A look at control of process uniformity across the wafer during plasma etch processes.
BY STEPHEN HWANG and KEREN KANARIK, Lam Research Corporation, Fremont, CA
Controlling process variability to achieve repeatable results has always been important for meeting yield and device performance requirements. With every advance in technology and change in design rule, tighter process controls are needed. In all of these cases, there are multiple sources of variability, often generalized as: within die, across wafer, wafer to wafer, and chamber to chamber. Typically, less than one third of the overall variation is allowed for variation across the wafer. For example, at the 14 nm node, the allowable variation for gate critical dimensions (CDs) is less than 2.4 nm, of which only about 0.84 nm is allowed for variation across the wafer . At the 5 nm node, the allowable variation across the wafer may be less than 0.5 nm, or equivalent to two or three silicon atoms. In this article, we will discuss control of process uniformity across the wafer during plasma etch processes, its evolution in the industry, and some key focus areas.
A fundamental challenge in controlling uniformity in etch processes is the complexity of a plasma. Achieving the desired etch result (e.g., post-etch profile with selectivity to different film materials) requires managing the ratio of different ions and neutrals (e.g., Ar+, C4F8, C4F6+, O, O2+). Since the same plasma generates both types of species, the relative amount of ions to neutrals is strongly coupled. As a result, the impact of parameters typically used to control the plasma (e.g., source power and chamber pressure) are also interdependent.
Improving uniformity through design
Since the start of single-wafer processing in the early 1980s, etch chambers have been designed to produce similar plasma conditions on every location on the wafer to achieve uniform process results. This is especially challenging since there can be inherent electrical and chemical discontinuities at the edge (FIGURE 1) that affect uniformity across the wafer. Voltage gradients are created at the wafer edge due to the change from a biased surface to a grounded or floating surface. This bends the plasma sheath at the wafer edge, which changes the trajectory of ions relative to the wafer. The chemical potential discontinuity is analogous and produces concentration gradients for different species across the wafer. The gradients are caused by multiple phenomena, including variation in reactant consumption and by-products emissions rates at the center relative to the edge, as well as differences in temperature between the chamber and wafer that cause different absorption rates of chemical species.
Many chamber design changes have been implemented over the years to improve radial symmetry (FIGURE 2a). For example, a key hardware parameter for capacitively coupled plasma (CCP) chambers is the gap between the cathode and anode. Historically, the gap would be designed to provide the most uniform etch for a given power, pressure, and mixture of gas chemistries. On inductively coupled plasma (ICP) chambers, the gas injection location was a key design feature that would vary by process. In aluminum etch chambers, the reactant gas was delivered from a showerhead above the wafer. For silicon etch, the reactant gases were injected from around the perimeter of the wafer, but then evolved so that the gas was injected from above the center of the wafer.
With continuous optimization of chamber design, non-radial patterns became more apparent. On a uniformity map, the average of all the points within every radius can be taken and subtracted from the map, which leaves the more difficult asymmetric portion (FIGURE 2b). With this awareness, focus shifted toward elimi- nating asymmetries in the chamber design.
In retrospect, some of these improvements seem obvious. For instance, up to the late 1990s, it was not uncommon to have etch chambers with the turbomolecular pump located to the side of the wafer. This design created a side- to-side pattern due to the convective flow of reactants and by-products laterally across the wafer. By moving the pumps under the wafer, the flow became radially symmetric, thereby eliminating the process asymmetry.
In other cases, the source of asymmetry was more subtle. One interesting non-uniformity corrected with design was a problematic side-to-side pattern on the wafer that had a seemingly random orien- tation chamber-to-chamber. After extensive investigation to eliminate possible sources in the chamber hardware, the pattern was correlated with the Earth’s magnetic field (FIGURE 3). This example demon- strates the sensitivity of plasma processes, even to minor external influences. Although not specifically a chamber issue, the problem was corrected by applying special shielding with high magnetic-permeability materials around the chamber.
Development of process tuning capabilities
As etch processes became more varied and complex, fixed chamber designs were not sufficiently flexible to meet increasingly stringent requirements since it was not practical to provide a specific uniformity kit optimized for each etch process. Moreover, it was more challenging to achieve uniform results when etch technology transitioned from processing 200 mm to 300 mm wafers in the early 2000s. As a result, tuning capabilities were developed to deliver the uniformity control needed for a wide range of processes and larger wafer sizes.
