Improving spin-on-glass processes by using pump and filtration technology

Abhay Ramrao Deshmukh, National Semiconductor

The planarization of interlevel dielectric (ILD) is critical to the success of multilevel metallization processes. In multilevel metallization, patterned conductors and dielectric layers at one level can create severe topography problems for the layers at the next level. Topography problems can produce opens in the interconnects or voids in the dielectric layers because of step coverage issues associated with deposited dielectrics. Topography-related lithography issues can also cause shorts between adjacent lines. The purpose of planarizing multilevel metallization layers is to eliminate or smooth out the topographic undulations caused by conductors, dielectrics, contacts, and vias. Planarization ensures that metal, at any level, is deposited on a flat surface, preventing step coverage issues.

ILD layers are primarily planarized with spin-on glass (SOG) to avoid processing problems such as poor step coverage of deposited metal or void formation in deposited dielectric films. However, the use of SOG is not without drawbacks. Gel or SiO₂ particles can easily form in the SOG solution. Even in sealed bottles, SOG has a finite shelf life because of the danger of particle formation. Extended sit times, even at room temperatures, can lead to high particle levels in SOG materials. Although SOG is shipped from manufacturer sites with very low particle counts, shipping, material storage and handling, and manufacturing processes can affect the material. Particulation becomes even more pronounced after an SOG bottle is opened, since solvent evaporation enhances particle formation. A more troublesome source of particle generation is that excess SOG spun onto the walls of the spinner bowl dries quickly and forms particles.

Ever-shrinking device geometries mandate increasingly stringent defect-reduction goals to decrease the sizes and numbers of particles, pinholes, and microbubbles created during the manufacturing process. Consequently, improvements in the quality of semiconductor process materials, such as SOG, are necessary. This article discusses tests involving the use of a pump and point-of-use filtration system to filter out the large particles generated during SOG processing. Designed experiments were carried out to understand pump characteristics, process parameters, and process outputs. Data from those tests were compared with data collected from a pressurized-bottle system, the old method used for dispensing SOG.

SOG Pump and Microfiltration Technology

Tests to optimize SOG dispense used a Model 450 system from Integrated Designs (Carrollton, TX), which dispenses chemicals in very accurate and repeatable amounts using patented pressure-on-demand technology. The unit's all-Teflon flow path is compatible with a variety of chemicals used in the semiconductor industry. Comprehensive software drives the system's sophisticated electronics.

A user-friendly system, the pump provides the operator with complete control over the dispense function. It can be integrated into most commercially available process equipment. Additionally, it can dispense fluids at varying rates within a single dispense cycle. The unit operates with or without filtration in the chemical reservoir, depending on the chemicals used and process demands.

The Dispense Function. Under operating conditions, the pump maintains vacuum, atmosphere, and pressure conditions in a programmed idle state until the dispense unit receives a trigger signal from the process equipment requesting that the dispense function begin.

At that point, the unit activates a pulse-width-modulated (PWM) pressure-regulation mechanism, which applies pressure to the system reservoir. PWM pressure regulation consists of a pressure-application solenoid valve (N₂ on) and a pressure-release or exhaust solenoid valve (N₂ exh). When a dispense command is given and the reservoir is at atmospheric pressure or vacuum, the N₂ on valve opens to bring the reservoir to the programmed dispense pressure. An output pressure transducer monitors the reservoir, and as the reservoir approaches the programmed pressure, the N₂ on valve and the N₂ exh valves begin to open and close alternately to maintain a stable dispense pressure.

As the unit is being pressurized, an autovent function is enabled, allowing the unit to purge any air trapped in the filter and return it to the source. Changing the state of the autovent three-way valve and opening the source valve for a brief period causes the autovent to function. The pressure in the reservoir pushes air in the filter to the autovent valve through an internal vent tube. The autovent function terminates when either the vent time or the dispense delay time expires.

Once the unit has reached the programmed dispense pressure and the dispense delay timer has expired, the output valve opens and dispense occurs. By manipulating time and pressure, the operator can achieve accurate dispense volumes. The pump dispenses chemicals for the programmed dispense time unless the trigger signal is disengaged for any reason, causing the dispense function to terminate.

The pressure-on-demand pump uses helium pressure to push chemicals through the system and onto the wafer. The helium blanket is crucial in SOG coating processes to prevent crystallization.

During dispense, the unit's output pressure transducer monitors the programmed dispense pressure. Feedback from the transducer is used to adjust the pressure and regulate the chemical output. The difference between the applied pressure and the desired output pressure is the differential pressure. Differential pressure is monitored during each dispense cycle and is used by the unit to monitor chemical filter loading. A user-programmable differential limit function sends the user a warning when the filter must be replaced. Constant monitoring of the output pressure transducer during dispense allows the pump to compensate for filter loading, ensuring that dispense remains constant throughout the life of the filter.

