MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 1 of 11 TI 35ES image reversal resist Technical Data Sheet revised 10/2003 General Information MicroChemicals GmbH Schillerstrasse 18 D-89077 Ulm Fon +49 (0) 731 36080409 Mobil +49 (0) 178 7825198 Fax +49 (0) 731 1517024 Email info@microchemicals.com Internet www.microchemicals.com The TI 35ES resist is specially designed for the application in the so called image reversal technology for: subsequent lift-off of deposited layers with a thickness up to 5 µm plasma etching (very low resist erosion) Wet chemical etching in HF-solutions (for this application, however, we recommend TI 35E) The viscosity of the resist leads to a thickness range depending on the spin-speed from 2.5-3.5 µm. The typical aspect ratio of the structured features achievable is in the range of 1.6.. 2.0. This technical data sheet intends to give you a guide-line for process parameters for various applications. However, the optimum values for e.g. spin profile, exposure dose, or development depend on the individual equipment and need to be adjusted on each individual demand. For general information and trouble shooting on image reversal resists, consult or ask for our latest work sheet "Processing of image reversal resists" Zur generellen Prozessierung von Umkehrlacken und dabei häufig auftauchenden Fragestellungen konsultieren Sie bitte die aktuelle Version unseres Datenblattes "Prozessierung von Umkehrlacken" bzw. fordern Sie dieses ggfalls. an
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 2 of 11 'Image Reversal' A Short Introduction What 'image reversal' generally means soluble inert resist substrate (1) Exposure using an inverted mask (the exposed areas finally remain) (2) The resist now would behaves like an exposed positive resist. (3) The reversal bake cross-links the exposed area, while the unexposed area remains photo-active soluble (4) The flood exposure (without mask)...... (5) makes the resists, which was not exposed in the first step, soluble in developer (6) After development, the areas exposed in the first step now remain... and for what image reversal is good for: fi Adjustable undercut for lift-off of thin and thick sputtered, CVD, and evaporated films like metals, a-si:h, a-sin:h etc. fi fi High thermal- and chemical stability allow the inverted TI 35ES as mask for wet-chemical and plasma etching under harsh conditions
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 3 of 11 TI 35ES Fields of Application Lift-off of PECVD layers Amorphous Si layer deposited in device quality at 110 C Amorphous SiN x Lift-off of sputtered layers Due to non-directional deposition, lower thickness achievable (approx. 1 µm) Possibly ultra sonic treatment necessary Lift ability dependent on operating pressure Etching of SiN x in buffered HF SiN x deposited at 400 C with PECVD Hardbake at 125 C (Hotplate) for two minutes No visible degradation of the resist after 10 min etching time Enables structuring of SiN up to at least 100 nm (tested) Use ammonia during SiN X deposition to increase adhesion of the resist on the surface (For harsh chemical attack prefer TI 35 E) Etching of thick thermal SiO 2 with 20 % HF No visible degradation of resist after 10 min Possible etching in HF conc. (50%) for several minutes (For harsh chemical attack prefer TI 35 E) Direct mesa grooving TI series resists allow direct Si etching without SiO 2 masking Hardbake at 120-145 C for 2 min necessary Use HNO 3 (70%) : HF (50 %) : H 2 O = 75 : 10 : 25 as etching solution and TI 35ES resist in reversal mode, groove up to 4 µm in 45s (For deeper mesa grooving prefer the use of TI 35 E up to 15 µm possible) Technological Requirements Since the TI 35ES yields an inverted structure in the image reversal mode, an inverted exposure mask is needed. Beyond this, compared to standard positive resist processing, no further upgrade in existing technological infrastructure is necessary. Also compared to standard positive resist processing, applying the TI 35ES implements just two further process steps: The reversal bake and the flood exposure without mask, both very easy to be performed and explained in the following.
