SE516361C2 - LC tank formed on a low resistivity substrate for use in resonators used in microwave filters, oscillators etc., has adjacent strips leading current in opposite directions and arranged so that substrate currents balance out - Google Patents

Abstract

An LC tank is formed on a lossy substrate with adjacent strips leading current in opposite directions and arranged so that substrate currents relating to individual strips (1) induced in the substrate (3) balance one another out leading to high Q-values. A pair of back to back coupled diodes (18,19) are between the tank input terminals and a pair of DC leads (16,17) are arranged between the midpoint of the inductor structure and the midpoint between the diodes.

Description

20 25 30 35 516 361 2 o u: n u ø v I »n o oo luster in the polysilicon capacitor and the inductive part of the tank. US-A-5 959 515 forms the basis for the preamble of claim 1.

The losses for a given tank can be expressed using the Q-factor, which in a simple case can be considered as the ratio between the conserved magnetic energy and the losses. In particular, losses in the substrate reduce the Q value of known components.

Silicon, which has many good properties, including a relatively low price, is not an ideal material for substrates in LC tanks, as its loss properties are in the range 0.0001 - 20 Qm. The relatively low resistance of this material causes eddy currents to be induced in the substrate, which in turn gives rise to losses in the inductor or tank.

Tank losses can be overcome by means of semiconductor substrates such as gallium arsenide or other substrates with high resistivity, p> 100 Qm, but these substrate materials are relatively expensive.

At microwave frequencies, the total losses become the sum of the resistive loss from the currents flowing along the bands and the dielectric losses in the surrounding dielectric, for example in the substrate.

The losses and the total efficiency of the inductance depend not only on the geometry and the materials, but also on how the inductors are connected in the current application. These effects shall be briefly discussed below, with reference to appropriate models for an inductor structure with certain inherent characteristics.

Figure 1 shows a single loop on a dielectric or semiconducting substrate, possibly with a ground plane on the back of the substrate. Figure 2 shows the inductor from the 1 clock 1 in cross section.

Figure 3 shows a known meander structure, which requires a ground plane for the return current. Figure 4 shows the inductor from fi clock 3 in cross section. The connection points of the inductor can be connected in different combinations.

Figures 5, 6 and 7 show three main models that correspond to different ways of connecting the inductor and possibly using the ground plane for the flat inductors according to Figures 1 - 4. In Figure 5, the inductor has a ground plane and is connected as a double port, ie the inputs are between the outlet band and the ground plane, while the exit is between the socket at the other end of the belt and the socket to the adjacent ground plane. 10 15 20 25 30 516 361 s In the configuration according to clock 5, the return currents pass through the ground plane and a parasitic capacitance Cp and resistance Rp, respectively, arise between the inductor band and the ground plane. In fi guren, rsmp denotes the losses in the bands and the ground plane. Additional ohmic losses (absorption of free charge carriers) occur if the substrate consists of a semiconductor with free charge carriers. These free charge carriers give rise to substrate currents between points on the belts that have different potentials (cf. fi clock 2). The losses associated with this correspond in the av gure of the shunt losses Rs. Cs is the parasitic capacitance due to capacitive coupling between the bands and through the dielectric substrate.

Finally, the losses due to parasitic currents, in fi guren 1a marked with dashed lines perpendicular to the direction of the main current, correspond to the losses in rsubsi, in the model according to fi guren 5.

Figure 6 shows a single-port configuration, where the structure has a ground plane on the back of the substrate and where the output port is short-circuited.

The components correspond to those in Figure 5.

There is no ground plane for the inductor in the 7 clock 7 or none of the belt connections has been earthed. In this case there is no parasitic capacitance Cp or resistance Rp.

It can be shown that the circuits according to Figures 5 - 7 can in many cases be transformed into the simpler circuit depicted in Figure 8.

