[Tex/LaTex] How to get align or similar equation environment to use the whole line instead of centering around the equality sign

Question

I'm creating a Beamer presentation in which a line in the equation environment should fill the whole screen and should not only be centered arround the equality sign. I think this looks nicer and is much more readable for the audience.
Actually using \multline on just one line gives me exactly that. How can I have that behaviour with \align or any other 'equation aligning' environment?
Here is a comparison, I want to achieve the above with an alignment like below:

\documentclass{beamer}
\usepackage{amsmath}
\usepackage[thicklines]{cancel}
\usepackage{color} 
\definecolor{Blue}{rgb}{0,0,1}
\definecolor{Red}{rgb}{1,0,0}
\newcommand{\ccancel}[2]{\renewcommand{\CancelColor}{\color{#2}}\bcancel{#1}}

\begin{document}
\begin{frame}
\begin{multline*}
                                    \frac{\partial s_{\mu}}{\partial x_\mu}=
                                    A \;[\ccancel{ \frac{\partial \phi^*}{\partial x_{\mu}}
                                    \frac{\partial \phi}{\partial x_{\mu}}}{Red}
                                    -\ccancel{\vphantom{\frac{1}{1}}(\square \phi^*)\phi}{Blue} 
                                    +\ccancel{\vphantom{\frac{1}{1}(kk)}\phi^* \square\phi}{Blue} 
                                    -\ccancel{\frac{\partial \phi^*}{\partial x_{\mu}}
                                    \frac{\partial \phi}{\partial x_{\mu}}}{Red}] 
\end{multline*}
\\[2em]
\begin{align*}
            \frac{\partial s_{\mu}}{\partial x_\mu} & = A \;[\ccancel{ \frac{\partial \phi^*}{\partial x_{\mu}}
                                    \frac{\partial \phi}{\partial x_{\mu}}}{Red}
                                    -\ccancel{\vphantom{\frac{1}{1}}(\square \phi^*)\phi}{Blue} 
                                    +\ccancel{\vphantom{\frac{1}{1}(kk)}\phi^* \square\phi}{Blue} 
                                    -\ccancel{\frac{\partial \phi^*}{\partial x_{\mu}}
                                    \frac{\partial \phi}{\partial x_{\mu}}}{Red}] \\
                                                    & = 0 
    \end{align*}

\end{frame}
\end{document}

Answer 1

I don't like very much cancellation signs. However here's a proposal; notice that I defined two new commands: one for simplifying the input, the second one to get the right spacing for the Laplacian.

\documentclass{beamer}
\usepackage{amsmath}
\usepackage[thicklines]{cancel}
\usepackage{color} 
\definecolor{Blue}{rgb}{0,0,1}
\definecolor{Red}{rgb}{1,0,0}
\newcommand{\ccancel}[2]{\renewcommand{\CancelColor}{\color{#2}}\bcancel{#1}}
\newcommand{\pder}[2]{\frac{\partial#1}{\partial#2}}
\DeclareMathOperator{\lapl}{\square}

\begin{document}
\begin{frame}
\begin{align*}
\pder{s_{\mu}}{x_\mu} &=
   A\biggl[
     \ccancel{\pder{\phi^*}{x_{\mu}}\pder{\phi}{x_{\mu}}}{Red}
    -\ccancel{\vphantom{\dfrac{1}{1}}\bigl(\lapl\phi^*\bigr)\phi}{Blue} 
    +\!\ccancel{\hphantom{\big(}\vphantom{\dfrac{1}{1}}\phi^*\lapl\phi\hphantom{\big)}}{Blue}\!
    -\ccancel{\pder{\phi^*}{x_{\mu}}\pder{\phi}{x_{\mu}}}{Red}
  \biggr]
\\[3ex]
&=0
\end{align*}

\end{frame}
\end{document}

I made also some adjustments for getting the same inclination of the blue bars.