KR19990067493A - RF transformer using multilayer metal polymer structure - Google Patents

Abstract

Transformers for high frequency applications having a multilayer thin film structure and a dielectric material disposed between the coils increase the inductive coupling between the coils and at the same time reduce the capacitive coupling between the coils. A first metal spiral 301 is disposed on the glass substrate 201, and a polymer insulator 601 is deposited on the glass substrate 201 to enclose the first metal spiral. A second metal spiral 701 is deposited on the polymer layer and the process continues with the next polymer material. The interconnection of metal spirals in different layers is effected by the use of selectively etched grooves with vias 902 therein. The present invention specifically provides a balun transformer. The use of polymers as dielectrics, preferably BCB, has an extremely low loss of functional benefits due to absorption of microwaves and rf frequencies, which significantly reduce capacitive coupling while allowing good inductive coupling between the various layers of the instrument. Let's do it.

Description

RF transformer using multilayer metal polymer structure

The present invention relates to a transformer for high frequency applications made by standard etching and deposition techniques.

Advances in high frequency and functional integrated circuits have led to interconnect and package technologies that have become limiting factors for system performance. Thus, the focus has been on system performance by promoting the interconnection and packaging of the mechanisms of the system.

In the range of rf, microwave and millimeter wave technology, the performance of the system has been improved by various means. The present invention focusing on the transformer will be described in comparison with the prior art. To this end, recent technologies in transformers and inductors have gone through thin film technology. The basics of the thin film technology applied to the transformer are as follows. In general, a dielectric substrate has a metal pattern deposited thereon to implement an inductor required for a transformer. Then, a dielectric layer is deposited. Transformers fabricated using thin film technology are known to have first and second coils in multi-level and parallel configurations. The driving factor in the design of the transformer is the Q factor of the system. This Q factor is a measure of energy conversion efficiency, where the Q factor is given by

Q = ω L / R

Where ω is the angular frequency, L is the total inductance and R is the resistance of the coil. As can be appreciated, the Q factor can be increased by increasing L and decreasing R to the maximum possible range for a given value of ω.

In thin film transformers and other transformers, the time dependent variation of the current in the first wire section or coil eventually becomes an induced current in the second wire section. This induced current is proportional to the number of turns of the second coil. In addition, the larger the number of turns of the first coil, the greater the magnetic field density generated by the first coil, leading to a larger induced current in the second coil. Therefore, in a transformer that induces current by the mutual induction between the second coil and the second coil, the larger the number of windings of the coil is, the greater the magnetic field strength generated by each coil is, so that the inductance between the coils is increased. Thus, the coupling between the coils and the corresponding energy conversion is increased.

An increase in L requires an increase in the number of turns of the first and second coils among other factors, which often serves to increase the area of the transformer itself. In addition to the obvious need to reduce the required space of individual elements of the circuit, such an increase in area also serves to reduce the high speed performance capability. In addition, the increase in the number of turns to increase the inductance increases the length of the conductor by increasing the resistance and lowering the Q value.

Thus, there is a need to provide a transformer with increased inductance and reduced resistance to optimize the Q factor at high frequencies.

An example transformer based on thin film technology is described in US Pat. No. 5,420,558 to Ito et al. The description of Ito et al. Shows a number of different types of transformers based on thin film technology. However, the transformers cited in Ito et al. Show the use of silicon dioxide (SiO ₂ ) as the dielectric material between the coil layers. SiO ₂ is limited to its application in thin film transformers due to its process thickness limitations. When using thin film deposition technology, SiO ₂ can be deposited to a layer thickness of 2 μm. The deposition of more than this amount of SiO ₂ eventually exerts an excessive mechanical load on the thin film, which adversely affects the structural reliability. Disadvantages of using SiO ₂ as an insulator are, among other disadvantages, a relatively high dielectric constant and the inability to provide a larger layer than mentioned above. To this end, the capacitance between the transformer turns of the different layers of a vertically stacked transformer is directly proportional to the dielectric constant and inversely proportional to the distance between the turns (the thickness of the magnetic coupling between the transformer coils), while the electrical coupling to capacitance Sex bonding is undesirable. For this purpose, since all inductors with time varying currents have inherent capacitance, the larger the capacitance, the smaller the bandwidth of functional use. Particularly at radio and microwave frequencies, capacitive coupling results in an equivalent circuit consisting of an inductor and a passive electronic resonator in parallel with the intrinsic capacitor. Resonance of the LC circuit tends to finally radiate losses, which eventually avoids resonance.