End of Dispense and the Fill Cycle. At the end of the programmed dispense time, the unit terminates the dispense function by turning off the dispense solenoid, closing the output valve. If the unit detects a low liquid level during the dispense operation, it ignores the signal until dispense is complete.

At the end of the dispense cycle, the unit either returns to its programmed idle state or begins the fill cycle. The fill cycle is initiated when an ultrasonic liquid-level sensor registers that the unit's chemical level has dropped below a certain point. The unit then shifts to the programmed vacuum state and opens the source valve to allow the chemistry to flow into the reservoir. The fill cycle continues until the sensor detects that the unit is full (at which time the unit returns to the programmed idle state) or until the process equipment sends out another dispense command.

Pump Features. For the purposes of the tests described in this article, the pump was analyzed from a day-to-day production point of view. As with the implementation of any technology, a primary concern was the unit's production friendliness. From that vantage point, its operating ease as well as pump interlocks and alarms were evaluated.

Alarm or Warning Condition Chemical source empty Source cannot draw any more SOG High differential pressure Filter clogged Low dispense pressure Low helium pressure Overfill Sensor was disconnected—no signal

Table I: Dispense pump alarms and warnings.

First, the pump is easy to install and its recipes can be modified to accommodate the addition of a 1.5-second pump-delay feature to optimize dispense accuracy. Second, it is equipped with alarms and warning lights to indicate potential or actual problems. In contrast to the old pressurized-bottle system, which is only capable of sending alarm signals when the chemical source is empty, the pump offers a variety of alarms and warnings, as listed in Table I.

Figure 1: Schematic diagram of the dispense pump showing all elements of the system, from chemical input to output.

Another feature of the system is that it offers true point-of-use filtration, which is achieved by an in-line filter. The schematic diagram of the system presented in Figure 1 shows all the elements of the pump, from chemical input to output.

Table II demonstrates the superiority of the pump and filtration technology over the old pressurized-bottle technology. The most important features of the pump are its versatility and ability to control the dispense process through Chemnet software, which allows the user to customize a wide range of dispense functions.

Testing the Pump

SOG Material. SOG represents a family of materials that consist of a silicon-oxygen backbone to which different side groups are attached. Inorganic side groups are called silicates while organic ones are called siloxanes.

The experiments described here used Accuglass 211 spin-on glass manufactured by Honeywell Electronic Materials (Sunnyvale, CA). Part of the Accuglass T-11 series of SOG materials, Accuglass 211 polymer combines organic groups (11% CH₃ by weight) on an inorganic polymer backbone (Si-O-Si). It has better dielectric properties, better crack resistance, and lower shrinkage on cure than silicate-type SOGs. Used primarily for metal partial-etch-back applications, the material has an organic content of 10%, a shelf life of 8 months at 4°C, a dielectric constant of 3.8 at 1 MHz, and gap fill capability of >0.3 µm. Accuglass 211's high thermal stability makes it compatible with aluminum and tungsten plug processing. It also provides good adhesion for top and bottom dielectric layers.

Experimental Details. Designed experiments were carried out to understand the effects of microfiltration on particle defect levels. Pump parameters such as dispense pressure, time, and speed were optimized to match the output of the process of record. Particle counts, SOG thickness, and film uniformity were the primary output parameters.

Two coats of SOG materials were applied to wafers using an 817 track from Semix (Fremont, CA) and then baked at different temperatures for 30 seconds per temperature. A Surfscan 6200 0.30-µm wafer-inspection tool from KLA-Tencor (San Jose) was used to detect and identify defects. Film thickness was measured with a Prometrix analysis tool, also from KLA-Tencor, using 49 measurement points to determine across-the-wafer thickness variations. Wafers were cross-sectioned to compare the degree of planarization.

That procedure was repeated until 20 bottles of SOG material had been consumed. Particle variation results were compared on a wafer-to-wafer, tool-to-tool, and SOG batch-to-batch basis. Data were collected when the bottle was full, half-empty, and empty.

Test Results

To measure the effectiveness of the microfiltration technology, particle performance, the degree of planarization, and SOG thickness and uniformity were analyzed.

Figure 2: Data showing a 50% reduction in average particle counts and substantially improved wafer-to-wafer variation after the introduction of the pump and filtration system.

Particle Performance. The primary objective of the project discussed here was to improve particle performance, one of the principal difficulties in SOG coating technology. As shown in Figure 2, average particle counts decreased by 50% after the introduction of the pump and filtration system, and wafer-to-wafer variation was reduced substantially, indicating a stable and sustainable process. A similar reduction in particle-count levels was observed across different tools and SOG batches.

