Metal etch in advanced immersion tank with precision uniformity using agitation and wafer rotation

Scott Tice1, a and Chan Geun Park1 1MEI Wet Processing Systems & Services LLC., 3838 NE Western Way, Albany, OR 97321, USA

a scott.tice@meillc.com

Keywords: metal etch, gold etch, copper etch, within wafer (WIW) etch uniformity, wafer to wafer (WTW) etch uniformity, bubbled N2, wafer rotation, immersion tank

Introduction

In semiconductor wafer fabrication, etching refers to the process of removing unwanted material from wafer surface through a subtractive process. Metal etching is most commonly used in the patterning of metal films for interconnects by establishing specific connection and conduction paths and can be classified by dry etching, de-plating and dissolution of the layers on various substrates such as silicon, SiO2, Si3N4, GaAs, germanium, and sapphire. Dry etching is used to produce very precise etching of vertical channels or vias forming the device features or lines which make up the conductive path because it is anisotropic or etching in one direction. Dry etching is achieved by using chemical gases and plasma in a process chamber so dry etching tools are very large, complex and expensive to purchase and operate. De-plating is a process of electro-chemically removing metal material from the surface of the wafer to an anode by creating a difference in electrical potential between the surface to be etched/de-plated (typically cathode) and the “target” or anode where the material is to be collected. De-plating in single wafer tools has also replaced immersion processing due to the better uniformity it provides. However, De-plating single wafer tools are also very large and expensive to operate and have low throughput (wafers per hour). Dissolution/Immersion is the used of recirculated chemical baths to perform the etching process. In an immersion bath chemical is used to dissolve the metal layer that is unprotected by the mask. Immersion metal etch process has been on the decline because of its isotropic etching property and poor etch uniformity caused by non-uniform chemical flow around wafers in the tank. For the most of etch processes lateral etching is undesired because it occurs on the walls of the features and makes them thinner or misshapen. As a result, most of critical etching steps are performed by dry etching systems. However, if etch uniformity is precise, immersion etching can be used for less critical features in place of complex dry etching and de-plating.

In this study, an immersion etch tank has been designed to improve etch uniformity and produce metal features or lines that are within the manufacturing tolerances. The performance was evaluated by conducting gold etch tests with iodine / potassium iodide mixture and copper etch with citric acid / ammonia / hydrogen peroxide mixture.

Experimental Metal etch uniformity evaluation was conducted with 1) 1600Å PVD (Physical Vapor

Deposition) Au film, 2) 5000Å PVD Cu film, and 3) 4.5um plated gold micro-coil pattern (Figure 1) in MEI Advance Metal Etch Tank on MEI RevolutionTM, Rotary Multi-Step Modular Wet Bench System. Its performance was compared with that of existing wet bench and spray tool on 200mm silicon wafer fabrication used to manufacture opto couplers, special RF ICs, or MEMS devices.

The etched amount of gold and copper was measured in PROMETRIX Omnimap RS-55 TC (KLA/TENCOR) and Micro X (Thermo Electron Cooperation), respectively. P-12 surface profiler (KLA/TENCOR) was used to measure step height difference before and after seed Au removal from plated gold pattern.

Figure 1 : (a) Scope image plated gold micro-coil pattern after seed Au strip in MEI Advanced Etch Tank (b) cross section diagram of the pattern

The etch operation, conducted within the MEI Advanced Metal Etch Tank is described in Figure 2. This shows the position of the cassette and wafers while processing. The Teflon cassette rests on the bubble barrier walls to force all of the agitation bubbles formed by the N2 gas through the cassette walls. The agitation plate is formed with 3 separate sparger plenums to provide complete coverage and uniform agitation up through the wafers. The wafers are lifted up off the bottom of the cassette by the distance of the roller to the bubble barriers being smaller than the distance from the wafer bottom to the contact point of the cassette and bubble barriers. The path of chemistry is also shown during processing in Figure 2. Chemical is injected into 2 inlets located on each side of the cassette. This chemical is forced through a series of small holes in the tank wall and then drawn through a slot in the bubble barrier and into the upward flow of N2 bubbles and across the wafer surface displacing depleted chemistry with fresh. The wafers rotate during processing due to the roller shaft which is being driven by the magnet coupled motor located outside the tank. This eliminates the need to breach the tank wall or place driving hardware on the deck top to turn the roller. The wafers are lifted out of the cradle area of the cassette to prevent shadowing of the wafer. When combined with wafer rotation and uniform micro-bubble agitation, excellent uniformity is achieved.

