PZT stack etch for MEMS devices in a capacitively coupled high density plasma reactor

Paul Werbaneth*, John Almerico, Les Jerde, Steve Marks Tegal Corporation

ABSTRACT

Ferroelectric thin films like lead zirconate titanate (PZT) are used to form several different families of MEMS devices. Moving mirrors for optical switching applications utilize the piezoelectric properties of PZT; varactors depend on its dielectric nonlinearity. The oxidizing environment during PZT deposition means that some material capable of resisting oxidation, like platinum, must be used as the metal electrode in any metal-ferroelectric-metal (MFM) stack. Ion milling has been used in laboratory applications for patterning MFM stacks. However, ion milling removal rates are low (~400Å/min), the throughputs are low, and the etched materials tend to redeposit along the edge of the etch mask, creating veils, or fences, after the etch mask is removed. These residues can lead to yield-limiting defects in finished devices. We report here on MFM stack etch results from a capacitively coupled high density plasma etch reactor. Using photoresist masks, we have demonstrated platinum and PZT etch rates greater than 1000Å/min at moderate (80ºC) wafer temperatures. Good etch profiles with no post-etch residue are produced for MFM stacks like those used for a MEMS- based Atomic Force Microscopy application, for example, which employs a bottom platinum layer 1500Å thick, 2800Å of PZT, and a platinum top electrode of 1500Å.

Keywords: PZT etch, platinum etch, MFM stack etch, plasma etch

1. INTRODUCTION

Plasma etch is employed in modern microelectronic fabrication processes to faithfully reproduce masking image features into the permanent layers of a device or circuit. The benefits of plasma etch include the ability to etch sub-micron structures without changing critical dimensions while maintaining good selectivity to the masking and underlying materials in the sustained production environment of a commercial wafer fabrication facility. Manufacturing processes for MEMS, and MOEMS, are similar to those processes used by the large integrated device makers for high-volume CMOS products: the basic sequence of thin film deposition, photolithography, and etch is repeated many times to make a finished MEMS device. However, the many novel structures and materials employed in MEMS fabrication cause it to diverge in several important ways from the family of standard silicon processing modules.

1.1. Uses of MFM structures for MEMS applications PZT is a useful material for many different MEMS applications, most prominently those applications requiring physical motion, like moving mirrors in optical switch fabrics (PZT is a piezoelectric material), and those for which the ability to adjust device capacitance, for example acoustic filters for RF communication applications, is important. The oxidizing environment found during the deposition and annealing of PZT films means that some electrode material capable of resisting oxidation (platinum) or capable of forming an electrically conductive oxide (iridium) must be used with PZT, creating what is called an MFM (metal – ferroelectric – metal) stack. This is very different from the capacitor structures and metalization schemes found in CMOS processing, where the dielectric materials are typically silicon dioxide or silicon oxynitride, and the electrode materials silicon, polysilicon, or aluminum and aluminum alloys.

* [email protected]; phone 1.707.765.5608; fax 1.707.773.3015; Tegal Corporation, 2201 S. McDowell Blvd., Petaluma, CA, USA 94954 1.2. Patterning MFM structures Patterning MFM structures is inherently challenging. Due to the nonvolatility of the etch products produced during MFM stack etch, it is difficult to successfully integrate MFM stacks into microelectronic devices1,2.

Some candidates for etching these difficult materials are wet etch, chemical-plasma etch, reactive ion etch, and ion milling. Wet etches suffer from feature size limitations; chemical-plasma etch offers only limited etch rates.

1.3. Ion milling limitations Ion Milling, or Ion Beam Etching, has been a versatile tool in laboratory applications for patterning almost any known material. The physical action of the ion beam can be made to be sufficiently energetic to remove atoms from solid thin film surfaces under temperature and pressure conditions where the vapor pressure of the material(s) to be removed is negligibly small. Ion milling lends itself perfectly to laboratory demonstrations of MFM capacitor formation, where the problem of patterning the noble metal electrodes and the heavy metals in the ferroelectric films, at the feasibility stage of device fabrication, does not warrant extensive exploration of other patterning techniques. Researchers have noted several drawbacks to ion milling: for example, when ion milling is used to pattern platinum, the removal rate of platinum is low (~400Å/min); the overall tool throughput, as a result of the combination of low etch rates and the need for long overetches, is poor; and the tendency of the etched material to redeposit on the side of the etch mask, and remain, causes the formation of fearsome veils, or fences, after the etch mask has been removed. These veils can lead to yield-limiting electrical defects in finished devices. Figure 1 is an example of the kind of veil observed post-etch ( and after photoresist mask removal) for a dry-etched platinum feature.