It should be noted that if one uses the simplified image according to figuren 8 for the configuration in figuren 7 then R is equal to Rs, C is equal to Cs and r is equal to rsubs., + Rsmp. Correspondingly, it can be shown that the parameters in the simplified variant according to figuren 8 can also be expressed by means of values which are calculated on the basis of the parameters according to figuren 5 and 6.

According to the prior art presented in "On-chip Spiral Inductors With Patterned Ground Shields for Si-based RFlCs", by Yue m fl, IEEE Journ. Solid State Circuits vol. 33, No. 5, page 743, 1998 (D3), one can express the Q-factor for a simplified circuit as above with the following formula: Q = É ._______ R. (1_r2_C ___ 0,2LC) r (DL L R + r (1+ T) 10 15 20 25 30 35 516 361 4 uu 'nu nu l the above formula, R is equal to Rs, and r is equal to rsubst, + rsm-p.

Several proposals have previously been made on how to reduce substrate currents in inductors on lossy substrates, in order to increase the Q value.

Many proposals are based on changes in the substrate which in turn change the resistances r and R. In US-5 757 243 (D4) an inductor is presented where layers are created in the substrate with low and high resistivity by diffusion or other relevant technology, to reduce the substrate currents. In the paper "Reducing the substrate losses of RF Integrated lnductors", Mernyei m fl, IEEE Microwave and Guided Wave Letters, vol. 8, No. 9, page 300, 1998 (D5), a helical planar inductor is presented, which in the present application is shown as 9 gures 9 and 10. The inductor according to the said writing has a star-shaped blocking structure 2, embedded in the substrate, with layers designated 4 - 7.

For the inductor corresponding to the above-mentioned document D5, grooves have been provided in the low-resistance substrate under the inductor, in order to reduce circumferential currents.

According to the older document "Large suspended inductors on silicon and their use in a 2 pm CMOS RF amp | i fi er", by J Y C Chang, IEEE Electron Device Letters, vol. 14, No. 5, page 246, May 1993 (D6), the silicon substrate under certain bands in the inductor structure has been removed by under-etching.

However, the techniques described above require additional masks and technical processes, which makes them expensive to very expensive, so they are not practical for large-scale industrial application. JP-A-06 224 042 (D7) presents a planar inductor consisting of two magnetic disks separated by a glass film, one of the disks having meander-shaped grooves, so that a copper inductor can be formed next to the glass menlm. The structure of the inductor according to this document has a set of inputs which are located close to each other. The inductor is said to provide improved high frequency properties and high Q-factor. However, the nickel-zinc-ferrite sheets have a high resistance factor. Furthermore, this inductor seems unsuitable for the microwave range above 300 MHz, where high losses can be expected. The inductor according to D5 requires a complicated manufacturing technology, which is not compatible with MMIC technology.

In an older document "A Q-factor enhancement technique for MMIC inductors", by M Danesh m fl, IEEE MTT-S Digest, 5/1998 (D8), a square helical microband inductor is presented as 10 15 20 25 30 35 516 361 manufactured with silicon technology for integrated circuits.

According to document D8, you can show that you get a significantly higher Q-factor by driving the microband structure differentially, compared to driving it "single-ended", ie connecting the source to one connection and grounding the other connection.

SUMMARY OF THE INVENTION An object of the present invention is to provide an LC tank which can be manufactured on a low-resistance substrate without requiring any special preparation of the substrate, the LC tank reducing the level of the induced currents in the substrate, and thus higher Q- values.

This object is achieved with a device according to claim 1.

Additional advantageous solutions are stated in the subclaims, which provide advantageous combinations of Q-values, inductances and resistances.

Another purpose is to offer a tunable tank with a high Q-value at high frequencies.

This object is achieved with a device according to patent claim 7.

Among the additional advantages of the invention are the high mechanical stability that characterizes the tank.