Figure 2 : A section view of the MEI Advanced Metal Etch Tank (left) and path of chemisty during process (right)

The splarger plate was devised to genetrate large quntities of very fine N2 bubbles in controlled and adjustable manner as shown in Figure 3 (Pattent Pending). The N2 bubbles assist the etch chemistry by creating a uniform and consistent etch effect across the entire surface of the wafer. The nitrogen pressure and flow are precisely controlled. Mass Flow Controllers enable accurate and repeatable process control. The sparger plate is divided into 3 sections to increase or decrease etch rate over different sections of the wafer to achieve the desired uniformity. A left, right and center chamber is built into the sparger’s bottom plate allowing the individual adjustment of the N2 bubble flow rates. The bubble barriers force agitation penetration into the cassette and through the wafers in a predictable and repeatable manner. Therefore, the bubbles obliterate un-uniform velocity of chemical flow caused by wafer cassette, tank design, chemical recirculation and wafer loading conditon.

Figure 3 : (a) top plate and 3 arrays of tiny diameter holes above each bottom plate chamber (b) O-ring seal between top and bottom plates (c) 3 chambers in bottom plate

Results and Discussion A PVD Au wafer was slightly over-etched in typical Au etch tank in order to understand how

chemical flow in the tank impacted to WIW etch uniformity. As shown in Figure 4, very obvious and unique pattern was found on wafer surface. It was perfectly matched to simulated chemical flow pattern and revealed shadow areas on 4 and 8 o’clock positions of wafer edge where fresh chemical flows were strongly restricted by cassette design. Meanwhile, those were not observed from another Au wafer etched in MEI Advance Etch Tank with wafer agitation by robot movement (robot agitation), agitation flow of bubbled N2 that carrying fresh chemical (N2 agitation), and wafer rotation.

Figure 4: (a) simulated chemical flow pattern for typical recirc.tank (b) flow pattern & shadow areas on PVA Au wafer over-etched in typical recirc. tank (c) etch amount maps of PVD Au wafers etched in typical and MEI Advanced Tank

WIW and WTW gold etch uniformity were evaluated with 25 PVD Au (1600Å) wafers loaded in MEI Advanced Etch Tank after process conditions for N2 / robot agitations and wafer rotation were optimized. Gold etch target was 500Å at 330Å/min with 25 wafers loaded to fully populate the cassette. Coefficient variance (%, [Standard Deviation / Average] x 100) of Au thickness after gold etch was used to calculate gold etch uniformity. Average WIW uniformity of all 25 wafers was only 4.4% and WTW uniformity was 2.9% (Figure 5). Meanwhile, existing spray tool has shown >15% WIW uniformity with only 12 wafers loaded in the cassette. This was required to improve chemical flow between wafers or etch uniformity was often >30%. Due to very poor uniformity the spray tools required a few hundred % over etch in gold etch chamber to complete seed Au strip. During the over etch time plated Au line patterns are being etched and gold etchant unnecessarily deteriorates. As a result, the chemical should must be exchanged for every 12 wafers processed. With plated Au pattern wafers (1000Å seed Au/4.5um plated Au), Au etching performance was evaluated again under the same process and wafer loading conditions. Etch uniformity was calculated by step height of plated gold line pattern after seed Au strip as shown in Figure 1. Both of average WIW uniformity of all 25 wafers and WTW uniformity were only 1.5% (Figure 6) and optical scope inspection confirmed that micro-coil patterns were well defined without any seed Au residue in dense pattern area (Figure 1).

Figure 5 : WIW and WTW Etch uniformity of 25 PVD Au wafers etched in MEI Advanced Etch Tank

Figure 6 : WIW and WTW Etch uniformity of 25 plated Au patterned wafers etched in MEI Advanced Etch Tank

Figure 7 : WIW and WTW Etch uniformity of PVD Cu wafers etched in MEI Advanced Etch Tank

200mm wafer Cu etch showed similar or better etching performance even with dual cassette-loading in the MEI Advanced Etch Tank. 5000Å PVD Cu wafers were loaded in left and right cassettes with dummy wafers in every other slot. Copper etch target was 2500Å at 2500Å/min and Non-Uniformity (NU, %, [Range / (2 x Average)] x 100) of Cu etch rate or amount was used to calculate copper etch uniformity. Average WIW uniformity of all processed wafers was 2.3% with ±100Å tolerance and WTW uniformity was only 0.9% with ±2.5Å tolerance (Figure 7). Given extremely high etch rate of the chemical, the uniformity results were as good as gold etch uniformity even if wafers were loaded in every other slot.

Conclusions The interaction of N2 bubbles, wafer rotation and chemistry works together to etch the metal

layer in the MEI Advanced Metal Etch Tank. The fine N2 bubble agitation provides interface mixing on chemical boundary layer constantly. This constant mixing also enhances the wetting of the wafer features ensuring a more consistent etch removal. The etch uniformity across the wafer is optimized by rotating the wafers and controlling the flow of the N2 sparger. In addition, the agitation and constant stirring of the wafer/metal chemical interface layer ensures consistent etch rates from wafer to wafer within the cassette regardless of slot position. By utilizing this right combination of hardware and software control capabilities the MEI Advanced Etch Tank achieves < 2% uniformity (coefficient variance) on 200mm gold micro-coil patterned wafers and < 5% (NU) on 200mm copper wafers, which are comparable to the much more expensive and physically larger alternative process tools previously described.