Pressurized-Bottle Technology Pump and Filter Technology • Uses pressurized helium bubbles to dispense SOG. • Has no in-line filtration. • Causes low SOG pressure problems because of pressure fluctuations. • Has high maintenance cost. • Uses helium pressure on demand to dispense SOG accurately. • Has 0.04-µm in-line filtration. • Offers differential pressure monitoringto signal need for filter replacement. • Has no moving mechanical parts andrequires low maintenance. • Has optical level sensors and liquid reservoir for better control. • SOG is kept under helium blanket, limiting the chances of crystallization.

Table II: Comparison between the pressurized-bottle system and the pump/filter technology.

Degree of Planarization. While considering the effects of microfiltration on SOG coating processes, the researchers evaluated its effects on the degree of planarization, since planarization is an important SOG process output characteristic. Although it was known that the pump and filter would not have any direct impact on the degree of planarization, that characteristic was investigated for the sake of experimental thoroughness.

Figure 3: Gap filling and global planarization. The left side of the chart shows metal lines with steep wiring spaces and small gaps that should be filled, while the right side shows metal lines with gradually sloping transitions.

The principal objective of the planarization process is to render the surface topography of the dielectric as smooth as possible. In other words, the surface should be free of small gaps, and transitions should be gradual and sloping rather than abrupt and vertical. Figure 3 explains the concepts of gap filling and global planarization. The left side of the figure shows metal lines with very steep wiring spaces and small gaps that should be filled, while the right side shows metal1, metal2, and metal3 lines with gradually sloping transitions. Proper planarization must accomplish both objectives or it must be supplemented with other methods. SOG generally produces short-range planarizations, which are often adequate for double-level metal devices.

Figure 4: Comparison between the gap fill and global planarization performance of (a) the old pressurized-bottle technology, and (b) the pump and filter system.

In addition to evaluating the pump and filter system's effects on the degree of planarization, the researchers also compared the gap fill and global planarization performance of the old pressurized-bottle technology with the new pump technology. The images in Figures 4a and 4b illustrate that there is no performance difference between the two technologies. The images are examples of gap filling in a double-level metallization structure, where Metal1 and Metal2 layers overlap a D2 base, D2 cap, and SOG pocket. That device region was selected for cross-sectioning because it is critical and because planarization problems, especially via poisoning from excess SOG, or metal bridging, are easy to identify on it.

Figure 5: Data showing that SOG thickness increased, and wafer-to-wafer variation decreased by 25% after the pump and filter system was installed.

SOG Thickness and Uniformity Results. The last output parameter to be investigated was as-deposited SOG thickness and uniformity, as measured by the Prometrix analysis tool. For the sake of experimental accuracy, data were collected from several tools and SOG batches over an extended period of time. As shown in Figure 5, a drastic improvement in thickness variation was observed after the pump was installed. Because the pump can dispense SOG materials consistently and accurately, wafer-to-wafer thickness variation was reduced by 25%. In addition, as-deposited SOG thickness nonuniformity was reduced by 40%. In further tests, similar results were obtained from wafers processed using recipes involving different SOG thicknesses.

Conclusion

The SOG coating process has high housekeeping needs because of its propensity to form particles. The process requires that fab personnel perform frequent chamber cleans, cup changes, exhaust-line cleans, and general maintenance on coating tools.

Using a pump and microfilter technology, the investigators studied the effects of true point-of-use filtration during SOG processing on highly critical output parameters such as particles, film thickness, and film-thickness uniformity. Process parameters such as dispense pressure, time, and speed were optimized to reduce defect levels. The beneficial pump and filter system reduced average particle defects by 50%, as-deposited SOG thickness nonuniformity by 40%, and wafer-to-wafer thickness variation by 25%.

The technology resulted in consistent, reliable process outputs for all measurable process output parameters. Point-of-use filtration improved tool cleanliness and reduced the frequency of chamber and exhaust cleans, increasing tool uptime.

Acknowledgments

The author wishes to thank Ronald LaPosa and Florizel Dharmaratnam of National Semiconductor (Arlington, TX) and Brian Kidd of Integrated Designs (Carrollton, TX) for their invaluable assistance with this project.

Abhay Ramrao Deshmukh is a process engineer at National Semiconductor in Arlington, TX, where he specializes in interconnect technology. He has worked at the company for three years in the areas of PVD systems and SOG planarization. He received an MS in materials sciences from the South Dakota School of Mines and Technology in Rapid City. (Deshmukh can be reached at 817/557-7602 or abhay.r.deshmukh@nsc.com.)