Facilities Technologies

Investigating static-charge issues in photolithography areas

Lawrence B. Levit and Arnold Steinman, Ion Systems

The institution of an effective static control program in photolithography areas can greatly reduce static-charge events, leading to a reduction in wafer scrap by eliminating repeating defects.

The semiconductor industry is seeking to increase productivity on two fronts: by shifting from 200-mm to 300-mm wafers and by shrinking feature sizes. However, because of overcapacity in 200-mm wafer fabs, the move toward 300-mm wafers has slowed considerably. Consequently, it has become clear that increased manufacturing efficiency in the short term will result primarily from the die shrinks that result from smaller feature sizes. It is widely believed that 0.1-µm features on 200-mm wafers will become a reality before 300-mm production begins in earnest. Chips with 0.25-µm feature sizes are already in mass production, and pilot lines for 0.18-µm devices are already in operation. Meanwhile, the IC industry has already demonstrated the feasibility of producing 0.13-µm devices.

The rapid decrease in feature sizes to below 0.33 µm has brought lithography to the forefront of technical innovation. However, photolithography is highly sensitive to static charge. The effects of static charge include increased levels of particulate contamination from the electrostatic attraction of airborne particles, damage to product and reticles from electrostatic discharge (ESD), and robotic lockup caused by the electromagnetic interference (EMI) generated by ESD. Discharge is driven by electric fields from charges on the reticle substrate and nearby charged objects. Reticle damage caused by electrostatically attracted contamination or ESD can cause repeating defects, leading to sudden and dramatic yield losses. Chrome lines on reticles are particularly susceptible to ESD damage.

In the lithography field, contamination and ESD problems become more acute as the dimensions on the wafer and photomasks become smaller. As dimensions shrink, smaller and smaller particles increasingly tend to be attracted to charged surfaces because of their size, causing defects. ESD damage to photomasks is the most serious charge-related problem facing the lithography field, both in mask houses and photolithography areas. As feature sizes on photomasks decrease, these masks become increasingly sensitive and susceptible to ESD damage.

At the current technology node, the cost of a single mask already exceeds $20,000; at the 0.1-µm technology node, its cost is expected to jump to $100,000. A single ESD event during manufacturing or handling in the photolithography area can destroy portions of the mask. At best, the damage is repairable once it has been identified. More importantly, step-and-repeat processing in photo areas means that many defective devices will result from ESD-damaged photomasks before the damage is detected. Improved inspection equipment can locate ESD-damaged masks, but their use requires removing the masks from the stepper and transporting them to an inspection tool. High-speed production operations discourage this type of interruption, making the impact of a repeating defect greater. Moreover, transporting masks from the stepper to the inspection tool increases the risk of ESD damage.

Extending the limits of photolithography production technology will increase the importance of controlling static-related problems. In fact, static charge may block the expansion of lithography operations in the next few years. Similar issues have arisen in disk-drive assembly operations using magnetoresistive heads. Because these small-geometry devices (~0.05-µm thick) are highly sensitive to ESD, it is no longer possible to assemble disk drives without a static control program in place. While many major semiconductor and photomask manufacturers have already implemented such programs, many more believe that their operations can escape the basic laws of physics. As geometries shrink, competitiveness will depend on maintaining static control. This article investigates the effects of static charge in the semiconductor environment, concentrating on studies in the field of photolithography to control it.

Contamination, Process Interruption, and ESD Issues

Cleanrooms are remarkably capable of generating and holding static electricity. The levels of static charge typically found in the cleanroom are easily an order of magnitude higher than what would be found in a conventional manufacturing area. Several factors account for this. Most cleanrooms must be maintained at low humidity, typically 40­45%. This limits the conductivity of the air and minimizes its ability to dissipate static charge. In this environment, materials with very different electronic affinities are quite often brought into contact with one another. For example, in semiconductor manufacturing, Teflon, which is highly electronegative, is frequently brought into contact with p-type silicon wafers. This produces large charge transfers from the one material to the other.

