A novel technique for planarization has drawn more attention, since conventional planarization methods such as reflowing-oxide-layers do not give the required global planarity for advanced processes. Multilevel interconnects and the use of 3-D packaging requires sophisticated methods to planarize the surfaces of wafers for subsequent device processing. (e.g. For multi-layers of a logic device at least one layer should be perfectly planar.) Lack of planarity may lead to severe problems for lithography (insufficient focus depth). Chemical Mechanical Planarization (CMP) evolves as the only way to achieve global planarization in IC fabrication. In this chapter, CMP will be explained from the views of system/configuration and process.
2.1 Chemical-Mechanical Polishing System
As implied by the name, Chemical-Mechanical Polishing, the system must perform mechanical abrasion and chemical erosion to wafer surface of various materials. Mechanical abrasion is mainly provided by contact force of fluid or solid objects, i.e. slurry, abrasive and pad. However, chemical erosion is provided by slurry which is composed of specific chemical agent. These two chief effects are coupled to each other. It is this reason why CMP technique is treated as an engineering ‘art’ rather than an engineering ‘science’.
2.1.1 Schematic CMP System
The design goals of all the CMP polishers include high throughput (multi-heads), “dry-in/dry-out”, easy control, endpoint detection and high quality of polished surface. The CMP equipments/systems are fabricated by ten more suppliers and all of these equipments have different configurations. However, the main mechanism of these polishers can be separated into three forms. Fig. 2.1.1 shows the most common kind of polisher framework-rotary tool.2 The wafer is held on a rotating carrier (holder) while the face being polished is pressed against a resilient polishing pad attached to a rotating platen disk. The abrasive contained in slurry and the slurry is carried to the wafer by the porosity of the polishing pad. Another configuration which is different to rotary tool is orbital tool, as shown in Fig. 2.1.2.
The major differences between these two frames are slurry supply system and platen motion. The slurry is fed from platen and the platen is oscillating during polishing.
The third configuration is announced by Lam Research, Linear Planarization Technology TM (LPTTM), as shown in Fig. 2.1.3. These three equipments have respective advantages and disadvantages.
2.1.2 Consumables
CMP combines both chemical action and mechanical forces to planarize wafer surface. The critical components required for CMP are a reactive liquid medium and a polishing pad surface to provide the mechanical control required to achieve planarity.
Either the liquid or the polishing surface may contain nano-size inorganic particles to enhance the reactive and mechanical activity of the process. Because of the great quantity of consumables requirement, CMP turns into one of the most expensive steps
2 In this work, the rotary tool will be investigated.
in the IC production. Besides, it is well known that consumables, slurry and pad, play significant roles in CMP. Therefore, consumable suppliers and many researchers are devoted to improve the stability and efficiency of consumables.
Slurry
Typically, slurry consists of two phases, namely, liquid and solid phases. Liquid phase consists of deionized (DI) water with several additives like oxidizers, complexing agents, inhibiting agents, and surfactants (Different components have distinct characteristics and are used to polish different materials). Solid phase is comprised of abrasives, which are usually inorganic oxides, e.g., alumina, silica, ceria, zirconia, titania, magnesia (Different sizes and types of abrasive have different effect on polishing). While recent advances in abrasive-free slurries and fixed abrasive CMP offer great potential, several challenges remain. In CMP process, slurry is dispensed in-between polishing pad and wafer to play a part in planarizing. Slurry flow rate is an important factor in supplying slurry. Fig. 2.2 shows the effects of slurry flow rate on the CMP process. According to that figure, slurry flow rate affects removal rate. In oxide CMP, removal rate will increase with increased slurry flow until saturation effect occurs. In metal CMP, removal rate will increase with increased slurry flow first and then decrease when lubrication effect occurs, i.e. slurry forms hydroplane between wafer and pad.