Therefore, in the case of an inductor or a transformer, it is required to increase the resonance band of the resonator by more than the usual frequency, thereby increasing the band for the transformer as much as possible to greatly reduce the intrinsic capacitance. In order to reduce the effect of capacitive coupling of the transformer as much as possible, it is desired to increase the distance (and thickness of the insulator) between coils at different levels and to reduce the dielectric constant of the insulator. Therefore, what is desired is to obtain a transformer having a certain small dimension and performing a certain inductive coupling between the coils but not sensitive to the capacitive coupling between the coils.

According to the transformer described above for high frequency applications, the transformer is described as a multilayer thin film structure having a low dielectric constant material between the coils which minimizes capacitive coupling between the coils of the transformer.

To this end, a first metal spiral is deposited on the glass substrate. After this deposition, a layer of polymer insulator, preferably benzocyclobutyne (BCB), is deposited to enclose the first metal helix. A second metal helix is deposited on the BCB layer and the necessary interconnects are effected by the use of vias selectively open to the BCB. This process of metal spirals, selected by BCB and interconnected by rig vias, continues until the desired number of turns of the first and second coils is obtained. Importantly, the described transformer comprises three metal windings, a substrate, a first metal coil winding, a first polymer layer, a second coil winding layer, a second polymer layer, a third coil winding layer and a third polymer layer. Have Depending on the desired circuit, it may have more or fewer layers. That is, this manufacturing process is also applicable to other circuits and structures.

The use of a polymer, preferably BCB, as the dielectric between the coils has functional rf and microwave frequencies, and this insulator allows good inductive coupling while greatly reducing capacitive coupling. In addition, the polymers make use of process steps in the manufacture of transformers, conferring ease and reliability of manufacture. To this end, the polymers utilize standard large-scale IC processing techniques to allow for relatively smooth metal deposition and easy processing.

The transformer finally obtained through representative material and processing steps has very small dimensions and is capable of large scale manufacturing. The adverse effects of eddy current elements such as capacitance with reduced operational transformer frequency at rf and microwave spectra are easily realized.

It is an object of the present invention to provide a high frequency, low cost, compact transformer manufactured in large quantities.

It is a feature of the present invention to provide a multi-layered vertical transformer selectively interconnected by vias.

It is a feature of the present invention to provide a transformer having a low capacitive coupling and a high inductive coupling between transformer windings, and to provide a low loss insulator at the high frequency of the transformer.

An advantage of the present invention lies in a transformer made of a material that can be easily applied to conventional processing techniques.

The present invention will now be described with reference to the accompanying drawings.

1 is a perspective view of a balun transformer;

2 is a plan view of a balun transformer made by the present invention.

3-10 are cross-sectional views illustrating manufacture as a continuous step procedure selected by the present invention.

11 is a schematic circuit diagram of a mixer using a balun transformer;

As mentioned above, the present invention relates to a transformer fabricated using a multilayer treatment technique with a polymeric dielectric material, and to a transformer having the advantages of dimensions, cost and reliability of equipment and ease of manufacture on a large scale. All transformers mentioned in the present invention are used in multi-layered devices, the scope of the present invention being balun transformers, directional couplers, spherical hybrids, and power dividers. A feature of the present invention lies in its ability to realize various circuits by applying the basic process of the present invention. That is, by selective placement by metal lines, vias, bond bands and other necessary elements, the processing procedure enables the fabrication of instruments using high frequency with the advantages of high inductive coupling and low capacitive coupling. .

An embodiment of the present invention will be described as a balun transformer. As mentioned, the present invention is also applicable to other applications, and the balun is described for illustrative purposes only. To this end, the circuit of a given transformer is implemented by circuit elements such as spirals, vias, bonding and capacitors by the process of the invention. Clearly, certain interconnections are implemented by selective placement of devices, vias, and circuits to the extent that those skilled in the art can readily manufacture. An overview of a typical balun transformer is shown. The input of the single end of the transformer to be handled is connected to a pair of outputs having the same amplitude and opposite phase. To this end, when voltage V is applied to the input terminal 101 having a single end, the voltage is the output value at the first output terminal 102 and the same voltage of opposite polarity is located at the second output terminal 103 position. Is the output value of. For time varying inputs, the outputs of terminals 102 and 103 have opposite phase values. For an unbalanced input signal on the ground plane (the reference terminal with zero potential to ground), the instantaneous output average voltage is therefore zero. In contrast, a balanced signal applied across two output terminals appears as an unbalanced signal across the input terminal and is therefore referred to as a "blun". Balun transformers are available for single and double balanced mixer applications as well as for other heterodyne transceiver systems.