Damage Caused by Electrostatically Attracted Contamination and ESD. Charged surfaces attract and bond with airborne particles, thus reducing the effectiveness of the clean environment. Electric fields extending from charged surfaces attract randomly charged airborne particles. In addition, the polarization of particles close to a charged surface makes the surface attractive to even neutral particles. Increased levels of particle deposition on surfaces obviously increase the number of particles that land on uncharged surfaces. Electrostatic attraction and bonding action are greater for smaller particles. In the submicron range, it is often impossible to remove a contaminating particle once it has attached itself to a surface. As wafer and reticle geometries shrink, smaller particles become killer particles. In lithography areas, a charge on wafers, reticles, and stepper optics can create contamination issues.^1­3

A discharge to or across the surface of a product or reticle deposits only a tiny amount of energy (10^­9 to 10^­6 J). Since an ESD event deposits its energy into a tiny volume of space (~10^­1 µm³), it can melt or vaporize the silicon or chrome. While this volume was not large enough to cause difficulties when feature sizes were 5­10 µm in size, in the era of submicron feature sizes the damage caused by ESD energy can easily destroy product or reticles. With the advent of 0.18-µm geometries on the wafer, mask dimensions will also shrink to the submicron level. Thus, the level of static charge required to deposit enough energy to cause serious damage will continue to decrease over time.

Reticles are particularly susceptible to ESD damage. The conductive chrome structures on reticles have sharp edges that concentrate electric fields, encouraging the breakdown of the air gap. The lines are created on a quartz substrate, an exceptionally good insulator that is easily charged. The areas between chrome features on reticles can be damaged by ESD as a result of electric fields from charges on the insulating quartz substrate or from other nearby charged objects. In photolithography areas, ESD damage occurs when reticles are transported from reticle stockers to steppers and in inspection and cleaning areas. Insulated or isolated reticle carriers and other charged objects in the photolithography area present an ESD hazard to reticles.

Figure 1 is an image of a damaged chrome structure on a reticle. Local heating has damaged the chrome, causing it to lose its adhesion to the quartz surface. This creates two problems: first, the missing chrome can result in an incorrect geometry on the wafer surface; and second, the dislodged chrome serves as a contaminating particle that will produce a problem whenever it lands on the quartz. It has been documented that small chrome fragments can damage pellicles.

Figure 1: Image of a chrome structure on a reticle damaged by ESD.

ESD events on reticles occur when the voltage of one chrome feature differs significantly from that of another. Such events most often occur when a reticle is placed near a charged object such as an insulating minienvironment wall or a reticle pod with a highly charged handle. An ESD event on a reticle causes an electrical breakdown of the air gap between two structures, local heating, and metal transfer across the gap. Of particular importance is metal transfer, which can easily result in a printable defect in the form of a short circuit. An example of a printable defect is shown in the Nomarski microscope image in Figure 2. A typical ESD signature, this image clearly depicts damage to both the chrome and material in the gap between the chrome structures.

Figure 2: Nomarski microscope image of a printable defect caused by ESD depicts damage to both the chrome and material in the gap between the chrome structures.

The same region of the reticle is shown in the transmission microscope image in Figure 3, which demonstrates that the damage to the chrome is quite visible while the material in the gap is not. In fact, the image clearly shows that chrome is missing. This type of damage would certainly have an adverse effect on yield. Although the material in the gap is not visible, the phase distortion caused by the material could well be an additional source of yield loss. The structure in the damaged chrome area is evidence that this was not a single ESD event, but rather the result of many low-level events that occurred over the life of the reticle. This is important: ESD events occur many times as reticles are used, and their negative impact increases over time, resulting in escalating levels of damage and yield loss.

Figure 3: A transmission microscope image of an ESD-damaged reticle demonstrating that the damage to the chrome is quite visible while the material in the gap is not.