Pad
All polishing pads used in IC production are polyurethane pads. Based on the structure/composition, a polishing pad can be classified into the following types: solid polyurethane pad, polyurethane with filler or void, fiber felt impregnated with
polyurethane, and poromeric pads. Each type of pad uses a unique manufacturing process and therefore has its own unique characteristics. For example, poromeric pads are widely used for buffing and tungsten plug polishing because of their capability to deliver good local uniformity and low cost. For the applications required long range planarity, pads consisting of polyurethane with filler, such as Rodel’s IC pads, are the most commonly used pads. Because of lack of long-range planarity and/or excess dishing for large geometry features, older pads made of fiber felt impregnated with polyurethane are limited to sub-pad and tungsten plug polishing applications [27]. According to the same literature [27], pads are investigated for groove effects and there are different influences by different forms of groove. They also mention that pad elasticity is also an important factor in CMP processing.
Conditioner
Pad conditioning, or pad “dressing”, is a critical component of the CMP process.
It refers to the process of refreshing the polishing pad surface during CMP. In the process, a pad is conditioned by contacting its surface with a diamond abrasive disc or wheel. Generally, the conditioning disc is mounted on a powered rotating chuck that can be lowered onto the pad surface. Well conditioning of a pad can increase pad life and performance of polishing. Fig. 2.3 shows the conditioning effect schematically.
2.1.3 Endpoint Detection System
To date, the design of CMP system is quite different from before, including the design of “dry-in/dry-out”, multi-platen and so on. In spite of the advances of configuration setting, robust sensor for end point detection (EPD) of polished material
is still a problem for process control of CMP. Endpoint detection is an in-line method of determining the termination point of wafer polishing based on metrological (i.e. film thickness) or physical (i.e. tool hardware) signals obtained during the CMP process. The end of the polishing step is traditionally determined by setting a time limit in the process. Changes in removal rate due to the normal pad life cycle, variation in slurry and pad lots, conditioning issues and a myriad of other potential variables, can result in under- or over-polish3 errors. Additionally, the incoming initial oxide or metal layer thickness may fluctuate from wafer to wafer. All the possible errors have to be compensated by valid in-line sensors [29]. To control CMP process well, a good and accurate in-line sensor is indispensable.
In past few years, endpoint detection is developed through many methods, optical, thermal, vibration, acoustic, electrochemical and motor current (frictional).
Some commercialized transducers were announced, ISRMTM (Applied Materials), SentinelTM (Novellus) and PRECICETM (KLA-Tencor) and so on.
2.2 Chemical-Mechanical Planarization Process
After introducing the configuration of CMP system, the CMP process itself will be discussed briefly.
2.2.1 Process Parameters
To improve CMP performance, understanding this process comprehensively is necessary. Finding out what parameters and how they contribute to the mechanical-chemical actions is critical for understanding of CMP. There are so
3 Under-polish and over-polish mean that the process is proceeding before oxide is exposed and until
many parameters related to CMP process. Dozens of parameters are dominant in the process. On the polishing tool side, down pressure/force, platen temperature, slurry flow (supply) rate, platen and carrier speed are all dominant factors in CMP process.
On the slurry side, chemical components, PH value, concentration of Oxidizer, kind and size of particles and stability of suspension are all very important factors of the process. On the polishing pad, elasticity, hardness, groove, pore structure, construction, age and conditioning of pad are very critical in CMP process. Finally, materials and structure (pattern) of wafer surface, wafer carrier and endpoint detection system et al. all influence CMP process. It is important to note, coupling effects exist among these parameters, between chemical and mechanical factors. It is a very complicated task to investigate CMP.
2.2.2 Performance Indices
Because CMP is a planarization method, it is clear that the most important index of polishing performance must be “how flat” of the polished wafer. The planarity is usually represented by within-wafer non-uniformity:
∆mean
where ∆ is removed thickness per minute.