Brief Description of the Figures Figure 1 shows a first known inductor with a simple loop, seen from the side; Figure 2 shows the first known inductor with a loop seen from above; ur gures 3 and 4 show a second known inductor with meander inductor; Figures 5 to 8 show known equivalent circuits for inductors in different connection models; ur gures 9 and 10 refer to a third known structure according to an older publication (D5); Figures 11 and 12 schematically show how the substrate currents are balanced according to the invention; The uren clock 13 shows an LC tank consisting of a varactor, according to a first embodiment of the invention, the fi clock showing two band layers shown isometrically and diodes in a symbolic image; 10 15 20 25 30 35 516 361 6 fi clock 14 is a cross section along the line A-A in fi clock 13; G 15 is a cross section along the line B-B in 13 13; The 16 groove 16 is a cross section along the line C-C in the fi groove 13; uren guren 17 is an equivalent circuit for the tank according to fi guren 13; Figure 18 shows another embodiment of an LC tank recovery; Figure 19 shows a third embodiment of the invention; The clock 20 shows a fourth embodiment of the invention; and the 21 guren 21 invention. according to an LC tank according to an LC tank according to shows a fifth embodiment of an LC tank according to a more detailed account of the invention In order to facilitate the understanding of the invention we will discuss the general definition of the Q factor as above, namely as the ratio between the stored average energy and the loss per unit of time, which ratio is multiplied by the frequency.

The stored energy is given by inductances and capacitances, and can be written as a sum of self-inductances and mutual inductances between the bands. As a first approximation, neglecting the energy in the capacitances, the inductively stored energy is proportional to Z (Li + Mij), where Li is the self-inductance in band numbers i and Mi, - is the mutual inductance between the bands i and j. For opposite currents the mutual inductance is negative. The losses correspond to the resistances in the equivalent circuit according to fi gure 8. More specifically, the losses due to the currents in the substrate can be written as Psubs1, = Gfisubsvz-rsubslf, where G1 is a geometry factor determined by the geometry of the bands and the distribution of current in the substrate. Similarly, the losses in the bands can be written as Ps "= Gg-istf-rs fl. The reduced current density in the semiconductor substrate and the possible ground plane results in lower microwave losses and higher Q-factor for the inductor.

According to the invention, the currents in the substrate and thus the losses of the inductor are reduced by arranging the bands in such a way that the induced currents in the substrate balance each other out. Figure 12 is a schematic cross-section through an inductor structure relating to a preferred group of inductors according to the invention, the direction of the currents induced in the substrate being marked (+ into the plane of the paper; o out therefrom). You can see that the currents in nearby bands are opposite.

Figure 11 shows the current density for a given depth in the substrate, as a function of the lateral position. The x-axis of figure 11 corresponds to the extent of the surface of the substrate according to figure 12, and the individual curves lm in figure 11 show the current density in the substrate that would occur at a given current if the other bands did not transmit current. The obtained current density lm when all bands convey the same current level has also been indicated.

Figure 11 shows that the resulting current density is much lower than the current densities that would occur if each band transmitted the same current one at a time. This effect is due to the fact that the currents in the substrate generate opposite magnetic fields around them. These magnetic fields in turn induce opposite currents in the semiconductor substrate according to Figure 2.

Since these currents are also opposite, they partially balance each other out, so that the resulting current in the substrate is lower than the individual currents in the substrate.

The most effective lowering of the resulting currents is achieved when the bands have the same cross section, ie the same width w, and where the distance bs between the bands is small enough to optimize the Q value, ie the values of above all rsum, rsmp and L are optimized, but also Rp, Cs and Cp.

If the distance between the bands becomes too small, the inductance L will deteriorate and the effective resistance rsubsl, will decrease, so that currents leak between the bands. If, on the other hand, the distance becomes too large, the induced magnetic fields will not affect each other, nor will there be any reduction in the substrate currents.

To optimize the design of the coil for a given parameter set, such as current frequency, substrate thickness, substrate resistivity and band conductivity and cross section, one must change the distance between the bands, by trial-and-error or according to calculations, until the currents in the lossy substrate extinguish each other and the maximum Q-factor is obtained. 5 15 20 25 30 35 516 361 a Practical experiments have shown that the balancing of currents in the substrate is strongest for substrates with resistivity up to about 10 Qm.