2, a plan view of a balun transformer produced by the process of the present invention is shown in this figure. For this purpose, an input bond pad 201, an input ground pad 202, an output ground pad 203 and an output bond pad 204 and 205 lie on the surface of the substrate. The transformer windings of FIG. 2 are those of the upper layer of the transformer, which are located directly on the windings of the lower two layers of the transformer (not shown). This structure constitutes a certain number of turns which elevates a high induction value, and the current crow The ding does not sacrifice Q value. In other words, if the opening passing through the plane of the spiral portion is reduced too much, there is also a corresponding decrease in the Q value. Thus, the benefit of reduced dimensions is disadvantageous for the reduction of the Q factor for the above reasons. Finally, the number of turns actually increases the inductance, or the capacitive coupling between the two layers, which decreases with the autotuning frequency of the transformer. Thus, the number of turns must be determined by balancing the benefits of increased inductance against damage of increased capacitive coupling.

Details of the above descriptions are published in the April 1995, International Conference on Multi-Chip Modules, in Processing and Electrical Characterigation of Multilayer Metalligation for Microwave Applications.

FIG. 10 shows a transformer in cross section taken along line XX 'of FIG. 2. To this end, the transformer windings are shown in three layers, labeled 1001 to 1011, and the medium 1012 provides an interconnect between the layers of the transformer.

Referring again to FIGS. 3-9, the fabrication procedure of a representative embodiment of the balun transformer of the present invention is described. As mentioned above, other transformers are possible and the balun transformers are mentioned for illustrative purposes only. Glass substrates preferably have borosilicate dielectric constants of about 4.1 and loss tangents of magnitude 0.002 at microwave frequencies. An amorphous glass substrate shows the metal deposition for the bond pads 301 of the balun transformer and the capacitor 302 to add fine line deposition for the first metal layer. All of the metal layers of the present invention are made by canonical lift-off techniques, eliminating the drawbacks of undercutting and the need for metal etching that occur through etching. This lift-off technique requires the use of an appropriate photoresist with an intaglio profile, and standard evaporation techniques are used to deposit the metal. The negative profile of the photoresist is obtained by inverting the polarity of the post-xposure bake and the resist. Since the evaporated metal is deposited in a direction perpendicular to the substrate surface, the negative profile of the resist prevents breakage between the metal on top of the resist and the metal on the substrate. The metal of the resist is removed with a water jet, and the resist is shaved leaving a clean metal pattern on the substrate. The metal wire of the transformer of the present invention is 25 microns wide and has a thickness of 2-5 microns. The metal layer of the first layer is preferably used for the wire bond pad 301, the lower capacitor plate 302, and the in-pass conductor as Ti-Pt-Au-Ti. Ti is used to enhance adhesion, Pt is used as the diffusion economic layer and Au is used for compatibility with standard wire-bond processes. For the coil windings and the capacitor top plate, the preferred metal is Ti-Pt-Ag-Dt-Ti. Although not highly desirable, gold can also be used instead of silver, but affects the processing price. In addition, other metals may be used as these metal wires.

Along with the deposition of the first metal layer shown in Fig. 3, an appropriate dielectric 401, preferably silicon nitride, is performed. As shown in FIG. 4, the SiN film serves as an insulator of the capacitor 302 and for crossover. SiN films are deposited by Plasma Enhanced CVD (PE CVD) at 300 ° C. Standard photolithography and plasma etching techniques are then used to etch the SiN film 401 where electrical contact is required. After the first metal layer is completed, the metal transformer winding 501 of the first layer of the transformer is implemented by the above technique.

