The mag-lev TMP provides clean, reliable, cost effective vacuum for thin film deposition processes in PV manufacturing applications
Kate Wilson, Michael Boger, Edwards, Santa Clara, CA USA
The mag-lev TMP provides clean, reliable, cost effective vacuum for thin film deposition processes in PV manufacturing applications
Vacuum pumps are used extensively in PV fabrication to create the vacuum conditions required by many of the manufacturing processes, particularly those that deposit the thin films of various materials that make up most PV devices. It is worth noting that many of these processes are also used in semiconductor device manufacturing, which adopted mag-lev turbomolecular pumps (TMPs) more than a decade ago for the same reasons that are now driving a parallel transition in PV manufacturing.
Although many different PV technologies are in various stages of development, two types dominate current production: crystalline silicon and thin films. In crystalline silicon devices, the essential p-n junction that separates charge carriers generated by the absorption of solar radiation is a homojunction created by differentially doping adjacent regions within a bulk silicon substrate, typically a monocrystalline wafer or a multi- or poly-crystalline wafer or ribbon. In thin film devices, the junction is typically a heterojunction created between adjacent thin films of different semiconducting materials.
Crystalline silicon devices still account for the largest proportion of PV production; however, thin film production is growing, driven primarily by its potential for significantly lower production costs. While the ability to create high quality thin films is an obvious requirement for thin film devices, it is also essential for crystalline silicon devices where thin films are used for contacts and reflective/antireflective coatings.
A variety of thin film deposition processes are used in PV production. Included are an alphabet soup of vapor phase processes: physical vapor deposition (PVD), chemical vapor deposition (CVD), low pressure CVD (LPCVD), plasma enhanced CVD (PECVD), and various evaporation, sublimation, sputtering and epitaxial processes. All of these share a common need for high grade vacuum to ensure the quality of the deposited film. Because of this pervasive need for vacuum throughout the manufacturing process, optimal choices for vacuum generation can have significant impact on production cost, primarily through reductions in energy consumption and maintenance costs.
Several different technologies can be used to generate a vacuum in the process chamber, including mechanical, cryo, oil diffusion and TMPs.
Mechanical pumps. These pumps are not usually capable of attaining the required levels of pressure and cleanliness alone, but they are widely used as forepumps to create the sub-atmospheric pressure that is prerequisite for other high vacuum technologies. Optimization of mechanical pump size and configuration can have a significant impact on overall vacuum costs.
Cryo pumps. These pumps use low temperature to condense gases from the vacuum environment. They have relatively high maintenance costs because the collector must be regenerated periodically to remove collected material.
Oil diffusion pumps. These pumps use directed jets of hot oil vapor to propel gas molecules toward the pump outlet (Fig. 1). The oil vapor condenses when it contacts cooled pump surfaces and flows back to the oil reservoir where it is reheated and recycled through the vapor jets. Oil diffusion pumps have high energy costs for heating the oil and cooling the pump. The pump oil becomes contaminated over time and must be replaced periodically, entailing significant maintenance costs for equipment downtime and expensive specialized pump oil.
Figure 1. Cross-section of a typical oil diffusion pump. Inset: Rotors from a magnetically levitated turbo pump.
Turbo molecular pumps. These pumps are based on the principle that gas molecules can be moved in a specific direction by repeated collisions with a moving solid surface. In the case of a TMP, the collisions are enabled by rapidly rotating, carefully shaped blades arranged in a series of stages. Collisions with the rotating blades (rotor) propel the gas molecules between stages of increasing pressure through openings in the stationary stator until they eventually exit the pump (see inset). A TMP requires only a fraction of the power used by an oil diffusion pump to generate similar levels of vacuum. Changing from a diffusion pump to a TMP can result in a significant amount of power savings, on the scale of kilowatts per tool. The solar industry is increasingly moving to TMPs for their low operating cost, high performance, and reduced downtime.
As with any rotating system, TMPs require bearings to reduce friction. These can be either ball bearings or magnetic bearings. Traditionally, ball bearings have been considered the low-cost option. However, they have relatively high maintenance requirements: they wear and must be changed, typically every two to three years; they require lubricants, which can contaminate the vacuum and must be changed periodically. Additionally, ball bearings limit pumping capacity because rotational speed cannot exceed a certain maximum without the risk of overheating and they are subject to degradation in harsh or corrosive process environments.
Mag-lev TMPs use magnetic fields to suspend the rotor, eliminating all contact between it and the pump during operation. As a result, there is neither wear nor any requirement for lubricants, eliminating the risk of contamination, and significantly reducing both periodic maintenance requirements and the incidence of unplanned maintenance due to bearing failure.
The lack of contact between the bearings and the rotor also means that the pump can operate at higher speeds and temperatures. Higher speeds permit higher flow capacities and better pumping of light gases such as hydrogen. Higher temperatures can be used to reduce deposition of process materials and by-products on pump surfaces. In addition, mag-lev TMPs are better suited for corrosive processes because there is no bearing surface to be degraded and all exposed surfaces can be coated to resist attack.
Mag-lev TMPs pumps can be flexibly configured to optimize efficiency and minimize footprint. They can be mounted in any orientation and are available with integrated controllers. All TMPs require a controller to create the high frequency electrical signals needed to control rotational speeds of tens of thousands of revolutions per minute. Traditionally, the controller has been rack mounted and connected to the TMP with a cable. The length of this cable must be limited to ensure reliable transmission of high frequency signals in the electrically noisy production environment. Mounting the controller on the pump eliminates the cable, frees up valuable rack space and results in a compact package that is easily replaced. Removing the cables greatly simplifies total tool design and eliminates the cost of cable itself, which can be surprisingly high for coating tools that may be tens of meters in length.