To maintain a high throughput of CMP process, removal of excess material of polished wafer must be accomplished in a specific time, i.e. the removal rate must be sustained on a reasonable level. For Cu CMP4, thickness of Cu film is calculated by dividing the film resistivity with its measured sheet resistance. The relation between thickness and resistivity is
4 In this thesis, copper material will be considered.
ρ∝R TS⋅ (2.2) where ρ is the resistivity ( µΩ⋅cm unit-area), RS is the sheet resistance
( mΩ unit-area) and T is the copper thickness (kÅ). Sequentially, CMP removal-rate can be represented by
( ) ( )
In fabricating Cu damascene interconnects, CMP of Cu and barrier metals is one of the most important techniques. SiO2 erosion and Cu dishing are serious problems of CMP. This erosion is defined as oxide loss around the patterned Cu area and dishing is defined as Cu thickness loss inside the Cu pattern. Fig. 2.4 shows schematic dishing and erosion. Any dishing and erosion and the resulting decrease in cross section area due to thinning (dishing plus erosion) of the long interconnect line during polish has a significant effect on circuit delays due to the increase in the RC constant. Therefore, to achieve the benefits of the low RC intended with the use of Cu, there is an increased need to minimize and control dishing and erosion across the wafer during Cu CMP process. In the previous literature, Steigerwald et al. [28] found that Cu dishing is a strong function of line width, but is only weakly dependent upon pattern density. Moreover, to specify dishing, they defined planarization efficiency (PE)
( )
[
1- ∆down ∆up]
100%PE= × (2.4.1)
where ∆down and ∆up are the thickness differences of the inside and outside of the feature after planarization processes, as shown in Fig. 2.5. In other words, planarization efficiency can be expressed as
r_bottom r_top
where Rr_bottom and Rr_top is removal rate of lower and higher area, respectively.
Wafer roughness which is measured by AFM (Atomic Force Microscope) can be defined by
where L is the distance of scanning and y is the difference between probe point and average height. The defects after copper CMP are shown in Fig. 2.6. In that figure, Cu puddle, corrosion and scratch are not caused by CMP, they just might appear as after effect of polishing [39].
2.2.3 CMP Models/Mechanisms
Establishing model/mechanism, much more works are focused on mechanical effects, just like what mentioned at section 1.2.1 and 1.2.2. Based on Preston’s equation (1.1), the generalized form
R k P V= ⋅ α⋅ β (2.6)
is widely accepted and used [30]. The value α and β are fitted parameters that vary depending on the process and the consumable set (pad, slurry) used. However, the models derived from Preston’s equation are ineffective in providing detailed fundamental understanding and locally relevant information regarding a chosen process. To date, there are very few wafer-scale, quantitative models for CMP removal rate and uniformity. This fact, combined with the resiliency of Preston’s
equation, makes a strong argument for a locally relevant expression for material removal rate [9, 31]
( ) ( ) ( )
R x, y = ⋅k P x, y V x, y⋅ (2.7)
where the removal rate R at point
( )
x,y on the wafer surface is a function of the local pressure P( )
x,y and the local pad/wafer relative velocityV( )
x,y . It has to be mentioned again that Preston equation is nonetheless flawed because of the neglect of chemical effects.Except for mechanical effect modeling, the chemical effect is also a very important part of CMP process, especially in polishing metal. Early in the works of F. B. Kaufman et al. [2] and R. J. Gutmann et al. [11], the mechanism related to chemical effects was discussed. The mathematical model including chemical effects was first established by Q. Luo et al. [34], that is
( )
CR= KP B V R+ + (2.8)
where B is a modification term of velocity V and RC represents the purely chemical removal rate. Equation (2.8) was established by linear regression of experimental data and has greater dependence on the velocity compared to the pressure. (It may be different on different polishing tools.) J. Luo and D. A. Dornfeld [8] also derived a model with mechanical and chemical effects of CMP process. In their model, an interface between chemical effect and mechanical effect has been constructed through a fitting parameter Hw, a “dynamical” hardness value of the wafer surface. It reflects the influences of chemicals on the mechanical material removal.
