Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-03-31
2003-11-25
Nguyen, Vinh P. (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S761010
Reexamination Certificate
active
06653852
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to plasma diagnostic apparatuses for semiconductor wafer processing systems. More particularly, the present invention relates to a wafer integrated plasma probe assembly array.
In the semiconductor industry, plasma, generally comprising of partially ionized gas is employed in etching and deposition processes whereby films are etched from or deposited onto wafer surfaces. In these processes, plasma can be characterized in terms of characteristics of the interaction between the surface to be processed and the plasma which is important in order to control the etch or deposition rate and consequently, the desired dimension of the etch depth or deposited film. These characteristics include the rate of flow of charged particles impinging upon the surface to be processed, the potential distribution of the plasma, the ion current flux, the electron temperature and density, and the ion energy.
In plasma etching systems, knowledge of the potential distribution of the plasma is useful because the energy with which particles impinge upon the surface to be processed depends upon the potential distribution. In addition, the plasma potential determines the energy with which ions strike other surfaces in the chamber. High-energy bombardment of these surfaces can cause sputtering and consequent redeposition of the sputtered material upon the surface to be processed. In addition, process uniformity is related to the uniformity of the plasma.
Similarly, the ion current flux is an important characteristic of the plasma generated within a reaction chamber of a semiconductor wafer processing system. This characteristic generally defines the effectiveness of the semiconductor wafer processing system. Specifically, the ion current flux affects the uniformity of the etch process and indicates potential damage to a wafer. The measurement of ion current at various locations within the chamber is therefore important to characterize the effectiveness of the plasma in processing a wafer.
It is thus desirable to diagnose instantaneously from outside the processing chamber the various characteristics of the interaction between the surface to be processed and the plasma. Prior art
FIG. 1A
is an illustration showing a conventional wafer
1
having probe structures
2
formed thereon. The conventional wafer
1
consists of a Si wafer with the probe structures
2
fabricated using three levels of masks such as substrate contact, metal pad and oxide layer. The wafer
1
is processed using conventional wafer manufacturing techniques.
Prior art
FIG. 1B
shows the probe structures
2
under greater detail. The probe structure
2
includes a semiconductor substrate layer
4
, which is on the semiconductor wafer
1
on which are comb-like structures
6
made from metals such as Cu or aluminum and with or without layers of insulators such as oxide layers
8
. In the fabrication of semiconductor IC's where advanced MOS devices require multiple levels of metal interconnections, the size of the comb-like structures is such that the height of the structure could be less than 0.5 micron and the space between the structures could be less than 0.4 micron wide such that the aspect ratio could be greater than two. The aspect ratio is defined as the height of the comb-like structure divided by the width of the space between the comb-like structures of prior art FIG.
1
B. The presence of tall structures on the substrate of a semiconductor wafer sometimes causes a differential charging of the surface due to the difference in electron and ion currents crossing the plasma sheath to the closely spaced structures.
The differential charging of the surface (prior art
FIG. 1C
) is mostly indicative of a non-uniform plasma which includes fluctuations in the electron and ion densities, and also indicates differences in surface potentials and charge flux densities. If plasma is non-uniform it is anticipated that the depth of etching or the depth of deposition would be variant across the surface of the wafer. Differential charging also could cause oxide damage in semiconductor devices due to differences in charge flux densities. This is very important as plasma is in contact with smooth and not so smooth surfaces on the wafer.
It is a purpose of the plasma diagnostics to ensure that the plasma is uniform across the wafer surface so that the different processes taking place in the plasma chamber would result in high yield for the device output.
Prior art
FIG. 1C
is an illustration showing a probe structure
2
on a conventional wafer
1
. As shown in prior art
FIG. 1C
, the presence of probe structures causes shaded regions
10
where there is charge accumulation and unshaded regions
12
where there is no charge accumulation. The sign of the charge depends on the surface potential of the structure. Local inequality of positive and negative charge fluxes reaching the wafer surface results in a net charge. Local charge-flux imbalances result in circulating currents through the wafer that generate charging damage in gate oxides as in IC process equipment.
This calls for application of sufficient RF power for better gap fill capability. If the plasma is not uniform across the substrate, then the resulting current imbalance causes a voltage to build up in the substrate. This voltage allows the current from the plasma to flow in the substrate to the gate oxides of underlying MOS transistors. However, application of sufficient RF power could cause damage to the gate oxides leading to gate leakage or oxide breakdown when the amount of current exceeds the capacity of the gate oxide.
In general, there are two conventional methods of diagnosing the characteristics of interest, a probing method, and an electromagnetic wave method. In the probing method, the electrodes
204
usually made from metal (Prior art
FIG. 2A
) are on a support
206
are directly introduced into the plasma
202
to detect the electric current in the plasma which is then analyzed to determine the characteristics of the plasma. The probe is also called the Langmuir probe
200
. The characteristic curve
250
(Prior art
FIG. 2B
) is obtained by varying the voltage on the electrode and measuring the current when the probe or the electrode is placed in the plasma. The I-V curve
252
indicates that for a large negative value of the probe potential, all electrons are essentially repelled and only ions contribute to the current leading to an ion saturation current (Isat). This ion saturation current or Isat simply determines the electron density provided electron temperature can be determined. Conversely, Isat is also a product of electron charge, disk surface area and ion flow.
In the electromagnetic wave method, electromagnetic waves including microwaves and lasers interact with the plasma and the results of the interaction are detected. By way of example, a beam reflected from the plasma is detected by spectroscopy and analyzed.
The probing method is limited to probing plasma of relatively low temperature and density. For plasmas of electron density Ne on the order of 10{circumflex over ( )}14 cm-3 and above and electron temperature of a few tens of electron volts and above, the probing method is of limited use. The electromagnetic method suffers from being complex and expensive to manufacture.
In view of the prior art that has been done on Langmuir probes and probe structures that are also charge monitors, what is needed is a diagnostic tool capable of taking simultaneous measurements of plasma characteristics like uniformity, electron or ion flux densities, potentials and ion energy in real time across a wide area of the wafer surface while the wafer is inside of the plasma chamber.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention includes an array of electrical probes formed upon an upper surface of a semiconductor wafer. The array of electrical probes provides simultaneous measurement of plasma characteristics in real time across a wide area of the wafer surface. The plasma
Lam Research Corporation
Martine & Penilla LLP
Nguyen Vinh P.
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