EMI-Induced Equipment Lockup. Another consequence of static charge in the cleanroom is equipment lockup. Although the amount of static charge is typically only ~10^­9 C, it discharges rapidly. Figure 4 shows an oscilloscope trace of a voltage pulse induced by static charge on a cleanroom antenna. The bandwidth of the signal is ~108 Hz. The initiating discharge is so fast that it radiates EMI very efficiently. Transient EMI resulting from the discharge of an object, perhaps within process equipment, can be broadcast throughout a facility as a radio wave and is referred to as radiated emission. ESD-caused EMI, called conducted emission, can also be picked up and transmitted from one piece of equipment to another through electrical conductors, often through the power lines. If the EMI pulse is induced on a critical control or data pulse within an equipment microprocessor or a metrology instrument at just the right time, the device either exhibits unpredictable behavior (such as lockup or movement in the wrong direction) or produces erroneous measurements.

Standards and Techniques for Controlling Static Charge

SEMI E78-0998. Anticipating the need to control static charge throughout the semiconductor manufacturing environment, SEMI released Document E78-0998, Electrostatic Compatibility: Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment.⁴ SEMI developed this standard to help equipment makers partner with chip manufacturers to achieve higher productivity through increasing tool uptime and reducing the damage to wafers and reticles caused by static charge.

Figure 4: Oscilloscope trace of a voltage pulse induced by static charge on a cleanroom antenna.

The document provides a matrix of maximum recommended levels of static charge on products, reticles, carriers, and the input and exit ports of production equipment or minienvironments. While theoretically investigating electrostatic particle attraction, it defines the types of ESD damage experienced by semiconductor devices and the methods for testing them. In addition, SEMI E78-0998 contains case histories detailing ESD-related equipment malfunctions. One section describes grounding techniques, static-dissipative materials, and air ionization—the static control methods commonly used in semiconductor manufacturing. With documented tests demonstrating the effectiveness of equipment static control methods, the guide will be used primarily by equipment manufacturers when they initially design their products and by end-users to verify compliance with equipment specifications.

Grounding Conductors and Static-Dissipative Materials. The first step of an effective static control program involves grounding all isolated conductors to assure that they do not hold static charge. The static charge on insulators is unaffected by grounding. Wherever possible, modified insulators, called static-dissipative materials, should be used. These materials are created by lowering the electrical resistivity of the insulator using a variety of additives. Minienvironment walls and wafer cassettes using static-dissipative materials are commonly available. Both conductors and dissipative materials should be grounded, and all materials should be cleanroom and process compatible.

Because many of the insulating materials used in high-technology manufacturing—such as quartz reticles, oxide-coated wafers, and Teflon cassettes—cannot be obtained in static-dissipative versions or be eliminated altogether, a potential static-charge hazard remains in many applications. It is impossible to ground all conductors (for example, the chrome lines on a reticle). When the use of dissipative materials or grounding is either inappropriate or not cost-effective, ionization can be implemented.

Air Ionization. By installing ionizers in the processing environment (either in the room or the tool itself), a low density of air ions (charged gas molecules) are mixed into the ambient air. The purpose of ionization is to increase the electrical conductivity of the air so that the time to neutralize the charge on an object drops from days to seconds. An ionizer must produce equal numbers of positive and negative ions to ensure a net effect of discharging both positive and negative surface charges to zero. Ionization is the only technique that can remove surface charge from insulators. Various ionizers are available offering a range of performance levels in respect to balance accuracy, ion delivery, and cleanliness class. Some ionizers are available for use in Class 1 or better cleanrooms.

Photolithography Case Studies

Contamination Control. In order to measure the ability of a static control program to minimize static-induced contamination on reticles, an experiment was conducted in a rather large photolithographic bay that measured the effects of static control on the rate of contamination of pellicles. The program involved the installation of air ionizers around an E-beam maskmaker, a reticle inspection area, and reticle metrology tools. As recorded in Figure 5, the cost in terms of both parts and labor of replacing pellicles on reticles before, during, and after the implementation of the program was calculated over a three-month period. While the October data served as the baseline information on the monthly cost of replacing pellicles without a static control program in place, the November data represent pellicle contamination costs during the installation of the ionizers, and the December data represent the performance of the system after the ionizers had been installed and operating. The study indicates that pellicle replacement costs dropped $300,000 annually as the system was implemented—a savings of 58%.

