There is a multi-billion dollar global market for semiconductor and other surface defect management which is growing both in absolute terms and as a percentage of capital equipment investment. In general, there are two factors that determine the economics of a semiconductor fabrication facility at a given utilization level, namely throughput and yield.
As complex new technologies such as 300 mm semiconductor wafers, copper interconnects, and reduced feature (circuit) sizes drive the margin of error in fabrication ever lower, new inspection technologies are critical to keep yields high and bottom-line economics attractive. Detection and elimination of chemical contamination and other types of defects is a constant concern for semiconductor manufacturers and equipment suppliers, and is an increasing concern as the nanometer scale of transistors and other devices continues to shrink.
A need exists to increase the overall accuracy, speed, and efficiency of current inspection systems at the nanoscale. Current systems do not meet the increasing demand from the industry to provide a method of testing a wider variety of products in a more efficient and faster manner.
Qcept Technologies, Inc. (Atlanta, GA) inventors M. Brandon Steele and Jeffrey Alan Hawthorne reveal an inspection system that can detect defects as small as one atomic layer to meet semiconductor metrology needs in U.S. Patent 7,634,365.
According to Steele and Hawthorne, the system is a fast, inexpensive, and effective means of detecting, locating, and classifying relatively small quantities of chemical content, and physical features on materials, such as semiconductor wafers, integrated circuit devices, liquid crystal display panels, or any material which may benefit from such inspections, while allowing for a minimization of the complexity of the sensor control mechanisms and an improvement in signal processing.
According to U.S. Patent 7,634,365, the Qcept system characterizes microscopic and macroscopic defects through imaging and visualization of the contact potential difference(CPD) topology on the wafer surface through the use of a non-vibrating contact potential difference sensor in combination with a vibrating contact potential difference sensor.
Contact potential difference (CPD) refers to the electrical contact between two different metals and the electrical field that develops as a result of the differences in their Fermi energies. When two metals are placed in contact, the Fermi energies of each will equilibrate by the flow of electrons from the metal with the lower Fermi energy to that of the higher.
Qcept’s system uses a combination of vibrating and non-vibrating CPD (vCPD & nvCPD) measurement modes which allows rapid imaging of whole-sample uniformity, and the ability to detect the absolute work function at one or more points. by detecting changes in work function across a surface via both vCPD and nvCPD sensors. It utilizes a non-vibrating contact potential difference (nvCPD) sensor for imaging work function variations over an entire sample.
The data is differential in that it represents changes in the work function (or geometry or surface voltage) across the surface of a sample. A vCPD probe is used to determine absolute CPD data for specific points on the surface of the sample. The combination of vibrating and non-vibrating CPD measurement modes allows the rapid imaging of whole-sample uniformity, and the ability to detect the absolute work function at one or more points.
FIG. 2 illustrates the Qtec concept of the contact potential difference methodology
Contamination of a whole wafer is a relatively common problem. One common defect of this type is where a chemical or elemental layer is uniformly deposited on the wafer, either where it was not intended to be or where an additional layer than was intended is applied. Conversely, wafers are produced which lack entire layers that were intended to be applied, such as where an oxide layer is not deposited as it should have been. Both of these, and particularly the last, provide distinct and difficult challenges for the inline inspection of wafers. Currently, offline, destructive techniques are needed to detect this category of defect.
For example, manufacturers apply very thin (on the order of 30 nanometers) organo-silane films to wafer surfaces to function as lubrication. The minute amount of material is undetectable by optical inspection tools. The industry must utilize difficult and time-consuming tests to specifically address this issue.
Thus, there exists a need to determine whether the wafer has a layer deposited thereon, and if so, to determine if the layer is of uniform thickness. Another application where current inspection devices fail involves the removal of a thin film from a wafer or other electronic device surface. For example, it is sometimes desirable to remove a photoresist layer from a wafer.
Current systems cannot efficiently determine whether the thin layer of photoresist has been removed. A need exists for a technique which can detect the presence or absence of a thin layer, while still providing information regarding surface defects.
Yet another industrial application which is in want of an improved inspection system involves the use of "witness wafers". Witness wafers are used to monitor the air quality in a semiconductor manufacturing clean room. The witness wafers are left exposed in the environment for a period of time, during which the volatile organic compounds present in the air are deposited on the wafer surface.