Figure 1 Veil formed after dry etch of platinum

This shortcoming has motivated us to develop alternative dry patterning processes for MFM stacks.

2. EXPERIMENTAL APPARATUS

2.1. Plasma reactor description The process module used for this work is a dual frequency, magnetically confined plasma etch reactor capable of producing high density plasma conditions at low (1mTorr - 100mTorr) discharge pressures. In this dual frequency reactor MHz RF power produces the dissociated, reactive, and ionized components of the etching plasma. A second RF frequency, in the kHz range, acts to control the energy of ions impinging on the wafer surface. A schematic of the reactor is shown in Figure 2. Figure 2 Dual frequency capacitively coupled plasma reactor

2.2. Effect of RF frequency in plasma reactors The general effect RF driving frequency has on the density of the reactive plasma, and on the energy of ions within the reactive plasma, is shown in Figure 3. A dual frequency plasma reactor, when the separation between the two RF frequencies is large, can be made to act independently on plasma density and ion energy. For etching materials like platinum and PZT in a plasma reactor the physical component (ion energy) of the etch will be one of the chief means by which the thin films are removed from the wafer surface. 100kHz, 400kHz, and 450kHz are examples of standard frequencies used by plasma tool manufacturers for this task. 13.56MHz is the most commonly encountered RF frequency used to generate and sustain reactive etching plasmas.

Figure 3 Effect of RF frequency on plasma density and ion energy

2.3. Other characteristics RF power in this dual frequency reactor is capacitively coupled to the plasma; it is the nature of capacitively coupled plasmas that power transfer, and hence process stability, is unaffected by deposits formed on the inner surfaces of the reactor even if those deposits are electrically conductive, as will be the case when etching noble metal electrode films like those found in MFM stacks. The reactive plasma generated in the dual frequency reactor is augmented through the use of magnetic confinement. Permanent rare-earth magnets mounted on the reactor sidewall and on the top electrode are placed to reflect escaping free electrons back into the active plasma region, thereby increasing the electrons’ useful life in the reactor, and extending their contributions toward sustaining the plasma overall. It is this magnetic confinement that creates the high density plasma conditions within the reactor. 3. STRATEGY FOR PLASMA ETCH OPTIMIZATION OF MFM STACKS

The basic concept of plasma etching is very straightforward. A glow discharge is used to generate chemically reactive species (atoms, radicals, ions) from a relatively inert molecular gas. The etching gas is chosen so as to produce species which react chemically with the material to be etched to form a reaction product which is volatile (at room temperature preferably). The etch product then spontaneously desorbs from the etched material into the gas phase where it is removed by the vacuum pumping system. The key requirement is the volatility of the etch product3.

3.1. Physical properties of platinum and PZT etch products Table 1 summarizes some of the physical properties of the important chlorides formed when platinum and PZT are etched in chlorine-containing plasmas.

Element Etch Product Melting Point (ºC) Boiling Point (ºC)

Pb PbCl2 501 950 Pt PtCl4 370 --- (decomposes) Ti TiCl4 -25 136 Zr ZrCl4 331 --- (sublimes) Table 1 Etch product physical properties

The volatility of the Pb, Pt, and Zr etch products is considerably less than those of other elements, like silicon and aluminum, commonly etched in plasma reactors. (And much less than for TiCl4, which behaves “normally”.) Low volatility of the etch products here suggests that etch rate issues and post-etch residues created from nonvolatile etch products landing on adjacent surfaces are likely to be encountered when plasma etch techniques are used to pattern these films. (Another issue that will arise is the question of MFM stack etch process manufacturability: how many wafers etched before the plasma reactor must be cleaned is directly related to the nature of the etch products formed during the plasma etch.)

3.2. Effect of ion bombardment on Pt and PZT etch A key finding obtained from historical efforts to optimize MFM stack etch in plasma reactors is that, in a reactive environment like an argon-chlorine-based plasma, the etch rates of platinum and PZT increase when their surfaces are bombarded by ions of increasing energy, and that veil formation can be controlled by adjusting the reactive (halogen) component mole fraction, either by increasing its flow, or by increasing the dissociation of the etchant gas in the plasma

(Cl2 → 2Cl). For example, platinum and PZT etch rates in an ECR tool in O2/Cl2 and Cl2/Ar plasmas, respectively, are reproduced in Figure 44.