Particularly high Q-values are also achieved when the sockets for the inductor are placed close to each other with adaptation to the wavelength for which the inductor is intended, ie when the distance between the sockets meets the relationship bt sk / (IOE), where: ef is the substrate's effective dielectric constant and A is the wavelength corresponding to the operating frequency of the inductor. You also get good results when the inductor is connected differentially, ie when none of the inputs are connected to the ground plane.

According to the invention, the belts 1 form at least one loop 13, with one or more segments running in pairs in parallel and having substantially the same length, and are substantially aligned with each other.

Furthermore, adjacent bands are arranged to conduct current in opposite directions, so that currents induced in the loss-generating substrate of the respective legs in the segment balance each other out.

Figure 13 shows a first embodiment of the tank 20. Figures 14, 15 and 16 show a cross section of the tank 20 along the lines A-A, B-B and C-C, respectively. Note that the fi gures are only schematic and do not reflect the actual measurements.

The tank 20 consists of four parallel bands 1 plus other bands which are perpendicular to and connected to the former via corner parts, so that the bands form two loops 13 and 14 which are symmetrical and connected to each other via a bridge part 15, which is formed by a band in a second tape layer 10.

The bands 1 have the same width. Two input connections 12 are formed by an extension of the two interiors of the four parallel bands. The loops are elongated and rectangular. The surface enclosed by the respective loop 13 and 14 and the bridge part 15 has an aspect ratio d: a of about 3, where a = b = c, approximately. Fig. 13 shows the transitions obliquely in relation to the bridge part 15. In reality, they are arranged perpendicular to the bands.

The bridge part 15 comprises the transitions 8 which connect the band which constitutes the bridge part with the bands 1 in the respective loops 13 and 14. The bridge part crosses the upper and underlying bands at right angles. The belts 1 and the bridge part form the inductive part of the tank 20.

As a result of the structure presented in the ur gures, the balancing is based on the distances a, b and c between the bands in the respective loop 10 15 20 25 30 35 516 361 9 and on the properties of the substrate. The aspect ratio between the length d of the parts with adjacent legs running parallel and the distance a, b and c between them is also important. This aspect ratio d: a is about 3, where a is approximately equal to b and c. Testing shows that you get good values if the aspect ratio of the loop is above 2 to 1 or below 1 to 2. Even better results are obtained if the aspect ratio is above 3 to 1 or below 1 to 3. You can determine through experiments where the balancing is optimal.

The high Q value is also calculated to derive from the symmetry of the described structures and the central location of the sockets in relation to the overall inductor structure.

Note that adjacent legs in the different loops in all the described structures conduct current in opposite directions, which means that the currents are balanced between the loops.

Figures 14 to 16 show cross-sections through possible arrangements of substrates and strips for inventive tank structures. In the present exemplary embodiment, the substrate 3 has a first silica layer 4 on top of a second dioxide layer 5, with a total thickness of 45 μm. Three conductive layers 1, 10, 11, preferably of gold, are provided for the bands, with a thickness t of 1 μm.

The width W_1 of the bands 1 is 20 μm. The width W_2 of the bands 11 is advantageously smaller than the width W_1. The first and the second layer of silica are so-called lossy substrate layers with the conductivity 2.5 (mm) -1, while the underlying silicon layer 17 with a thickness of 360 μm has a resistivity of less than 100 omm.

As can be seen from the gurus 14 and 15, a pair of capacitive bands 23 and 24 of length h are formed, extending from the bridge portion 15 of the layer 11 vertically below the bands 1. The two capacitive bands 23 and 24 together with the bands 1 form above a shunt capacitance. whose size depends on the length h, the dielectric properties of layers 4 and 5 and the height |.

A pair of back-to-back-connected varactor diodes 18 and 19 are connected between the inputs 12. The varactor diodes are integrated in the third silicon substrate via a p + -doped region. Suitable transitions 27 form connections between a band 10 and the diodes and thus to the bands 1, via the transitions 8.