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 4 of 11 Processing the TI 35ES (in chronological order) Put the substrate on the hotplate at a minimum temperature of 120 C for 10 minutes to remove adsorbed water from the substrates surface. Alternatively, use a furnace at same temperature for 30 min. Standard HMDS procedure (only from vapor phase at an optimum substrate temperature of 125 C!) or recommended TI PRIME adhesion promoter is also an adequate preparation. Spin-coat the resist after cooling down the substrates, spin at the final speed level for 30-40 seconds. Softbake the coated substrate at 100 C for 2 minutes on the hotplate (when using a furnace, 100 C for 20 min is recommended) Exposure the coated substrate with the mask. This 1 st exposure dose adjusts the negative wall profile ( undercut ). 200 mj/cm² will be a good choice for most applications, lower 1 st exposure doses increase the undercut. A too low 1 st exposure dose will increase the erosion of the resist not to be cleared (see appendix). Note: Exposure doses refer to i-line (365nm). A standard mask aligner with a 350W Hg light source has approx. 6-15 mw/cm 2 i-line intensity, while in many cases 20-30 mw/cm² are measured meeting the total (g-, h- and i-line) intensity! Delay time: Keep the coated substrate at room temperature after the exposure for at least 10 minutes. In this delay time nitrogen, generated during exposure, will diffuse out the resist. If square shaped substrates are used, the resist thickness on the edges is significant thicker than 3-4 µm. In this case the N 2 needs more time to diffuse out, in this case double the delay time. After the delay bake the coated substrate at a temperature of 120.. 130 C on the hotplate for two minutes (when using furnace try 20 minutes at 115 C- 120 C. Because this step is very temperature critical furnace baking is not recommended). This step is the reversal bake where the image is reversed due to cross-linking of the exposed areas making them insoluble in the developer. Exposure the coated substrate for the 2 nd time without a mask (flood exposure). Use a dose (very uncritical) of 540 mj/cm 2 (300-800 mj/cm 2 has no dominant effect). When UV-light is present during a subsequent deposition (plasma coating, thermal evaporation), especially when the temperature raises beyond approx. 60 C, use a high flood exposure dose to avoid an exposure during deposition with bubbles/foaming as a consequence. Develop in AZ developer such as 826MIF (metal ion free) or potassium-/sodium based (AZ 400K 1:4) developer. When the structure is through-developed (cleared), add another 10-30% of the time in the bath of the total development time to finalize the side wall profile (see appendix). Hardbake the coated substrate only when using the resist as an etching mask under harsh conditions (For hard etch attack, prefer TI 35E). For mesa grooving hardbake at 135 C to 145 C for 2 minutes on the hotplate. The side-wall profile will loose the undercut during this step if the hardbake temperature is higher than the reversal bake temperature. In this case lift-off processes become more problematic. If you need a high hardbake temperature and subsequent lift-off or very high temperatures during deposition, make a UV-curing to harden the resist after development (or contact us for further information).
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 5 of 11 Processing the TI 35ES - Overview Substrate preparation TI PRIME recommended Resist thickness (µm) 2.6... 3.5 Exposure Broadband or g, h, i (mj/cm -2 ) Typical Exposure time (sec)* 100.. 350, 200 (typically) 10-35, 20 (typically) Delay 5 min.. 20 min dependent on resist thickness and substrate shape Reversal Bake Time (Hot plate Temperature) Flood exposure (mj/cm -2 ) (without any mask) Developer Hardbake 135... 145 C (only for hard etching, avoid it for lift-off ) Lift off media 2 min (120-130 C) 540 (typically) 300-800 Clariant AZ 826MIF, AZ 400K 1:4, AZ 351B 2 min NMP, Acetone, MMP, EEP, DMF, ethyl lactate, PGMEA Remover (Stripper) AZ 100 remover, acetone * i-line (365nm) exposure dose. A standard mask aligner with a 350W Hg source has approx. 6-15 mw/cm 2 i-line intensity, in many cases 20-30 mw are measured meeting the total (g-, h- and i-line) intensity!
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 6 of 11 Lift-off with the TI 35ES The adjustable undercut allows lift-off of metal films with thicknesses exceeding the resist thickness 5 µm Figure 1: Resist with typical undercut on a Si substrate Firgure 2: Metal layer with 4.5 µm in thickness, evaporated and structured via lift-off. Aspect ratio more than one.
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 7 of 11 Dry-etching with the TI 35ES Extraordinary stability of TI 35ES in reversal mode (resist erosion by a factor of approx. 2 reduced as compared to most positive resists) allows dry etching of several µm of SiO 2 Both REM pictures: TI 35ES in reversal mode No plasma etch residual remover Etching of 3.2 µm SiO 2 with CHF 3 + O 2 Resist SiO 2 MicroChemicals gratefully acknowledges Mesa+ Research Institute, University of Twente, Netherlands for verifying TI 35ES results and providing the pictures shown in this data sheet.