Analytical techniques used to evaluate the witness wafer are slow and only able to spot check the wafer. An inspection method which is able to defect check the entire surface as well as to check for deposited layers of volatile organics is needed. In the manufacturing situation, one common case of uniform contamination involves the use of a contaminated solution or deposition chamber.
This may occur, for example, if the wafer is placed in a contaminated solution (for example during cleaning or plating) or in a contaminated deposition chamber (such as a Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) system). In this case the contamination may be uniform across the entire wafer. Thus, processes which detect changes on the surface (e.g. nvCPD and optical) would not detect these defects. A critical need therefore exists for a fast, inexpensive, and effective means of detecting, locating, and classifying relatively small quantities of chemical content or features and physical features on samples (and any surface which dictates ultimate performance of an electronic or chemical device), including the presence or absence of completely or nearly complete missing or additional layers on a wafer or other material.
There is also a need for a system which minimizes cost and complexity of the sensor control mechanisms, such as height control. Furthermore, there is a need for methods and systems that have improved signal processing.
FIG. 1 illustrates one embodiment of the Qtec nvCPD scanning method and system
FIG. 9 illustrates a Qtec flow diagram for the image acquisition process of a radially scanned nvCPD imaging system
FIG. 10A illustrates an optical image of a 100 mm diameter silicon wafer after application of a vacuum pick-up device; FIG. 10B illustrates an nvCPD image of the wafer of FIG. 10A, which is an optical image that shows no defects, while the nvCPD system detects defects
The CPD images allow the visualization of chemical and geometrical defects missed by optical imaging and thereby enables classification of the type of defect present on the wafer surface. Some examples of these CPD images can be seen in an optical image (FIG 10A) compared with a CPD image (FIG. 10B) which are taken from the same 100 mm wafer. The Qtec inspection system is capable of generating image maps of one atomic layer thick patterns, for example it has detected sputtered gold defects on a wafer at densities less than a single complete atomic layer.
Nanolithography Equipment For It, Electronics And Photonics – A Technology, Industry And Global Market Analysis, a recent report from Innovative Research and Products (iRAP) Inc, estimates that the global market for equipment, including metrology equipment, used to fabricate semiconductor, photonic and other nanoscale devices is expected to grow at nearly 10% a year for the next five years and grow from an estimated $65.8 billion in 2009 to $105.6 billion in 2014.
Companies involved in nanofabrication materials, apparatus, metrology and testing for the IT and electronics, photonic and other industries had sales in excess of $80.013 billion in 2007 and more than $73.558 billion in 2008, reflecting the worldwide economic downturn.
Defect detection and characterization systems, such as in the semiconductor wafer industry, can usually be divided into in-line and off-line systems. "In-line" refers to inspection and measurement that takes place inside the clean room where wafers are processed. "Off-line" refers to analysis that takes place outside of the wafer processing clean room, often in a laboratory or separate clean room that is located some distance from the manufacturing area.
In addition, many of these analytical techniques are destructive, which requires either the sacrifice of a production wafer or the use of expensive "monitor" wafers for analysis. In-line inspection and measurement is crucial for rapidly identifying and correcting problems that may occur periodically in these types of manufacturing processes.
A typical semiconductor wafer can undergo over 500 individual process steps and require weeks to complete. Each semiconductor wafer can have a finished product value of up to $100,000. Because the number of steps and period of time involved in wafer fabrication are so large that substantial work in process can exist at any point in time. It is critical that process-related defects be found and corrected immediately before a large number (and dollar value) of wafers are affected. Such defects, regardless of the nature of the wafer, semiconductor, IC, or other device, are detrimental to performance and diminish productivity and profitability.
Many types of defects and contamination are not detectable using existing in-line tools, and these are typically detected and analyzed using expensive and time-consuming "off line" techniques such as: Total Reflectance X-ray Fluorescence (TXRF), Vapor Phase Decomposition Inductively Coupled Plasma-Mass Spectrometry (VPD ICP-MS) or Secondary Ion Mass Spectrometry (SIMS). Since these techniques are used off-line (outside of the clean room used to process wafers) and usually occur hours, or even days, after the process step that has caused the contamination, their value is significantly limited