Figure 4 Pt and PZT etch rate vs. RF power 4. MFM STACK ETCH DATA

Etch rates and post-etch veil formation for photoresist masked platinum films were characterized in the dual frequency capacitively coupled plasma reactor described in Section 2. This characterization was performed using Design Of Experiment techniques. These experimental methods are useful in that they reveal process trends and process interactions for multi-variable systems. The principle process variables, or experimental factors, in a plasma etch reactor are pressure, temperature, type of reactant used, reactant flows, reactant fraction (in multi-reactant chemistries), plasma density, and ion bombardment energy and flux at the wafer surface.

Figure 5 represents a model of the platinum etch rate response to changes in two experimental factors: Cl2 flow and MHz (high frequency) power.

Figure 5 Response of platinum etch rate to changes in Cl2 flow and MHz power

Figure 6 shows the response of platinum etch rate to changes in kHz (low frequency) and MHz (high frequency) RF power.

Figure 6 Response of platinum etch rate to changes in kHz and MHz power The effect MHz power and Cl2 flow have on post-etch veil height for a photoresist masked platinum film is presented in Figure 7.

Figure 7 Response of post-etch platinum veil height to changes in MHz power and Cl2 flow.

5. OPTIMIZED MFM STACK ETCH RESULTS

5.1. Typical MFM stack structure A schematic drawing of a representative MFM stack for MEMS applications is shown in Figure 8.

Figure 8 Representative MFM stack for MEMS applications

Plasma etching of stacks like this one are often performed using two separate masking steps. The two-masking step process can be used to create a common bottom electrode for an array of MFM structures, should this be desirable.

By keeping the wafer temperature low during the MFM stack etch, something like 80ºC, photoresist masks are readily accommodated.

5.2. MFM stack etch results Figure 9 is an SEM micrograph showing a completed MFM stack (Pt/PZT/Pt) etch performed in the dual frequency capacitively coupled plasma reactor. The micrograph shows the etch profile after photoresist mask(s) removal. Figure 9 Etched MFM stack (Pt/PZT/Pt) with photoresist mask(s) removed

The complete stack has been etched veil-free using photoresist masks.

Table 2 summarizes the MFM stack etch results obtained with an optimized plasma etch process in the dual frequency capacitively coupled plasma reactor for platinum and PZT films using photoresist masks.

Process Result Platinum Etch PZT Etch

Etch Rate 1000Å/min 850Å/min Etch Profile ≥ 70º ≥ 70º Delta CD ≤ 0.2µm ≤ 0.3µm Post-etch Veils and Fences None None Endpoint Optical Optical Table 2 Summary of MFM stack plasma etch results

6. CONCLUSIONS

We have characterized plasma etch processes for PZT-based MFM structures used in MEMS device fabrication in a dual frequency capacitively coupled high density plasma reactor. Experimental models derived from DOE methodology demonstrate platinum and PZT etch rates greater than 1000Å/min at moderate (80ºC) wafer temperatures. Post-etch veils or fences can be eliminated using process conditions optimized with the same DOE techniques. Good etch profiles with no post-etch residues have been produced for MFM stacks like those used for a MEMS-based Atomic Force Microscopy application, for example, which consists of a bottom layer of platinum 1500Å thick, 2800Å of PZT, and a platinum top electrode of similar thickness to the bottom electrode5.

7. REFERENCES

1 Thomas C. Taylor and Holli A. Harper, “Chemically assisted etching of PZT-based ferroelectric dielectrics and noble metal capacitor electrodes”, Proceedings of the 20th Annual Tegal Plasma Seminar, pages 41 – 81, 1994. 2 Hidemi Takasu, “The innovation on semiconductor devices by using the ferroelectric memory technology”, Proceedings of the 24th Annual Tegal Plasma Seminar, pages 11 – 17, 1998. 3 John Coburn, Plasma Etching and Reactive Ion Etching, p. 5, American Institute of Physics, Inc., New York, 1982. 4 Yasuyuki Ito, et al., “High temperature etching of PZT/Pt/TiN structure by high density ECR plasma”, Proceedings of the 20th Annual Tegal Plasma Seminar, pages 83 – 86, 1994. 5 H. Nam, et al, "Fabrication and Characterization of Piezoelectric PZT Cantilever for High Speed Atomic Force Microscopy", Proceedings of the Twelfth International Symposium on Integrated Ferroelectrics, Part VI, R. Waser, D. Wouters, R. Whatmore, editors, pages 185 – 197, Gordon and Breach Science Publishers, 2001.