A process of forming a conductor pattern.

0001This invention relates to a process of forming a conductor pattern on a substrate by a metal lift-off technique.

0002U.S. -A- 3,964,908 discloses the use of resists containing dimethyl­glutarimide units. U.S. -A- 4,524,121 refers to positive photoresists containing preformed polyglutarimide. U.S. -A- 4,568,411 describes a process for metal/semiconductor deposition using a two-layer photoresist mask. U.S. -A- 4,569,897 describes a negative photoresist composition with polyglutarimide. For the purposes of that invention, the poly­glutarimide has a molecular weight from 2,000 to about 500,000. U.S. -A- 4,606,998 describes a barrierless high-temperature lift-off process using two layers of polyimide.

0003It should be noted that none of the above cited prior art shows the sequence of steps required by the process of the present invention. In particular, none of the prior art shows a process, in which the PMGI is not exposed to radiation.

0004Metal lift-off has traditionally been achieved through producing undercut profiles in resist films. Undercut profiles are typically generated with use of a negative resist opaque at the exposing wave­length, image reversal of a positive photoresist, or with the use of bilayer or trilayer schemes consisting of a thin cap and a thick bottom layer. These systems are useful when metals are deposited by evaporation. However, sputter deposition results in a serious fencing problem, i.e. side walls of the undercut structures are coated with metallic films, which make the lift-off very difficult and the metal edge definition very poor. The multilayer systems mentioned above are quite complex and involve many coating steps and multiple exposures or the use of etching tools. Furthermore, they do not avoid the fencing of sputter-deposited metals.

0005The present invention seeks to provide a simple process for metal lift-off that is devoid of fencing and achieves clean lifting without difficulty.

0006The invention provides a process of forming a conductor pattern on a substrate comprising the successive steps of (a) coating the substrate with a layer of low molecular weight poly(methyl glutarimide); (b) overcoating the poly(methyl glutarimide) layer with a layer of photoresist; (c) exposing the photoresist to radiation through a mask; (d) developing both the photoresist and the poly(methyl glutarimide); (e) depositing a conductor; and (f) lifting off the remaining portion of the poly(methyl glutarimide) by means of a solvent.

0007For use in the present invention, the PMGI should preferably have a weight average molecular weight (polystyrene as a standard) within the range of 3,000 to 40,000. The choice of the molecular weight depends on the depth of undercut desired for specific applications, which is also a function of bake temperature and time as well as of developer strength and development time. Thus, it is possible to produce desired undercut by varying the above-mentioned parameters. An absolute weight average molecular weight of approximately 30,000 (19,000 as determined by GPC with polystyrene as a standard) is most preferred in the examples given below. Such low molecular weight material is commercially available from the Shipley Company. Alternatively, high molecular weight PMGI could be used if blanket deep UV exposure is performed to lower the molecular weight to the desired range prior to coating the photoresist.

0008How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:-­ <ul id="ul0001" list-style="none"><li>Figs. 1 - 4 illustrate cross-sections, not to scale, of the structures formed during the steps of a process embodying the invention.</li></ul>

0009Fig. 1 shows a substrate (1) coated with a layer of PMGI (2) and then with a photoresist (3). The resist is exposed to radiation through a mask. Both the resist and the PMGI are then developed, leaving the structure as shown in Fig. 2, with the PMGI undercut below the resist. Conductor metal is next deposited, e. g. by sputtering, leading to the formation of a covering (4) for the substrate and for the resists as shown in Fig. 3. Finally, lift-off is carried out using, for example, an organic solvent or aqueous alkali. The result is shown in Fig. 4, wherein the substrate has been selectively coated with a conductor without any fencing problems.

0010The top imaging resist includes aqueous base developable photo­resists either positive or negative working. Such examples are diazonaphthoquinone/novolac positive photoresists, the dual tone resist systems described in U.S. -A- 4,491,628, and the image reversal negative resists described in U.S. -A- 4,104,070.

0011The imaging radiation is not limited to UV light but electron beam, x-ray, or ion-beam can be employed.

0012The process described in this specification is useful in the metal lift-off employing sputter deposition and is particularly useful when a long metal edge taper is desired for better step coverage. In addition, application of the invention provides a simple method of producing well defined metal patterns when evaporation or evaporation/sputter hybrid deposition is employed. Thus, the process described herein is useful when metals are deposited either by evaporation or by sputtering.

0013The thickness of the imaging layer determines the degree of taper when sputtering is employed. A thicker imaging layer results in a longer taper.

0014The thickness of the PMGI layer depends on the desired thickness of deposited metals. The PMGI layer should be slightly thinner than deposited metals for better metal edge definition and ready lift-off when metals are deposited by sputtering. In the case of evaporation, the PMGI layer should be thicker than the deposited metals.