A pair of bias conductors 16, 17 establish a connection to a source of bias DC, Vma $ + and Vbiaf, and to the varactor diodes 18, 10 15 20 25 30 516 561 10 19, allowing the varactor diodes' capacitance values to be regulated. The first bias conductor 16 connects to a center point of the inductor structure, more specifically the center point of the bridge part 15. The second bias conductor 17 is connected to the center point between the diodes 18, 19 on the band 10.

Figure 17 shows a circuit equivalent for the system. C_varactor1 and C_varactor2 correspond to the controllable capacitances of the varactor diodes 18 and 19, determined by the magnitude of that DC bias voltage. r is the total resistance and consists of the resistance r_strip of the bands and the resistance r_substrate of the substrate, to the longitudinal substrate currents. C_t is the total capacitance which is the sum of the capacitance between the capacitive conductors 23 and 24 and the bands 1 and the built-in shunt capacitance C J) between the bands and the loss substrate layer 6, respectively.

R__shuntp is the parasitic shunt resistance of the loss substrate. The total inductance is composed of the inductances in the loops 13 and 14 and the parasitic inductance in the bands 8, 15, 16, 23, 24. This also includes the inductive reaction to the longitudinal currents in the substrate.

It will be appreciated that the resonant frequency can be approximated by the following formula: f = 1, / L_t - (C_t + C_var) By suitable dimensioning of, for example, the distance h and tuning of the varactor diodes by means of a suitable direct bias voltage, the desired resonant frequency can be obtained.

Since the capacitive conductors 23 and 24 have been arranged below the band 1 of the inductor, the electric field lines extend mainly along the dielectric low-loss layers 4 and 5, which gives lower total losses. Due to the narrow width W_2 of the bands 23 and 24 in relation to the width W_1 of the band 1 in the loop, a further reduction of the current density in the bands 1 is obtained, so that the current can be distributed more homogeneously in the bands 1 above the capacitive conductors 23 and 24. which also contributes to lower resistance in the bands in associated parts.

It is realized that the tank is symmetrical about a line through the bias conductors 16 and 17 and between the inputs 12. See also 14 gures 14 - 16. 10 15 20 25 30 35 516 361 11 The inventive tank makes it possible to achieve very high Q-values . Practical experiments have shown that Q values above 20 can be achieved for 20 GHz. The tank also provides low noise due to the reduced losses.

The advantages of the tank of the invention are fully achieved if the inputs 12 are connected differentially, ie the connection points are given equal potentials in relation to earth but with different signs.

The third substrate layer 6 in the substrate 3 can form a ground plane. An additional metal layer on the back (not shown) can also act as a ground plane.

If the inputs are connected differentially, a virtual ground plane arises in the plane of symmetry (according to fi gures 14 - 16), along which the DC bias connections 25 and 26 and the bias conductors 16 and 17, respectively, have been placed. Since the impedance in the bias connections appears to be zero, when the potential is zero, no matching networks or decoupling networks are required, such as low-pass filters. The performance of high microwave frequencies and the high Q value are thus not affected by the bias circuit.

Figure 18 shows a second embodiment of a tank 28 according to the invention. The tank 28 is similar to the tank 20 as above, with the difference that no varactors are provided. For applications where no tunable tank is required, and the capacitance can be set with acceptable precision, this embodiment can be used. In practice, it is often difficult to calculate the shunt capacitance, but for some applications this design may be appropriate.

Figure 19 shows a third embodiment 29 of the tank of the invention.

This is also similar to tank 20, but there are no capacitive conductors here. For applications where sufficient capacitance can be achieved by means of varactor diodes, this embodiment may be suitable.

Fig. 20 shows a fourth embodiment, with even higher capacitance values, in that capacitive conductors 23 and 24 are arranged along a larger part of the first and second loops 13 and 14 in the tank, respectively. According to this embodiment, the capacitive conductors from the first bias conductor 16 run close to the first DC connection 25.