By the early 2000s, the first uniformity tuning knobs focused on controlling the chemistry over the wafer. This was done in several ways, for example by splitting the main reactant gases into different locations or by adding tuning gases at separate locations from the main reactant gas. Since then, a number of tunable parameters have been identified for etch processes (Table 1). Ideally, orthogonal (independent) tuning knobs are used in order to match compensation as closely as possible to root causes. This provides the greatest impact on the process while limiting impact on other parameters. For example, in many dielectric etch processes, the etch rate is limited by the flux of ions from the plasma. Since gas injection doesn’t significantly impact plasma density uniformity, Lam Research developed tunable gap technology for CCP chambers to achieve uniform flux of ions across the wafer for a given set of process conditions.
Over the years, continued development has focused on increasing the spatial resolution for better control across the wafer. For example, gas was at first only injected from the center location above the wafer. Then, additional capability was added that allowed controlling the ratio of gas directed to the center or edge of the wafer. Several years later, an additional gas injection location was added around the periphery of the wafer. To use wafer temperature as a control knob, different heating or cooling zones can be added to an electrostatic chuck (ESC), which holds the wafer. Historically, the number of temperature zones has increased from one to two (by 2002) to four radial zones (by 2006) to improve the radial uniformity of CDs. Since temperature directly affects CD uniformity (CDU), this is an effective way to tackle one of the most critical uniformity challenges.
Some of the most complex process flows today rely on these sophisticated tuning capabilities. Innovations that drive continuous scaling, such as 3D FinFET devices, advanced memory schemes, and double/quadruple patterning techniques, add to the challenge of reducing variability due to the increasing number of steps within the integration flows. Even if the uniformity for individual unit processes (including etch) are relatively good, their combined impact can be significant, and there is need to compensate somewhere in the flow.
When the uniformity profile of a step in the sequence, upstream or downstream, is known and difficult to correct, the profile of an etch step can be modified. For example, if one step is center fast, etch can compensate by being edge fast. This may sound simple, but it is actually quite difficult to achieve the level of process control that can essentially provide a mirror image of the non-uniformity in another process. Fortunately, plasma etch is one process that has matured to being capable of this level of control.
Uniformity control today
After many years of innovation, uniformity control capabilities now have the following characteristics:
• A high degree of granularity (numerous independent tuning locations across the wafer)
• Active tuning of both radial and non-radial patterns
• The ability to compensate for non-unifor- mities upstream and downstream of the etch process
One strategy being used at Lam to achieve the degree of control now needed is providing numerous independent heaters or micro-zones to control the wafer temperature, which is a critical parameter impacting CD uniformity. For example, using more than 100 localized heaters on one etch chamber delivers significantly higher spatial resolution than a system using only two or four heater zones for the entire wafer. Control of numerous individual heaters tunes both radial and non-radial patterns, whereas only center-middle-edge tuning was possible in previous generations (FIGURE 4).
With such high granularity, it is challenging for an individual engineer to manually determine the appropriate settings for so many heaters that will achieve a target thermal pattern across the wafer. To address this issue, advanced algorithms and controls with special temperature calibrations were developed so that the system automatically controls the heaters. Moreover, it can be difficult to determine the thermal map profile that will achieve the required process uniformity. Sophisticated software algorithms have also been developed to use process trends, chamber calibration data, and wafer metrology information to automatically create the appropriate thermal maps. With this capability, incoming non-uniformity can be reduced to less than 0.5 nm CDU after etch (FIGURE 5).
Future focus areas
Beyond the uniformity challenges discussed, performance at the edge of the wafer – the outer 10mm, where up to 10% of the die may be located – is an increasingly important area of future focus for improving yield. In this region, uniformity control is dominated by the electrical discontinuities at the edge of the wafer that can cause sheath bending. The impacted region of sheath bending is much smaller (~10-15 mm from the edge) compared to chemical or thermal effects (50-70 or 30-50 mm, respectively). While fixed edge hardware can be redesigned for optimal uniformity, new technologies are in development to provide in situ tunability of the sheath at the wafer edge.
Looking ahead, we can expect more types of control knobs and further granularity for finer tuning along with a greater focus on automation. Compensatory process control should continue to develop and be used as process modules become increasingly complex.
1. ITRS 2013: Table FEP 12 Etch Process Technology Requirements