Figure 2. Normalized cumulative costs (acquisition plus operating cost) of an oil diffusion pump compared to a TMP. The cumulative cost of a diffusion pump exceeds that of a TMP in year 3 due to higher operating costs.
Although the purchase price for mag-lev pumps remains higher than for ball-bearing pumps, the price differential has been reduced by large scale manufacturing to a level where the mag-lev’s lower operating costs can result in a total cost of ownership (CoO) that is significantly lower than that of a ball bearing pump (Fig. 2).
Crystalline silicon. While screen printing can be used to deposit some of the layers used in crystalline silicon solar cells, PVD is seeing increasing use for electrical contacts and reflective/antireflective coatings. It offers a low CoO, as well as high throughput and excellent film quality, making it a desirable processing solution as the solar industry moves into high-volume production. PVD processes must maintain vacuum conditions in the process chamber while accommodating the flow of process gases. Vacuum is also required for the silicon etch process used to increase light absorption by roughening the surface of crystalline solar cells.
Solar PVD applications typically use either diffusion pumps or TMP systems. While both require backing pumps to provide the sub-atmospheric foreline vacuum, TMP systems can tolerate twice the foreline pressure needed for diffusion pumps and therefore offer significant savings in the cost of mechanical foreline pumps. Due to their higher speed and capacity, a TMP-based solution can also reduce the number of pumps required for PVD processing by up to 50%, providing significant reductions in both capital expenditures and utility costs.
Using mag lev-based TMP technology for PVD solar applications provides additional reductions in maintenance costs and system downtime, and their high energy efficiency minimizes their environmental impact—a significant consideration in an industry that is particularly sensitive to environmental concerns.
Thin film applications. The primary advantage of thin film devices is the absence of high grade silicon, a primary contributor to the cost of crystalline silicon devices. Currently, thin film devices are less efficient in converting light to electrical energy, but reduced production costs may potentially result in lower cost/watt, the most important measure of PV device performance. Thin film devices also offer much greater flexibility in the types of substrates on which they can be fabricated.
In general, thin film deposition requires low pressure to ensure good process results. For thin film PV devices, the deposited films include the metal layer used for the solar cell’s back contact, the transparent conductive oxide (TCO) used for the front contact, the absorbing layer where light energy excites electrons to the conduction band and various other intermediate layers. A number of films can be used for the back contact film, including molybdenum. The TCO is usually indium tin oxide, although other films, such as zinc oxide, are also used. The absorbing layer may be copper-indium-gallium diselenide (CIGS) deposited via sputtering or molecular beam epitaxial methods, or cadmium telluride (CdTe) often deposited with closed-space sublimation. All of these methods require vacuum.
CIGS. CIGS is one of the more promising semiconductor materials used in thin film devices. Its high absorption coefficient and current density provide relatively high conversion efficiency. CIGS materials retain their performance properties longer than many other semiconductor materials and lend themselves to the production of large area panels in high-volume production. CIGS films can be manufactured by several different methods. The most common vacuum-based process co-evaporates or co-sputters copper, gallium, and indium, then anneals the resulting film with a selenide vapor to form the final CIGS structure.
Selenium can be a particularly troublesome gas to pump due to its corrosive attack of aluminum, a material used extensively in TMP to form the strong, light, complex-shaped surfaces of the rotor and stator. Aluminum corrosion can result in deterioration in vacuum performance or complete failure of a rotor with consequent total destruction of the pump. Nickel coatings applied to both the rotor and the stator have been found to prevent corrosion. The virtually inert nickel coating also provides effective protection against attack by typical chamber cleaning gases, such as plasma-activated NF3. Because the rotor of a mag-lev TMP has no bearing surface, there is no risk that bearing performance will be degraded by selenium corrosion and it is a simple matter to protect the full rotor surface with a nickel coating.
PECVD of a-Si/mc-Si Films. Silicon thin film technology seeks to combine the high efficiency of crystalline silicon with the lower manufacturing costs of thin films by using very thin films of amorphous or microcrystalline silicon. Devices based on silicon thin films are interesting because they can be fabricated directly for a variety of building materials, such as roof panels and window glass.
A number of solar manufacturers are using TMPs to improve the quality of silicon thin films, particularly the amount of hydrogenation, which affects the material’s electrical behavior and device lifetime. Pumping a light gas such as hydrogen can require significant modifications to the TMP rotor design to ensure sufficient compression, and hence pumping speed, in as small a package as possible. This must be balanced against performance requirements for heavier gases. Sometimes the best solution is a dedicated TMP optimized for hydrogen pumping. The high rotational speed of mag-lev TMPs improves their ability to pump light element gases.
The mag-lev TMP provides clean, reliable, cost effective vacuum for thin film deposition processes in PV manufacturing applications. Its low energy consumption and low maintenance requirements can reduce its COO below alternative technologies. It incorporates the coated components required for pumping fluorinated and other corrosive gases, such as the selenium gas used in some CIGS manufacturing processes. These benefits are driving a significant shift to mag-lev TMP.
Kate Wilson received her MEng in mechanical engineering from Brunel U, London, UK, and is a business development manager, solar, at Edwards; [email protected] Michael Boger is global product marketing manager, emerging markets.