0015In a preferred variation of the present invention, the process may be adapted to define a magnetic recording read track width. The process of the present invention leads to a beveled lead track (BLT). In the process described above, the total development time is the sum of the resist development time (t1) and the PMGI underlayer development time (t2). The undercut into the PMGI layer is a linear function of t2 time and thus, proportional to the overall development time since for a given resist system and a specific dosage, the resist endpoint time or t1 is constant. Thus, long development times (more than five minutes) generate a long undercut which is good for easy resist lift-off but bad for track definition due to a long conductor taper. A short development time (less than five minutes) generates a short undercut which is good for track definition but bad for resist lift-off. The process of the present invention can be adapted by a compromise in which a short undercut is achieved in the critical track region and a long undercut in the non-critical region in the conductor pattern. We have called the scheme the "beveled lead track" (BLT) process. The way to achieve different undercuts in a single patterning layer is by having the ability to vary the development time in different regions of the conductor pattern. In the BLT process, this is done by using two different masks to define the overall conductor pattern. The first mask defines non-critical regions of the conductor and the second mask defines the critical track region of the device. Both masks are exposed onto a single bilayer resist structure. In most resist systems the development endpoint (t1) is inversely proportional to the exposure dose. The t1 times at 400 and 150 mj/cm² are one and three minutes, respectively. When mask 1 (non-critical) is exposed at 400 mj and mask 2 (critical) is exposed at 150 mj and the total development time is five minutes, the PMGI development times (t2) will be four and two minutes, respectively. The resulting undercut in the low exposure dose will be 0.4 µm. In the high exposure area, the undercut will be 0.8 µm. The short undercut provides good track definition while the long undercut aids the resist lift-off step.

0016The following Examples are given solely for purposes of illust­ration and are not to be considered limitations of the present invention.

Example I

0017The following gives the process details for carrying out the invention in a preferred fashion. <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Prebake</entry><entry namest="col2" nameend="col2" align="left">5 min., 92°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Apply Resist</entry><entry namest="col2" nameend="col2" align="left">PMGI, 4000 RPM, 150nm thickness, spin 30 sec.</entry></row><row><entry namest="col1" nameend="col1" align="left">Soft Bake</entry><entry namest="col2" nameend="col2" align="left">Hot Plate, 30 min., 185°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Apply Resist</entry><entry namest="col2" nameend="col2" align="left">Diazoquinone photoresist, 3000 RPM, 0.7µm thickness, spin 60 sec.</entry></row><row><entry namest="col1" nameend="col1" align="left">Soft Bake</entry><entry namest="col2" nameend="col2" align="left">Hot Plate, 30 min., 92°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Expose</entry><entry namest="col2" nameend="col2" align="left">180 mJ/cm2</entry></row><row><entry namest="col1" nameend="col1" align="left">Develop</entry><entry namest="col2" nameend="col2" align="left">MP-2401, 1:5, 4.0 min., 21°C±1°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Spray Rinse</entry><entry namest="col2" nameend="col2" align="left">5 cycles</entry></row><row><entry namest="col1" nameend="col1" align="left">Spin Dry</entry><entry namest="col2" nameend="col2" align="left">Semitool</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Inspection</entry><entry namest="col2" nameend="col2" align="left">Scope, alignment and residual resist</entry></row></tbody></tgroup></table></tables>

0018This process yielded 0.4 µm lateral undercut. With this stencil, a metal thickness of 200nm was sputter deposited.

Example II

0019The following gives the process details for the beveled lead track process. <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Prebake</entry><entry namest="col2" nameend="col2" align="left">Hot Plate, 5 min., 92°C,</entry></row><row><entry namest="col1" nameend="col1" align="left">Apply Resist</entry><entry namest="col2" nameend="col2" align="left">PMGI, 4000 RPM, spin 30 sec.</entry></row><row><entry namest="col1" nameend="col1" align="left">Soft Bake</entry><entry namest="col2" nameend="col2" align="left">Hot Plate, 30 min., 185°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Apply Resist</entry><entry namest="col2" nameend="col2" align="left">Diazoquinone photoresist, 2750 RPM, 0.7µm thickness, spin 60 sec.</entry></row><row><entry namest="col1" nameend="col1" align="left">Soft Bake</entry><entry namest="col2" nameend="col2" align="left">Hot Plate, 30 min., 92°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Expose</entry><entry namest="col2" nameend="col2" align="left">Mask 1, 300 mJ/cm2 on PE center focus</entry></row><row><entry namest="col1" nameend="col1" align="left">Develop (Optional)</entry><entry namest="col2" nameend="col2" align="left">MP-2401, 1:5, 2.0 min., 21°C±1°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Expose</entry><entry namest="col2" nameend="col2" align="left">Mask 2, 180 mJ/cm2 on PE center focus</entry></row><row><entry namest="col1" nameend="col1" align="left">Develop</entry><entry namest="col2" nameend="col2" align="left">MP-2401, 1:5, 2.5 min., 21°C±1°C</entry></row><row><entry namest="col1" nameend="col1" align="left">Spray Rinse</entry><entry namest="col2" nameend="col2" align="left">5 cycles</entry></row><row><entry namest="col1" nameend="col1" align="left">Spin Dry</entry><entry namest="col2" nameend="col2" align="left">Semitool</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Inspection</entry><entry namest="col2" nameend="col2" align="left">Scope, alignment and residual resist</entry></row></tbody></tgroup></table></tables>

0020This process yielded a lateral undercut of 0.3 µm in the critical regions, and 0.5 µm in the non-critical areas.