Figure 5: Cost of replacing pellicles on reticles before, during, and after the implementation of a static-charge program.

ESD Damage. Induced static charge and subsequent ESD were suspected of causing printable defects on reticles. Previous studies had shown the link between an effective static control program and the elimination of reticle damage. A study was conducted to demonstrate the effectiveness of ionization in reducing damage of this type.

Figure 6 shows ESD damage data collected over a full year from an E-beam toolmaker before a static control program was in place. After the program, including air ionization, was established, the number of reticles that were damaged by ESD over the course of another year was recorded. The effects of the static-charge control program were dramatic. Figure 7 shows that the program all but eliminated ESD damage. While 35 reticles were destroyed in the manufacturing process before the static control program was instituted, only two were damaged the following year, after the program had been implemented. Since reticles cost more than $10,000 to replace and are becoming more expensive and more ESD-sensitive as the technology advances, the financial benefits of the program are significant.

Figure 6: Full year of ESD-damage information from a maskmaker before a static control program was established.

Figure 7: Data showing that ESD damage was virtually eliminated in the mask facility after the institution of a static control program.

Two similar tests were performed in other high-production areas. In one case, reticle life increased by as much as 100% and in the other, the number of reticles that were replaced per month decreased by a factor of 4.

EMI-Related Interference. In one facility, steppers were suffering lockup problems at the rate of four per day. An oscilloscope probe on the power bus of the robot controller revealed the presence of 20 V peak-to-peak of high-frequency noise (over 100 MHz). A study conducted to locate ungrounded conductors and to provide them with a path to earth ground found that the steppers had enclosure parts made of ungrounded static-dissipative material. The enclosures were discharging to ground, creating EMI that interfered with the operation of the steppers. Grounding all enclosure parts eliminated the EMI from these discharges, significantly reducing the incidence of lockups.

Since the photolithography area had already been equipped with ionizers and static-charge problems continued to occur, it was concluded that a complete static-elimination program was required in which ionization or earth grounding alone was inadequate. Several case studies demonstrated that the implementation of ionization reduces or eliminates tool lockup. In one study, a reticle inspection unit was locking up approximately five times per week. Theoretically, when highly charged reticles or reticle pods are placed in the tool, ESD events are inevitable. Assuming that the unexplained lockups were ESD-related, we installed an ionizing bar in the load/unload station of the tool. Because both the reticles and pods have excellent insulators (plastic and quartz), grounding them would not eliminate charge. Charge neutralization with ionizers was the only option.

Figure 8 shows the results of installing an ionization system. The red bars show several measurements of the rate of lockup without the use of ionization. The green bars show the rate of lockup after the installation of ionizers in the load station. The results indicate that there was a statistically significant improvement in the rate of lockups—a decrease of more than 50%—although they still persisted at a lower level.

Figure 8: Data illustrating the effect of ionization in a load station.

To investigate the origin of these residual lockups, ionizers were installed in the ceiling of the entire photolithography bay containing the inspection station. Figure 9 compares the effects of no ionization, ionization in the load station only, ionization in the whole room system, and ionization in both the station and the room. The lockup rate decreased significantly when ionization was installed in the tool, but it decreased even more dramatically when it was installed in the room as well. There were both radiated and conducted paths for the EMI to the microprocessor in the tool.

Figure 9: Data demonstrating the effects of implementing ionization in different parts of the lithography bay.

