
FIG. 2
shows a typical electron energy spectrum resulting from the incident electron beam of an SEM. A backscattering peak
18
is at near the incident beam energy. Auger peaks
20
and secondary electron emission peak
22
are at lower energies. Generally, secondary electrons
22
are detected by the SEM and employed for analysis. The highest energy peak results from the backscattered electrons, which have energies close to that of the incident beam, and which have undergone only elastic collisions with the target atoms. Peaks
20
seen at intermediate energies are the Auger electrons emitted due to relaxation of electrons between atomic energy levels. The lowest energy emitted electrons
22
, produced by inelastic collisions between the primary beam and the inner shell electrons of the sample are known as the secondary electrons and are generally useful for morphology studies. This is partially due to the short escape depth of secondary electrons, which yields high surface sensitivity. Additionally, since the incident electron beam undergoes beam broadening due to multiple collisions as it penetrates into the sample, the backscattered electrons originating from deeper into the sample reflect this broadening with degraded point-to-point resolution. The lower energy secondary electrons that escape the sample originate from the surface region above the penetration depth where beam broadening becomes influential, and therefore yield higher point-to-point resolution. than evidenced by backscattered electrons.
The detected electron current, typically chosen to be the secondary electron current as described above, is used to intensity modulate the z-axis of a CRT. An image of the sample surface is produced by synchronously raster scanning the CRT screen and the electron beam of the SEM.
The contrast of the image depends on variations in the electron flux arriving at the detector, and is related to the yield of emitted electrons per incident electron. The yield is dependent on both the work function of the material and the surface curvature. These factors allow the SEM to distinguish between materials such as metal, oxide, and silicon, and also to distinguish surfaces that differ in slop. Thus, CDs of patterned and/or etched lines and gaps can be measured.
Two factors affecting the accuracy of SEM measurements are resolution and charging effects. The resolution of the SEM depends on the type of sample under inspection and on the incident beam diameter or “spot size”. The high voltages of the electron beam required to produce small scanning spot sizes were historically on of the sources for charging of the surface when examining insulating surfaces. When incident beam energies exceeded the secondary electron crossover point, i.e., when the incident beam penetration depth was high enough that the number of emitted secondary electrons was less than the number of incident electrons, the surface in the region of the scanning beam would acquire excess negative charge. This would cause the incident beam trajectory to be disturbed and would therefore degrade the image. Grounding schemes such as coating the surface with gold and attaching a ground wire to the coating were used to attempt to reduce charging effects. However, these methods prohibited further processing following inspection of the wafers. More recent SEM machines have eliminated high energy accelerating voltages, thus eliminating that source of charging. High voltage SEMs with gold coated samples are still used to verify CDs as measured in low-voltage SEMs.
Semiconductor devices are often measured after or during photomasking processes. These photomasking processes typically include a resist or developer drying process wherein a wafer is spun at a high speed. The rapid acceleration and high speed of the drying step commonly causes high levels of static charge to build up. Aside from causing damage to the semiconductor devices on the wafer, the static charge can also prevent measurement of CDs. For example, the image focus or an SEM can be degraded due to deflection of the incident e-beam by static charge.
There exist a number of conventional ways to control and prevent static charge buildup. Fabrication stations may discharge static from equipment and operators by utilizing grounding wrist straps, antistatic garments, antistatic processing equipment and grounding work surfaces. Additionally, ionizers may be placed by filters to attempt to neutralize any static charge buildup on filtered air. Another way of reducing static charge is by manually rinsing a wafer or manually applying a solution to a wafer. A solution, such as a water, may be sprayed at a high pressure to remove statically attached particles. However, these conventional ways to reduce static charge may not work and do not detect high levels of static charge and prevent higher levels of static charge from developing. Furthermore, these conventional ways require manual intervention.
Therefore, there is an unmet need in the art for new and improved system and method for monitoring and controlling static charges in semiconductor and mask fabrication.
SUMMARY OF THE INVENTION
A system and methodology is provided for monitoring and controlling static charge during processing of semiconductor devices and masks.
An electrostatic discharge monitor and control system according to one aspect of the invention is disclosed. The system includes a cup holder, an antistatic solution dispenser, a sensor arm, a sensor and a controller. The cup holder holds a target device. The antistatic solution dispenser dispenses an antistatic solution on the target device. A sensor arm is attached to the cup holder. The sensor is attached to the sensor

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