After completion of the SiN film deposition, the first layer of polymer insulator 601 is deposited. The bilayer can also be a material such as photosensitive or non-photosensitive polyimide, but preferably BCB. After the BCB layer is deposited, vias for intermediate contact and bond pad connections are performed. The deposition of BCB and via etch was published on November 7, 1993, Issue 7 vol. "Processing and Mierowave Characteriqation of Multilevel Interconnect Using Benzocyclobutene Dielectric" by IEEE Transactions on Conponents, P. Chinoy and Janes Tajadod of Hybrids and Manufacturing Technology, 16. It is implemented by the technique described in "Processing and Electrical Characteriqation of Multilayer Metalliqation For Microwave Applications." As mentioned earlier, the BCB layer has a relatively low value and a dielectric constant of 2.65 magnitude and the advantages for the transformer are as mentioned above. The BCB layer may be 20 microns thick but is preferably deposited to a thickness of 3 microns. In addition, BCB has a low humidity coefficient (size of 0.1%) in addition to the advantages mentioned in the paper, Chinoy et al., And guarantees electrical properties that do not occur in the environment where the electrical properties are compatible with Ag metallization, which is highly reactive to moisture. Has the advantage.

Finally, these two types of BCB are of interest as described herein. The first invention is photosensitive as if via 602 is exposed to light and can be patterned immediately, and second non-sensitized via 602 is plasma etched through a photoresist or metal mask. to be. The former is easy to process and the latter is given good planarization and thick coating. Good planarization allows finer metal lines and thicker layers reduce capacitive bonding. In the present invention, a representative technique is to deposit an upper layer of non-photosensitive polymer, preferably BCB, in order to planarize the lower two layers of BCB polymer, which is easy to process, and the overall structure of the transformer.

The remaining layers of the transformer show the complete metallization of the technique. As shown in FIG. 8, the next layer of BCB 801 is deposited and etched via 802 is executed. Finally, the third layer of the winding is deposited as shown in FIG. 9, the vias are metallized as 902 and the last layer of BCB 1013 is etched as in 1014 as shown in FIG. 10. And is deposited. Metallized vias 303 and 902 etched into layers 601 and 801 provide electrical connections between coil windings 501, 701 and 901 as required as circuits in the transformer, respectively. Vias 1014 show how to open a wire bond pad and easily perform dishing or sawing of a wafer on each die or circuit. Finally, the ground plane 1015 of the transformer is implemented by the deposition of metal on the bottom surface of the glass substrate.

The balun transformer of a representative embodiment is made as follows. A layer of SiN is deposited on the lower plate and the bond pad 301 of the capacitor 302. Then, the coil winding 501 and the top plate of the capacitor 502 and the crossover 503 are deposited. This forms the V-output 103 of the transformer. Resonant capacitors are newly formed where the coil winding 501 and the cross-over 503 rest on the bond pad 301 and the lower plate of the capacitor 302. Metal coil 701 is then deposited to execute the input windings by via 3030.

Finally, via 902 runs V + output winding 102. 11 shows a typical use of a balun transformer in a transceiver circuit. For this purpose, the balun 1104 is connected to a silicon schottky ring quad. In operation, the high frequency signal from the rf input 1102 is mixed with some low frequency from the local vibrator 1105 and extracted and filtered the multi-frequency region at the IF output 1106 to extract the predetermined signal output. In practice, the packaged balun is installed on a common substrate such as BCB and connected to where necessary for other active, passive components on the common substrate.

Claims (8)

The first metal spiral having a substrate, a first metal spiral portion deposited on the substrate, a dielectric material layer deposited on top of the substrate and the first metal spiral portion, and a second metal spiral portion deposited on top of the dielectric layer; In the transformer and the second metal spiral portion is selectively electrically connected, The dielectric material is a one-layer polymer insulator. 2. The transformer of claim 1 wherein said polymer insulator is benzocyclobutyne. 2. The transformer of claim 1 wherein the polymer insulator has a loss tangent of approximately 0.002 and a dielectric constant of approximately 2.65. The transformer according to claim 1, wherein the dielectric layer is made of photosensitive benzo cyclobutene and a non-photosensitive polymer layer is deposited on the benzo cyclobutene and the second metal helix. 10. The transformer of claim 1, wherein electrical contact between the metal spirals is effected through selectively opened vias. 6. The transformer of claim 5, wherein the thin polymer insulator layer has a dielectric constant of approximately 2,65. 6. The transformer of claim 5, wherein the polymer insulator has a loss tangent of approximately 0.002. 5. The transformer of claim 4, wherein the non-photosensitive polymer layer substantially surrounds the second coil.