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 8 of 11 Process Parameters and Undercut 1 st exposure dose A1 A2 C1 A1 A3 Development B1 B2 C2 B1 B3 A high 1 st exposure dose (A1) yields - after reversal bake, flood exposure, and development - to a steep resist profile with minor undercut (A2), while a small 1 st exposure dose (B1) not exposing the substrate-near resist layer leads to a strong undercut and sometimes to peeling of narrow resist structures in the developer (B2). Die optimum 1 st exposure dose therefore depends on the desired undercut and the minimum lateral feature sizes. At the beginning of new processes, an exposure series is recommended. Reversal bake Before the reversal bake is applied, the exposed resist needs a certain time depending on the resist type and thickness to outgas nitrogen (N 2 ) formed during exposure. This will avoid bubbling (irregular developed structures (C2)) and foaming (C1) of the resist by thermally activated N 2. Nitrogen preferentially accumulates near locations with inferior resist adhesion to the substrate, which has to be optimised The undercut forms in the last stage of development when the structures are already cleared (see p. 10). We recommend an approx. 30% over-developing.
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 9 of 11 Adjusting the undercut of TI 35ES TI 35ES (thickness 3.5 mm, reversal bake time 1 min) parameter study I (varying 1st exposure, reversal bake temperature, development time) AZ 726MIF Development time conclusion fl 1st exposure 100 mj/cm 2 Reversal Bake Temperature = 137 C 130 C 123 C 115 C Feasible only for very short develop. time or very high reversal bake temperature 200 mj/cm 2 137 C 123 C 115 C Wide process window allows adjustment of undercut 300 mj/cm 2 137 C 130 C 123 C Very stable conditions for undercut Please note: The gap widths are different for the individual pictures. This series intends to give a survey of the undercut process window.
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 10 of 11 Adjusting the undercut of TI 35ES TI 35ES (thickness 3.5 mm) parameter study II (varying the development time) 1 st Exposure 200 mj/cm 2 Reversal bake at 126 C for 1min 2 nd exposure 1.200 mj/cm 2 Development in AZ 726MIF 2 s 16 s 3 s 19 s 4 s 22 s 5 s 25 s 7 s 29 s 9 s 37 s 11 s 46 s 13 s Please note: The gap widths are different for the individual pictures. This series intends to deliver insight in the evolution of the undercut with development time at given process parameters.
MicroChemicals TI 35ES technical data sheet revised 10/2003 p. 11 of 11 Trouble Shooting Bubbles after coating (but before exposure) If the resist has been refilled before coating, a delay time for at least a few hours may be necessary to outgas air bubbles. If the substrate is textured, a spin profile starting with approx. 500 rpm for 5 seconds helps to completely coat the substrate without forming comets before the final spin speed determines the desired resist thickness. Bubbles after 1 st exposure (but before reversal bake) Nitrogen (N 2 ) formed during the exposure of the resist normally diffuses out of the film. Bubble formation during exposure may be caused by: A remaining solvent concentration too high with expansion of N 2 preferentially in substrate-near regions as a consequence. In this case, increase the softbake time (e.g. 100 C per µm resist film thickness) which, however, will increase development time, but reduce dark erosion. A sub-optimum resist adhesion to the substrate. In this case we recommend the application of TI PRIME adhesion promoter. Bubbles during reversal bake Nitrogen (N 2 ) formed during the 1 st exposure needs time to diffuse out of the resist film, strongly dependant from the film thickness, the temperature, and the first exposure dose. In this case, increase the delay time between exposure and reversal bake, and/or perform the delay at a moderately elevated temperature (approx. 30-50 C) Bubbles/holes after development I) Either the bubbles already have formed during any previous process step (coating, exposure, reversal bake) and finally become visible after development. In this case, consult the previous sections. II) Or the bubbles represent spatial inhomogeneous dark erosion. This points towards a 1 st exposure dose or/and reversal bake temperature/time too low. General information on processing of AZ and TI resists in our work sheets Processing of image reversal resists Reproducible litho-processes Baking and delays Exposure and development Processing of thick resists available on request.