In another application, a stepper operating with 300-mm wafers was behaving erratically. Rather than operate smoothly when raising wafers from the stage, it performed jerky motions. Occasionally the tool lost registration and had to be rebooted. A digital storage oscilloscope was used to determine the cause of the malfunction, and a wideband loop antenna was inserted into the tool. The pulse width discrimination trigger feature of the oscilloscope was used to trigger ESD-like EMI bursts. Figure 10 shows a typical recorded event in which the size of the voltage swing (14.65 V) was substantially greater than what would be required to toggle a digital chip in a microprocessor. Further investigation showed that there were discharges between entering and exiting wafers and from wafers to the grounded structure of the tool.

Figure 10: EMI radiated from a stepper ESD event.

Several remedies were required to address the problem. First, room humidity was increased from 30 to 45%, thus decreasing the magnitude of the EMI and, consequently, the lockup rate. This measure, however, did not eliminate EMI events. Next, several air ionizers were installed in the tool. As a result, wafer movements became smooth and the lockups ceased. As expected, static charges on 300-mm wafers were significantly higher than those commonly measured on 200-mm wafers, because larger surfaces tend to accumulate more static charge.

Conclusion

As the semiconductor industry shifts to smaller geometries, smaller die sizes, and larger wafers, the negative effects of static charge are likely to increase. Novel equipment and device designs will not eliminate the problem of contamination, ESD damage, and equipment lockup. At the leading edge of new semiconductor technologies, the photolithography area is particularly sensitive to the effects of static charge. This article has demonstrated that static-charge events in the photolithography area can be greatly reduced by instituting an effective static control program. The most obvious advantage of such a program is that it lowers the number of reticles that must be repaired as a result of particle contamination or ESD damage. More important, however, is that such a program reduces wafer scrap by eliminating repeating defects.

A complete static control program includes the grounding of all conductors, the use of dissipative materials wherever possible, and the installation of air ionizers to diminish the level of static charge on insulators. SEMI E78-0998 provides guidelines for static control in equipment designs and performance verification. Implementing an effective static control program will lead to increased equipment uptime and higher device yields.

References

1. RP Donovan, Particle Control for Semiconductor Manufacturing (New York: Marcel Dekker, 1990), 316.
2. BYH Liu and KH Ahn, "Particle Deposition in Semiconductor Wafers," Journal of Aerosol Science and Technology 6 (1987): 215­224.
3. M Yost et al., "Electrostatic Attraction and Particle Control," Microcontamination 4, no. 6 (1986): 18­25, 70.
4. Electrostatic Compatibility: Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment, SEMI Document E78-0998 (Mountain View, CA: Semiconductor Equipment and Materials International, 1998).

Lawrence B. Levit, PhD, is the director of technology development for Ion Systems (Berkeley, CA). He is responsible for planning technology implementation and applications engineering for the company's products both for cleanrooms and for advanced semiconductor, disk-drive, and flat-panel display industries. Before joining Ion Systems, Levit held technical support, sales, and marketing positions at LeCroy and Jandel Scientific Software. At LeCroy, he contributed to instrumentation designs for six experiments that earned Nobel prizes in physics. Levit taught physics and conducted research in high-energy and cosmic-ray physics. A member of the ESD association and a senior member of the IEST, he participates in several standards committees and is the vice chairman of the RP 22 standards committee on ESD control. He received a BS in physics from the Case Institute of Technology in Cleveland and a PhD in experimental high-energy physics from Case Western Reserve University in Cleveland. (Levit can be reached at 510/548-3640 or llevit@ion.com.)

Arnold Steinman is chief technology officer at Ion Systems, where he is responsible for designing the company's static control products. He joined the company in 1981 after nine years as an electronics consultant specializing in digital, analog, and microcomputer design. He also served at the Lawrence Radiation Laboratories (Berkeley) in the biomedical electronics and heavy-ion linear accelerator groups. Steinman holds four patents in the field of air ionization technology and has actively contributed to formulating standards for that field. He has BS and MS degrees in electrical engineering from the Polytechnic Institute of Brooklyn in Brooklyn, NY. (Steinman can be reached at 510/548-3640 or asteinman@ion.com.)

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