Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to decrease time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and, thus, higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
As semiconductor devices become smaller, it becomes more important to develop enhanced inspection and review tools and procedures to increase the resolution, speed, and throughput of wafer and photomask/reticle inspection processes. One inspection technology includes electron beam based inspection such as use of a scanning electron microscope (SEM). An SEM uses an electron source. Electron sources can be divided into two broad groups: thermionic sources and field emission sources. Thermionic sources are usually made of tungsten or lanthanum hexaboride (LaB6). In thermionic emission, electrons are boiled off the material surface when the electron thermal energy is high enough to overcome the surface potential barrier. Even though thermionic emitters are widely used, they typically require elevated temperatures (e.g., >1300 K) to operate, and may have several drawbacks such as inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness. The demand for more efficient electron sources has driven the research and development of Schottky emitters and cold electron sources such as electron field emitters.
In the Schottky emitters, thermionic emission is enhanced by effective potential barrier lowering due to the image charge effect under an applied external electric field. Schottky emitters are typically made of a tungsten wire having a tip coated with a layer of zirconium oxide (ZrOx), which exhibits a low work function (e.g., approximately 2.9 eV). Schottky emitters are currently used in some electron beam systems. Despite being quite successful, thermally-assisted Schottky emitters still need to be operated at high temperature (e.g., >1000 K) and high vacuum (e.g., approximately 10−9 mbar), and have wider than desirable electron emission energy spread due to the high operating temperature.
Cold electron sources, particularly electron field emitters, have been used in field emission displays, gas ionizers, x-ray sources, electron beam lithography, and electron microscopes, among other applications. Field emission takes place when the applied electric field is high enough to reduce the potential barrier on the tip-vacuum interface so that electrons can tunnel through this barrier at a temperature close to room temperature (e.g., quantum-mechanical tunneling). A typical field-emitter comprises a conical emitter tip with a circular gate aperture. A potential difference is established across the emitter cathode, the gate and the anode under an applied external field, resulting in high electric field at the surface of the tip. Electrons tunnel through the narrow surface barrier and travel towards an anode, which is biased at a higher potential than the gate. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitters.
Field emitters, because they can operate near room temperature, have lower energy spread than Schottky and thermionic emitters, and can have higher brightness and electron current than thermionic emitters. However, in practical use, the output current of a field emitter is less stable because contaminants can easily stick to the tip of the emitter and raise its work function, and hence lower the brightness and current. Periodic flashing (i.e., temporarily raising the tip temperature) is required to remove those contaminants. While the tip is being flashed, the instrument is not available for operation. Instruments in the semiconductor industry are required to operate continuously and stably without interruption, so Schottky emitters are usually used in preference to cold field emitters.
Previous field emitter arrays (FEAs) had multiple conically shaped electron emitters arranged in a two-dimensional periodic array. These field emitter arrays can be broadly categorized by the material used for fabrication into two broad categories: metallic field emitters and semiconductor field emitters.
Thermal field emitters (TFE) were previously used to generate electron beams. An individual electron source was used to form an array. Each electron source requires expensive XYZ stages. The cost of each individual electron source system was expensive and cost-prohibitive for a large array. In addition, the electron current density was low.
Photocathodes also have been used to generate electron beams. A single light beam incident on a photocathode system can generate a single electron beam with high brightness that is capable of delivering high electron current density. However, a problem with single electron beam systems is that even with high brightness systems, single electron beam systems still have relative low throughput for inspection. Low throughput is a drawback to electron beam inspection. With current available electron beam sources, thousands of beams would be required.
Splitting the single electron beam into numerous beams for a multi-beam SEM system required an array of aperture lenses and/or micro-lenses. The array of aperture lenses and/or micro-lenses are set in small, electrically-charged apertures that are substantially round in design to create lens fields. If the apertures are out-of-round, astigmatism is introduced in the lens fields, which results in a distorted image plane.
Therefore, what is needed is an improved system to generate electron beams.