Usually, the model/mechanism in CMP process investigation means material removal mechanism. To further improve the performance of CMP process, e.g.
copper dishing, the CMP process model has to be analyzed in more detail, too. Y.
Lin [42] established copper planarization (step height reduction) model and copper-dishing model based on the difference of removal rate between high and low area of wafer surface and that between copper and barrier material, respectively.
The copper-dishing model is shown as
( )
where hD is the dishing height; P is applied downward pressure; H is the pad thickness;
ξ is a constant defined as the conformity of the pad; KCu is the Preston constant for copper; E is Young’s modules of the pad; V is the relative velocity between the work piece and the pad; t represents the overpolish time; w and w0 are defined as the trench width and effective minimum width, respectively. (The word “effective” means that pad can contact with the lower feature.) The factor S represents the removal rate selectivity of copper to barrier material, like tantalum, defined as
Ta
The step height reduction model is
where hS is the step height of sacrificial copper; hS0 is the initial step height; t represents polish time of bulk copper here. Note that (2.9) and (2.11) are derived involving pad conformity. V.H. Nguyen et al. [32] also established a physical model for development of dishing during metal CMP. The model is
( )
where D
(
LW,t)
is dishing, LW is line width, R is the blanket removal rate, t is Bl over-polish time, Φ( )
rC is the distribution of contact size, r is the average contact C radius and K is the only adjustable parameter. Note that this model was derived including pad morphology. G. Fu and A. Chandra [33] also derived an analytical model for dishing and step height reduction in CMP.These models/mechanisms shown above are established in steady-state.
However, one appropriate dynamic model is necessary for setting the operation profile via strategy of dynamic-tuning. The only dynamic model is the conceptual model proposed by J.-B. Chiu et al. [24], shown as
dt R
represents the thickness of copper to be removed, R is the removal rate and u is the system input. For convenience, (2.13) and (2.14) are further changed into the following state equation:
( )
t, on the investigations of run-to-run controller of CMP, just like the works of L. Da et al. [15]. In their research, the removal rate would drift down as the process proceeds because of pad wearing and slurry dilution. Although the problem of pad wearing can be adjusted by conditioning during the CMP process, the slurry concentration can be kept constant by pre-mix before delivering onto platen and they might not be too obvious during a single wafer polishing, the disturbance d is also added into the simulation to model the uncertainties during CMP process processing. Note that the coefficients are assigned small values arbitrarily. This model will be used to designthe “dynamic tuning” operation profile via sliding-mode theory in chapter 3.
In following sections, some of these models will be used to verify the ability of dynamic-tuning operation by numerical simulations.
2.3 Process Control in Chemical-Mechanical Planarization Process
Most of people who are devoted to do research on the CMP process focus their works on process improvement, i.e. finding the perfect composition of process parameters, namely, “Golden Recipe”. “Golden recipe” means: through fixed perfect parameters setting the perfect performance will be derived. For example, in the CMP process, the perfect performance is complete flat. Nevertheless, it can not be realized at most case in real world because of existence of a lot of disturbances, noise and uncertainties. The inherent characteristics/flaws of equipments may cause some imperfections in products. (That may be one of the reasons why control theories were developed for.) Continuous control for CMP process seems necessary. Here, one question is raised: Is it more complete if the “Golden Recipe” contains an appropriate operation pattern?
CMP process control covers both consumables and equipments to improve performance of CMP process. Run-to-run control is the only one control method with specific control algorithm/law in the CMP process and is thought to be the only viable scheme in most semiconductor manufacturing processes because of the lack of in-situ measurements of the product quality of interest [22]. The method of run-to-run control is based on statistic data of process performance. Besides, the name “run-to-run” means the control object is a single run, one specific number of wafers (ten, five or less number of wafers). The concept of run-to-run control is shown in Fig. 2.7.
As the requirements of degree of planarization increases, within-wafer CMP process control becomes necessary. Little effort has been tried to move forward to within-wafer control because of the complexity of CMP process. In this thesis, it will be attempted based on the knowledge of CMP process.