Figure 21 shows a fifth embodiment of the tank of the invention, comprising both varactor diodes 18 and 19 and capacitive conductors 23 and 24, respectively, but with only one loop. It can be pointed out that the invention is not limited to the configurations of substrates and tapes described above. The invention also applies to a substrate with your epitaxial layers. In addition, dielectric films can be provided to such an extent that substrate currents are conducted through a loss generating portion of the substrate. As long as currents can potentially be induced in a loss substrate, balancing of currents in this can be accomplished using the principles described.

The tank structures of the invention can therefore be easily applied in a wide range of MMIC applications, such as balanced amplifiers, mixer stages and voltage controlled oscillators, which can lead to new levels of performance for such applications. 10 15 20 25 30 516 361 13 List of designations SCOGNOIUI-ÄOJN-Å -Å .A -xsx -ÄQON -xx-x fl- x (Oæ fl ßi fl l MIOIUN OJIO- * O IU -b OOOJNIUIQION -JCCOCb ict second block band third layer fourth layer transition ground plane second band layer third band layer input first loop second loop bridge portion first bias conductor second bias conductor first diode second diode tank third loop fourth loop first capacitive conductor second capacitive conductor first bias connection second bias connection transition second tank third tank now tank fifth tank fifth tank fifth tank n.

Claims (9)

10 15 20 25 30 35 516 361 14 I n v p »» n «I o n Patent claims: A tank, comprising conductive bands (1) formed on a substrate (3) of material, the inductor having at least two inputs (12) connected to the bands where the inputs are arranged close to each other in accordance with the wavelength for which the inductor is intended; wherein the bands (1) form at least one loop (13, 14), with one or more segments of adjacent parallel legs, the loops representing an induction value and said at least one loop being connected in parallel with at least one capacitor (23, 24, 18). , 19) formed in or on the substrate, characterized in that the substrate (3) consists of a loss-generating dielectric or semiconducting material with a resistivity of less than 10 ohms; and that the respective legs in the segments are of substantially the same length; are substantially aligned with each other and are arranged in pairs to conduct current in opposite directions so that currents induced in the loss-generating substrate (3) of respective legs in the segment balance each other; and that the bands (1) in at least one loop are further arranged so that no two adjacent bands in the loop or loops conduct current in the same direction. A tank according to claim 1, comprising a bridge part (15), wherein the loop (13, 14) and the bridge part (15) seen from above form an enclosed area. A tank according to claim 2, wherein the loop has an elongated substantially rectangular shape and wherein the proportions of the loop, expressed as the ratio of the length (d) to the width (a, b, c) of the area enclosed by the loop (13, 14), is more than 2 to 1 or less than 1 to 2. A tank according to claim 3, wherein said proportions are more than 3 to 1 or less than 1 to 3. A tank according to any one of claims 2 to 4, comprising at least one additional second loop (13, 14) which is substantially symmetrical with the first loop (13, 14) about an axis which intersects the bridge part (15). A tank according to any one of claims 2 to 5, wherein the tank is symmetrical about a line running parallel to and between the inputs (12) and parallel to the loop or loops (13, 14) formed by the belts (1). Tank according to one of Claims 2 to 6, in which the inputs (12) connected to the belts (1) are arranged near the bridge part (15). A tank according to any one of the preceding claims, wherein said at least one capacitor is formed by a pair of back-to-back coupled varactor diodes (19, 20) arranged between the inputs (12); Wherein the tunable tank is adapted to be supplied with a DC bias voltage between a center point between the diodes (18, 19) via a first bias line (16) and a second bias line (17) arranged on an opposite midpoint (V_bias +) on the tank. A tank according to any one of the preceding claims, wherein the capacitor is formed by a pair of capacitive conductors (23, 24) arranged below the belts (1) forming said at least one inductive loop, the capacitive conductors being arranged symmetrically with respect to the belts ( 1) in said at least one loop.