single_layer_lift-off

Single Layer Lift-Off: Image Reversal of Positive Resist, to create a negativesidewall profile for Clean Lift-OffDate: 08/22/2008Revisionist:Paul KimaniRevision #: 1In this process, we use single layer positive resist S1805 to enable a lift-off of 1000 Å of Au on 100 Å of chrome evaporated with e-beam on a patterned wafer.Background on Image Reversal for positive photoresists1With an image reversal technique, the exposed area remains after the process. This is as if you are using a negative resist. Therefore, in designing your mask pattern, you need to create a pattern that is the inverse of the one you desire.In a standard positive photoresist process, if you expose the photoresist as depicted above, the exposed area becomes more soluble in developer and would be washed away if you develop at this point.The addition of a reversal bake in an ammonia environment cross-links the exposed area. The unexposed area remains photoactive. The YES-310 oven uses anhydrous Ammonia (NH3) for the cross-linking process.A flood exposure then makes the previously unexposed resist to now be soluble in developer. After development, the areas exposed in the first step are now retained. An undercut profile will result when over-development is performed. A 30% over-develop is recommended (after the features are formed leave wafer in developer for another ⅓ of the time used to create a good undercut). A poor undercut makes it harder to remove unwanted resist during lift-off.Process in the lab1.Dehydrate a bare silicon wafer at 120 °C for at least a minute.2.Expose the wafer to HMDS vapor for 5 minutes, to promote adhesion.3.Spin S1805 at 3000 rpm for 30 seconds.The resulting resist thicknesses are recorded on the above image (measured using the Nanospec and a refractive index of 1.65 for the resist).4.Expose under MABA6 (12 mJ/cm2 output) for 4.5 s. This exposure energy is atleast 50% higher than a normal exposure would be for patterning if no imagereversal is being undertaken.5.Reverse the image by running the wafer in the YES 310 oven using the standardimage reversal program.6.Flood-expose the wafer under Oriel UV light for 14 minutes. At the 7 minutemark, change the orientation of the wafer by approximately 90° after blowing the surface with nitrogen/air gun. The Oriel UV energy is variable and depends on the age of the lamp. The 14 minutes duration has been found to be adequate under most circumstances.7.Develop the wafer in 1: 6 351 developer: DI water mixture, for 35 seconds.Alternatively, watch how long it takes for the developer to clear the features, and then add approximately 30% more time. This extra duration is necessary for a prominent undercut to form. A good undercut makes for a clean lift-off, withminimum problems in clearing the resist from small features. In this example, during the lift-off soaking after deposition, resist cleared from 2 µm features in less than an hour. Overdevelop also minimizes the need for ashing (in oxygen plasma to clean the features), which may cause the undercut to collapse in thicker resists.8.Deposit 100 Å of chrome as adhesion layer and evaporate 1000 Å of gold on topof the chrome. The metal layer thickness was approximately ¼ the thickness of the resist.9.Lift-off unwanted material by soaking the wafer in 1165 resist removermaintained at 80 °C for approximately 3 hours. An overnight soak in acetonehelps to clean the wafer further.Figure 1: Broad view of features after lift-offFigure 2: 2 µm serpentine features. The lift-off process leaves very little residue, thus providing clean features necessary for further downstream processing.Figure 3: Analysis of features obtained from lift-off using AFMThe images show that the width of the serpentine metal lines are wider than the 2 µm (it is ~ 2.5 µm), while the width of the spaces are less than 2 µm. In the design of the mask, if the goal is to obtain an even pitch of say 4 µm (2 µm spaces and 2 µm lines), it would be necessary to include some bias, i.e. decrease the width of the chrome on the mask by 0.5 µm and increase that of the space to 2.5 µm.The areas of photoresist under the chrome are not exposed during first exposure. They are flood exposed after image reversal. Over-development is applied to increase the width of subsequent features, as well as form the undercut, but this increase will have been counteracted by the bias included in the mask design, (an equivalent decrease in the width of the light absorbing chrome region).Figure 4: Analysis of features obtained from lift-off using Confocal Microscopy1 Adopted from: "image reversal, resists, semiconductor, structuring". Application notes for image reversal resists; Dr. Christian Koch - MicroChemicals GmbH。